ArticlePDF Available

Neural Correlates of Subjective Awareness and Unconscious Processing: An ERP Study


Abstract and Figures

The aim of the present study was to dissociate the ERP (Event Related Potentials) correlates of subjective awareness from those of unconscious perception. In a backward masking paradigm, participants first produced a forced-choice response to the location of a liminal target presented for an individually calibrated duration, and then reported on their subjective awareness of the target's presence. We recorded (Event-Related Potentials) ERPs and compared the ERP waves when observers reported being aware vs. unaware of the target but localized it correctly, thereby isolating the neural correlates of subjective awareness while controlling for differences in objective performance. In addition, we compared the ERPs when participants were subjectively unaware of the target's presence and localized it correctly versus incorrectly, thereby isolating the neural correlates of unconscious perception. All conditions involved stimuli that were physically identical and were presented for the same duration. Both behavioral measures were associated with modulation of the amplitude of the P3 component of the ERP. Importantly, this modulation was widely spread across all scalp locations for subjective awareness, but was restricted to the parietal electrodes for unconscious perception. These results indicate that liminal stimuli that do not affect performance undergo considerable processing and that subjective awareness is associated with a late wave of activation with widely distributed topography.
Content may be subject to copyright.
Neural Correlates of Subjective Awareness and
Unconscious Processing: An ERP Study
Dominique Lamy, Moti Salti, and Yair Bar-Haim
&The aim of the present study was to dissociate the ERP
correlates of subjective awareness from those of unconscious
perception. In a backward masking paradigm, participants first
produced a forced-choice response to the location of a liminal
target presented for an individually calibrated duration, and
then reported on their subjective awareness of the target’s pres-
ence. We recorded (Event-Related Potentials) ERPs and com-
pared the ERP waves when observers reported being aware vs.
unaware of the target but localized it correctly, thereby isolat-
ing the neural correlates of subjective awareness while con-
trolling for differences in objective performance. In addition, we
compared the ERPs when participants were subjectively un-
aware of the target’s presence and localized it correctly versus
incorrectly, thereby isolating the neural correlates of uncon-
scious perception. All conditions involved stimuli that were phys-
ically identical and were presented for the same duration. Both
behavioral measures were associated with modulation of the
amplitude of the P3 component of the ERP. Importantly, this
modulation was widely spread across all scalp locations for sub-
jective awareness, but was restricted to the parietal electrodes
for unconscious perception. These results indicate that liminal
stimuli that do not affect performance undergo considerable
processing and that subjective awareness is associated with a
late wave of activation with widely distributed topography. &
The search for the neural correlates of consciousness
(NCC) has become one of the most challenging issues
in neuroscience research in the last two decades. This
search relies on the premise that only some neural activ-
ity correlates with conscious experience (Crick & Koch,
1998). To isolate this neural activity, a condition in which
the observer reports being aware of a critical stimulus is
compared to a condition in which the observer reports
being unaware of it.
In real-life situations, consciously perceived stimuli
typically differ from stimuli that remain outside aware-
ness in their physical characteristics (e.g., high-acuity vs.
degraded stimuli), the time allowed to process them, or
the amount of attentional resources allocated to them.
However, to isolate the neural correlates of perceptual
awareness, one must experimentally produce a differ-
ence in subjective experience that cannot be attributed
to objective differences in stimulation, exposure time, or
attention. Researchers have endeavored to meet this goal
by designing paradigms in which visual input remains
the same, whereas conscious perception varies between
aware and unaware states. Such variations in awareness
might take the form of alternations between two differ-
ent interpretations of the same stimulus as in phenom-
ena of perceptual bistability such as binocular rivalry
(e.g., Logothetis, 1998; Tong, Nakayama, Vaughan, &
Kanwisher, 1998), between change blindness and change
detection (e.g., Fernandez-Duque, Grossi, Thornton, &
Neville, 2003; Koivisto & Revonsuo, 2003), or between
missed and seen targets in the attentional blink (e.g.,
Kranczioch, Debener, & Engel, 2003; Vogel, Luck, &
Shapiro, 1998) and in threshold detection tasks (e.g.,
Pins & ffytche, 2003).
In the present study, we used event-related potentials
(ERPs) recorded during a threshold detection task to
investigate the chronometry of neural responses elicited
by stimuli that participants report seeing (henceforth,
‘‘seen’’ or ‘‘aware’’ stimuli) and stimuli that participants
report not seeing (henceforth, ‘‘unseen’’ or ‘‘unaware’’
stimuli). Previous ERP studies of the neural correlates of
perceptual awareness have consistently found the am-
plitude of the P3 component, a large positive def lection
in the ERP occurring 300 to 600 msec after stimulus
onset, to be markedly reduced on unaware trials relative
to aware trials (e.g., Babiloni, Vecchio, Miriello, Romani,
& Rossini, 2006; Sergent, Baillet, & Dehaene, 2005;
Wilenius-Emet, Revonsuo, & Ojanen, 2004; Koivisto &
Revonsuo, 2003; Pins & ffytche, 2003; Vogel et al., 1998).
Differences in earlier ERP waveforms between seen
and unseen targets have also been reported, albeit with
less consistency across studies and with large variability
as to the earliest component found to be modulated by
conscious awareness. Some studies reported awareness-
related amplitude modulation as early as on the P1
Tel Aviv University, Israel
D2008 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 21:7, pp. 1435–1446
component (Pins & ffytche, 2003), the N1 component (e.g.,
Koivisto, Revonsuo, & Lehtonen, 2006; Hunter, Turner, &
Fulham, 2001; Kaernbach, Schroger, Jacobsen, & Roeber,
1999), the P2 component (Vogel et al., 1998), or the
N2 component (e.g., Sergent et al., 2005; Wilenius-
Emet et al., 2004; Koivisto & Revonsuo, 2003; Ojanea,
Revonsuo, & Sams, 2003). Other studies, however, found
no awareness-related modulation in ERP amplitudes prior
to the P3 component (e.g., Babiloni et al., 2006; Fernandez-
Duque et al., 2003; Kranczioch et al., 2003; Turatto,
Angrilli, Mazza, Umilta, & Driver, 2002; Niedeggen,
Wichmann, & Stoerig, 2001). Although these amplitude
differences in ERP waveforms could sometimes be at-
tributed to physical differences between the stimuli
presented in the aware and unaware conditions (e.g.,
Wilenius-Emet et al., 2004; Koivisto & Revonsuo, 2003),
most studies used identical stimuli in the two conditions.
Conscious and Unconscious Perception
In experiments designed to investigate unconscious
perception (e.g., Sidis, 1898; see also Merikle, Smilek,
& Eastwood, 2001), when participants report not seeing
a stimulus (unaware trials), a distinction is made be-
tween trials in which no perception occurs and trials in
which the stimulus is unconsciously perceived, that is,
influences behavior outside of subjective awareness. In
a typical experiment, the critical stimulus is presented
under conditions that prevent conscious perception. Two
types of measures are contrasted: one is an explicit re-
port of whether or not a stimulus has been subjectively
seen; the other is an indirect measure that bypasses
the participant’s introspection and reveals whether the
stimulus is capable of influencing the participant’s be-
havior. For instance, despite denying any perception of
a masked word, the observer may provide the correct
response more often than would be expected by chance
when forced to choose among alternative words. Such
above-chance performance for an unseen stimulus, how-
ever, is typically poorer than when the stimulus is seen.
Therefore, all unseen targets do not share the same fate:
Some undergo enough processing to elicit a correct re-
sponse, whereas others do not.
Previous ERP studies of the neural correlates of per-
ceptual awareness typically did not dissociate between
awareness and task performance. Indeed, they used only
one behavioral measure designed to index the partic-
ipants’ subjective awareness of the critical stimuli, and
did not provide a separate measure of the extent of
processing on unaware trials. In consequence, trials that
were classified as ‘‘unaware,’’ in fact included two dif-
ferent categories of trials, namely, ‘‘unconscious percep-
tion’’ trials and ‘‘no perception’’ trials. One important
implication of the failure to distinguish between these
categories is that differences in neural activity that were
attributed in previous studies to differences in process-
ing of seen versus unseen targets may have also included
differences between processed and barely processed
targets. Such differences do not specifically ref lect the
neural correlates of consciousness, as they can also
occur between unseen targets on different trials, namely,
between ‘‘unconscious perception’’ and ‘‘ no percep-
tion’’ trials. To illustrate, a participant will report not
seeing a stimulus when blindfolded, yet obviously, the
conclusion that retinal stimulation is correlated with
visual awareness is of little informative value because
retinal stimulation is also correlated with any type of
visual processing, whether conscious or unconscious.
Subjective and Objective Measures of Awareness
Within the framework of the debate surrounding the
existence of unconscious perception, the use of subjec-
tive measures of awareness to distinguish between con-
scious perception and absence thereof has been sharply
criticized (e.g., Draine & Greenwald, 1998; Holender,
1986). Objective measures of awareness were suggested
as a more accurate method for assessing whether stimuli
are perceived with or without awareness. With objective
measures, it is assumed that any ability to discriminate
between alternative stimulus states at a better-than-
chance level of performance indicates that the critical
stimulus was perceived with awareness. An inability to
do so reflects absence of awareness. Accordingly, the
neural correlates of consciousness derived from com-
paring the neural activity associated with a subjectively
seen versus unseen stimulus may amount to comparing
different levels of awareness and fail to capture potential
qualitative differences between conscious and noncon-
scious processing.
In the present study, we contrasted the neural activ-
ity evoked by ‘‘aware’’ and ‘‘unaware’’ stimuli while
addressing the potential confounds associated with the
distinction between perception with versus without
subjective awareness on the one hand, and between
subjective versus objective measures of awareness on
the other hand, within the same experiment. On each
trial, a target was presented in one of four possible
locations for a near-threshold exposure time determined
individually for each participant in a calibration phase,
such that under constant stimulus conditions, the tar-
get was subjectively seen on roughly half of the trials.
Participants were required to provide two separate
responses. They first made a speeded forced-choice
localization response to the target, and immediately af-
terward indicated whether their decision was based on
their conscious perception of the target or on guessing.
We derived ERP waveforms associated with three distinct
categories of trials: (1) trials in which participants were
subjectively aware of the stimulus and correctly localized
it (aware-correct condition); (2) trials in which partic-
ipants were subjectively unaware of the stimulus, yet
correctly localized it (unaware-correct condition); and
(3) trials in which participants were subjectively unaware
1436 Journal of Cognitive Neuroscience Volume 21, Number 7
of the stimulus and incorrectly localized it (unaware-
incorrect condition). Importantly, all three categories of
trials involved identical stimuli. We expected only a very
small number of trials in which participants were sub-
jectively aware of the stimulus, yet incorrectly localized it
(aware-incorrect condition). Therefore, this condition
was not included in the planned analyses.
On the one hand, we contrasted the neural activity
evoked by seen and unseen targets that underwent
enough perceptual processing to elicit a correct re-
sponse. This comparison between the aware-correct
and unaware-correct conditions allowed us to narrow
the potential differences in perceptual processing be-
tween subjectively seen versus unseen targets because
localization performance was equated between the two
conditions, and thereby to better circumscribe the differ-
ences in neural activity that are specifically associated
with subjective perceptual awareness.
On the other hand, based on previous studies of
perception without subjective awareness, we expected
localization performance for unseen targets to be better
than chance, that is, we expected unconscious percep-
tion to occur. Thus, because above-chance localization
performance indicates that participants are able to dis-
criminate between different states of the target stimulus
(i.e., its location), according to the objective-measure
approach, participants should be considered objectively
aware of the target in the unaware-correct condition (at
least on those trials in which the correct answer is not
arrived at by chance). By contrast, one can be confident
that participants are objectively unaware of the target
when they are unable to localize it (unaware-incorrect
condition). Thus, the neural correlates of awareness
defined according to the objective-measure approach
may be studied by comparing unaware-correct trials and
unaware-incorrect trials. It is important to emphasize
that this rationale is contingent on above-chance local-
ization performance for unseen targets. To illustrate, it
would be absurd to claim that in a task in which par-
ticipants perform at chance, correct trials are trials in
which they are objectively aware of the target and in-
correct trials are trials in which they are objectively un-
aware of it.
In this study, in order to avoid confusion, we will
adopt the terminology associated with the subjective-
measure approach of awareness and label above-chance
forced-choice performance in the absence of subjective
awareness ‘‘unconscious perception’’ rather than ‘‘ob-
jective awareness.’’
Twenty three right-handed students (6 men, 22–28 years
of age) participated for pay ($10). All reported normal or
corrected-to-normal visual acuity.
The fixation display was a cross subtending 0.58of visual
angle. The target display was a 15 15 matrix made up
of line segments tilted to the right, each of which
subtended 0.58of visual angle. On target-present trials,
a square region of 3 3 line segments was randomly
chosen at one of four possible locations: the upper-left,
upper-right, lower-left, or lower-right corner of the
matrix, and centered at an eccentricity of 48of visual
angle from the fixation point (Figure 1). Line segments
within this square region were tilted by 258, whereas line
segments in the remainder of the matrix were tilted by
158. The resulting percept was a square figure against a
background. On catch trials, all line segments in the
target display had the same orientation, thus no square
figure was visible. The masking display consisted of the
matrix with two line segments in the two possible
orientations (that of the background and that of the
square) superimposed in each cell. All stimuli were gray
on a black background.
Behavioral Procedure
On each trial, the fixation display appeared for 500 msec.
The target display was then presented for a variable du-
ration, as described below. The masking display imme-
diately followed and remained on the screen for 500 msec.
Participants were required to produce two responses.
First, they made a speeded forced-choice response to
the location of the target by pressing one of four desig-
nated keys with one hand (localization response). A ques-
tion mark appeared immediately after the first response,
prompting the participants to indicate by pressing one of
two other designated keys with their other hand whether
they had seen the target or merely guessed its location
(awareness response). A new trial began 500 msec after
the second response. Response-to-hand mapping was
counterbalanced between participants.
The experiment consisted of a calibration phase fol-
lowed by an experimental phase. The calibration phase
Figure 1. Example of the target (left) and mask (right) stimuli. In
this example, the target appears in the lower right corner.
Luminance polarity was inverted in the actual experiment (gray
lines on a black background).
Lamy, Salti, and Bar-Haim 1437
was designed to determine the target-display exposure
duration that would yield an approximately equal num-
ber of trials in which the target square would be seen or
not seen (a 50% detection threshold). We used a mod-
ified version of the threshold estimation procedure de-
scribed by Levitt (1971). Initial exposure duration was
set to 16 refresh rates (200 msec), and then exposure
duration changed every six trials by steps of one refresh
rate (12.5 msec) based on the participant’s awareness
responses. Exposure duration was shortened when the
participant reported seeing the target on more than
three out of six trials and was lengthened when the
participant reported not seeing the target on more than
three out of six trials. Exposure duration remained un-
changed when the participant reported seeing the target
on exactly three out of six trials. The calibration phase
included 130 trials. A participant’s detection threshold
was defined as the lowest target exposure duration that
was maintained over two consecutive blocks of six trials.
The experimental phase was similar to the calibration
session except for the following changes. It included 520
trials divided into four blocks, with a rest period allowed
between blocks. It included four types of trials randomly
mixed within the blocks: catch trials (7% of all trials);
above-threshold trials, in which the target display was
presented for a duration of 6 refresh rates (75 msec)
above the detection threshold individually determined
for each participant during calibration (7% of the trials);
short-exposure trials, in which exposure duration was
set at one refresh rate below the detection threshold
(43% of all trials); and long-exposure trials, in which
exposure duration was set at the detection threshold
(43% of all trials). Above-threshold trials were included
in order to verify that the participants indeed complied
with the instructions: They were expected to report
being aware of the target on a high proportion of such
trials. Short-exposure trials were included because pilot
data indicated that due to practice effects, some of the
participants tended to achieve higher percentages of
awareness with the same exposure times in the exper-
imental phase relative to the calibration phase.
EEG Recordings and Analysis
Continuous EEG was recorded from 20 scalp sites (Fp1,
Fp2, F7, F3, Fz, F4, F8, T3, T4, T5, T6, C3, Cz, C4, P3, Pz,
P4, O1, Oz, and O2, plus the left and right mastoids)
while participants performed the behavioral task. EEG
data were recorded using a stretch Lycra cap (Electro-
Cap, Eaton, OH) with pure-tin electrodes located accord-
ing to the International 10–20 System. All EEG channels
were collected referenced to the chin. Vertical and hor-
izontal EOG were recorded from above and below the
left eye and at the right and left outer canthi, respectively.
All electrode impedances were kept below 5 k. EEG
and EOG signals were amplified with Ceegraph IV bio-
amplifier (Biologic Systems), and were digitized onto a PC
using a 16-bit A/D converter and Ceegraph IV data ac-
quisition software. For both EEG and EOG, sampling rate
was 256 Hz with bioamplifier filter settings of 0.1 Hz
high pass and 100 Hz low pass. Further processing and
analysis of the EEG signal were carried out off-line us-
ing BPM software package (Orgil Company). Artifactual
EEG (±100 AV) was automatically removed from fur-
ther analysis. Eye blinks that appeared in the EOG sig-
nal were regressed out of the EEG using a procedure
based on the methods described in the literature (e.g.,
Miller & Tomarken, 2001; Lins, Picton, Berg, & Scherg,
1993). Overall, 5% of the trials were removed due to arti-
facts in the EEG signal, with similar percentages of trials
removed from each condition (aware correct, unaware
correct, unaware incorrect). Before derivation of the ERPs,
the EEG signal was subjected to a 30-Hz low-pass digital
Separate ERP waveforms were derived for each par-
ticipant by averaging trials in each of the experimental
conditions (aware correct, unaware correct, unaware
incorrect) and for each exposure duration (short and
long). ERP waveforms were measured relative to a base-
line epoch of 200 msec preceding the target matrix on-
set. Based on inspection of the grand-averaged ERPs,
mean amplitudes (AV) for all experimental conditions
were computed within the following time windows: P1
(109–150 msec), N1 (148–187 msec), P2 (178–261 msec),
N2 (230–304 msec), and P3 (375–550 msec).
Behavioral Responses
The data from three participants were excluded because
the ERP recordings for these were lost due to technical
failure. Thus, the data from 20 participants were analyzed.
The mean percentage of false alarms (‘‘aware’’ re-
sponses on catch trials) was 6.5% (SD = .05) on short-
exposure trials and 6.2% (SD = 0.07) on long-exposure
trials. The mean percentage of misses (‘‘unaware’’ re-
sponses on target-present trials) in the above-threshold
condition was 11.2% (SD = 0.12), confirming the reli-
ability of the participants’ self-reports. Because the false
alarm rate was low and because catch trials made up
only 7% of all trials, there were not enough data in the
relevant cells to perform a reliable signal-detection esti-
mation of sensitivity and criterion (Green & Swets, 1966).
The individual threshold exposure times ranged from
2 to 8 screen refresh rates for the short exposure (25
to 100 msec, respectively) and from 3 to 9 screen refresh
rates for the long exposure (37 msec to 112 msec).
Mean percentage of ‘‘aware’’ responses on target-present
trials was 25.8% (SD = 0.13) for the short exposure and
50.5% (SD = 0.17) for the long exposure, confirming that
the percentage of aware trials established in the calibra-
tion phase was generally maintained in the experimental
phase. Of these, 89.0% (SD = 0.15) were responded to
1438 Journal of Cognitive Neuroscience Volume 21, Number 7
correctly for the short exposure and 96.1% (SD = 0.06)
for the long exposure. On unaware trials, that is, on the
remaining 74.2% of the trials for the short exposure, and
49.5% of the trials for the long exposure, the percentage
of correct responses was 51.6% (SD = 0.16) and 59.3%
(SD = 0.15) for the short and long exposures, respec-
tively. Localization performance was therefore clearly
above chance (25%) when the participants reported
being unaware of target presence. The distribution of
trials per condition is summarized in Table 1.
An analysis of variance (ANOVA) was conducted on
localization reaction times data with condition (aware
correct, unaware correct, and unaware incorrect) and
threshold exposure (short vs. long) as factors. None of
the effects approached significance. There was no main
effect of threshold exposure [F<1(M= 526 msec, SD =
73 vs. M= 522 msec, SD = 78 for the short vs. long
exposure conditions, respectively)] and no main effect
of condition [F(1, 19) = 1.21, p>.3(M= 508 msec,
SD = 68 for the aware-correct condition; M= 538 msec,
SD = 56 for the unaware-correct condition; and M=
536 msec, SD = 70 in the unaware-incorrect condition)].
Specifically, despite a numerical trend, there was no sig-
nificant difference in reaction times between the two
correct-performance conditions [F(1, 19) = 1.55, p> .2],
and the interaction between condition and threshold
exposure was nonsignificant [F(1, 19) = 1.39, p> .2].
Event-related Potentials
Figure 2 shows grand-averaged ERPs for the aware-
correct, unaware-correct, and unaware-incorrect condi-
tions for each of the recorded electrode sites for the
short exposure (top) and for the long exposure (bot-
tom). Preliminary ANOVAs were conducted to examine
potential lateralization effects on the mean amplitudes
of each of the preselected ERP components. In one set
of analyses, electrode site (20), condition of awareness
(aware correct vs. unaware correct in one analysis and
unaware correct vs. unaware incorrect in another anal-
ysis) and side of target appearance (right vs. left) served
as factors. In another set of analyses, scalp region
(frontal, temporal, central, parietal, and occipital), con-
dition of awareness, and side of target appearance
served as factors. These analyses yielded no significant
interactions involving condition of awareness and side
of target appearance. Therefore, subsequent analyses
were carried out on mean ERP amplitudes over five
scalp regions: frontal (mean amplitude of Fp1, Fp2, F7,
F3, Fz, F4, F8), temporal (mean amplitude of T3, T4, T5,
T6), central (mean amplitude of C3, Cz, C4), parietal
(mean amplitude of P3, Pz, P4), and occipital (mean am-
plitude of O1, Oz, O2), and collapsed across sides of
target appearance.
Subjective Measure of Awareness (Subjective Report)
We compared the ERP waveforms associated with trials
that were identical in terms of physical stimulus, expo-
sure time, and participants’ responses to the target
(correct responses only), and differed only in the partic-
ipants’ subjective experience, that is, in whether they re-
ported being aware or unaware of the target. An ANOVA
with condition (aware correct vs. unaware correct), scalp
region (frontal, temporal, central, parietal, occipital), and
exposure (short vs. long) as within-subject factors was
conducted on the mean amplitudes of the P1, N1, P2,
N2, and P3 components of the ERP. Statistical data are
presented in Table 2.
The mean amplitude of the P3 component was signif-
icantly higher in the aware-correct condition (M= 4.36,
SE = 0.61) than in the unaware-correct condition (M=
1.99, SE = 0.80) [F(1, 18) = 18.04, p< .0001]. This effect
interacted with scalp region [F(4, 76) = 5.34, p< .001].
Follow-up comparisons showed that although significant
effects of condition were obtained for all scalp regions,
these were more pronounced over the central and pari-
etal regions (see Table 3 for statistics).
Visual inspection of the ERPs for the P2 component
suggested that for the short exposure (Figure 2, top),
there was a trend toward larger amplitude in the aware-
correct relative to the unaware-correct conditions over
the central and frontal scalp regions. This observation
was confirmed by ttests [t(18) = 1.97, p< .07 and
t(18) = 1.91, p< .08 for the frontal and central regions,
]. There were no significant effects or re-
markable trends involving condition (aware correct vs.
unaware correct) for any of the other ERP components.
In the present study, there were only four alternative
localization responses, such that a nonnegligible portion
of the unaware-correct trials were trials in which par-
ticipants responded to correctly by chance. Thus, the
observed unaware-correct ERP waveform, in fact, repre-
sented a mixture of the neural response to unaware
trials that were responded to correctly due to sufficient
perceptual processing and of the neural response to un-
aware trials that were responded to correctly by chance.
By contrast, the portion of aware-correct trials that were
responded to correctly by chance was inconsequential
because accuracy on those trials was very high. Brain ac-
tivity associated with chance performance can be indexed
by the ERP waveform on unaware-incorrect trials, for which
the amplitude of the P3 component was substantially
Table 1. Mean Percentage of All Trials by Conditions of
Awareness (Aware or Unaware, Subjective Measure) and
Localization Performance (Correct or Incorrect, Objective
Measure) for the Short and for the Long Exposure Durations
Short exposure 23.0% 2.8% 38.3% 35.9%
Long exposure 48.5% 2.0% 29.4% 20.1%
Lamy, Salti, and Bar-Haim 1439
smaller than its amplitude on correct-performance trials
(see Figure 2). It follows that the observed lower mean
amplitude in the unaware-correct waveform relative to the
aware-correct waveform on the P3 component may result
from the higher proportion of chance responding in the
latter relative to the former condition.One might therefore
argue that there is, in fact, no difference between aware-
correct and unaware-correct trials on the P3 component,
when chance responding is taken into account.
We conducted additional analyses to examine this pos-
sibility. Specifically, we sought to mathematically esti-
mate whether the amplitude of the P3 component of the
ERP waveform corresponding to unaware-correct trials
that were not responded to correctly by chance (hence-
forth, chance-free unaware-correct trials) would remain
lower in amplitude than the P3 amplitude of the aware-
correct waveform. Note that although the finding that
localization accuracy on unaware trials was well above
chance tells us that such chance-free unaware-correct
trials indeed occurred, one cannot determine whether an
individual trial was a chance trial or a chance-free trial.
Thus, we could only estimate the amplitudes of the wave-
form corresponding to chance-free unaware-correct trials.
The calculations used to derive a hypothetical approxima-
tion of the unknown chance-free unaware-correct wave-
form from the known waveforms corresponding to chance
trials (unaware-incorrect waveform) and to unaware-
correct trials are described in the footnote.
We con-
ducted an ANOVA on ERP mean amplitudes in the P3
time window with condition (aware correct vs. chance-
free unaware correct), scalp region (frontal, temporal,
central, parietal, occipital), and exposure (short vs.
long) as factors and found that the amplitude of the P3
component remained significantly larger in the aware
correct than in the chance-free unaware-correct condi-
tion in all scalp regions (see Tables 2 and 3 for statistics).
Note that although the estimated chance-free unaware-
correct waveform does not reflect the neural processes
that actually took place in the participants’ brains, it
should provide a reasonable approximation for the
purpose of rejecting the argument that chance respond-
ing alone accounts for the P3 difference attributed to
subjective awareness.
Unconscious Perception (Objective Performance)
To examine the neural correlates of unconscious pro-
cessing, we compared the ERP waveforms associated
with trials that were identical in terms of physical stimulus,
Figure 3. Mean of long and short exposures ERP waveforms at Pz
for the aware-correct, unaware-correct, and unaware-incorrect
conditions. The time window for the P3 component used for
analyses is depicted in light gray. Below are scalp current density
maps for the three experimental conditions during the P3 time
window. The aware-correct condition elicited a widespread
positivity across the entire scalp, the unaware-correct condition
elicited a positivity restricted to the parietal scalp region, and the
unaware-incorrect elicited little activation across the scalp.
Figure 2. Grand mean event-related potentials (ERPs) of the
aware-correct (red), unaware-correct (blue), and unaware-incorrect
(yellow) conditions for the short-exposure condition (top) and for
the long-exposure condition (bottom). The ERPs are time-locked to
matrix display onset and are calculated relative to a 200-msec baseline.
1440 Journal of Cognitive Neuroscience Volume 21, Number 7
stimulus exposure duration, and subjective awareness
(unaware trials), and differed only in the accuracy of the
participant’s localization response (correct vs. incorrect). It
is important to note that our operational definition of
unconscious perception or correct objective performance
departs in important ways from the one commonly used.
Typically, the objective performance threshold is defined
as the stimulation conditions (e.g., level of stimulus deg-
radation or stimulus-to-mask SOA) at which the observer
performs at chance in discriminating different states of
that stimulus. Thus, stimulation conditions necessarily
differ between a stimulus that an observer does not per-
ceive (incorrect objective performance) and a stimulus that
the observer perceives unconsciously (correct objective
Table 2. Significance Values of the Effects of Subjective Awareness (Condition: Aware Correct vs. Unaware Correct) and
Objective Performance (Condition: Unaware Correct vs. Unaware Incorrect) and Relevant Interactions on the Mean Amplitude
of Each Component of the ERP
Condition Condition
Region Condition
Exposure Condition
df(1, 19) df(4, 76) df(1, 19) df(4, 76)
ERP Component F p F p F p F p
Aware Correct vs. Unaware Correct
P1 <1 ns <1 ns <1 ns <1 ns
N1 <1 ns 1.10 ns <1 ns <1 ns
P2 <1 ns 1.32 ns <1 ns <1 ns
N2 1.26 ns 1.21 ns 1.93 ns <1 ns
P3 18.04 <.0001 5.34 <.001 <1 ns <1 ns
12.04 <.003 2.61 <.08 <1 ns <1 ns
Unaware Correct vs. Unaware Incorrect
P1 <1 ns <1 ns <1 ns <1 ns
N1 <1 ns <1 ns <1 ns 1.46 ns
P2 <1 ns 1.56 ns <1 ns <1 ns
N2 <1 ns <1 ns <1 ns <1 ns
P3 <1 ns 9.7 <.0001 <1 ns <1 ns
1.37 ns 3.75 <.003 <1 ns <1 ns
Chance-free unaware-correct condition.
Table 3. Significance Values of the Effects of Subjective Awareness (Aware Correct vs. Unaware Correct) and Objective
Performance (Unaware Correct vs. Unaware Incorrect) on the Mean Amplitude of the P3 Component in Each Scalp Region
Frontal Temporal Central Parietal Occipital
df(1, 19) df(1, 19) df(1, 19) df(1, 19) df(1, 19)
Fp F pFpFpFp
Aware Correct vs. Unaware Correct
8.67 <.007 8.43 <.009 19.24 <.0001 24.19 <.0001 22.91 <.0001
6.02 <.03 5.08 <.04 8.55 <.009 9.64 <.006 7.04 <.02
Unaware Correct vs. Unaware Incorrect
1.27 >.3 <1 ns 1.92 ns 6.76 <.02 2.99 <.1
1.61 <.3 <1 ns 2.00 ns 7.04 <.02 3.26 <.09
Chance-free unaware-correct condition.
Lamy, Salti, and Bar-Haim 1441
performance). Here, it was critical to keep the stimulus
constant across all conditions so as to ensure that differ-
ences in neural responses held to reflect differences in
objective performance were not confounded with differ-
ences in physical stimulation. Because localization perfor-
mance was clearly above chance, stimulus conditions were
such that observers unconsciously perceived the target on
the observers produced an incorrect response, it is rea-
sonable to claim that they did not perceivethe target. Such
trials were therefore defined as ‘‘no-perception’’ trials.
We conducted an ANOVA on ERP mean amplitudes
with condition (unaware correct vs. unaware incorrect),
scalp region (frontal, temporal, central, parietal, occipi-
tal), and exposure (short, long) as factors (see Table 2
for statistics). The mean amplitude of the P3 component
was higher in the unaware-correct than in the unaware-
incorrect condition only over the parietal region. There
was no main effect of condition (unaware correct vs.
unaware incorrect) or remarkable trends in earlier ERP
components (P1, N1, P2, and N2) and no significant in-
teractions involving this factor. Note that the difference
between the unaware-correct versus unaware-incorrect
waveforms was underestimated because the unaware-
correct condition included a nonnegligible portion of
chance correct trials. However, the same analyses using
the hypothetical chance-free unaware-correct waveform
instead of the raw unaware-correct waveform yielded
no additional significant effects (see Tables 2 and 3 for
detailed statistics).
The comparison between the ERP correlates of sub-
jective awareness and unconscious perception is illus-
trated in Figure 3.
Our procedure allowed us to isolate the ERP correlates
of subjective awareness by comparing the aware and
unaware conditions when these did not differ in the ob-
servers’ objective performance on a forced-choice local-
ization task, and the stimulus parameters were identical
in the two conditions. We could also distinguish be-
tween the ERP correlates of subjective awareness and
those of unconscious perception, which was ref lected
behaviorally in above-chance localization of the target
when the observers reported being unaware of its
presence. Contrasting subjective and objective measures
of perception is a widely used method to study percep-
tion without subjective awareness in healthy observers
(Merikle et al., 2001; Draine & Greenwald, 1998; Merikle
& Reingold, 1998; Marcel, 1983) and in patients with
neuropsychological conditions associated with impaired
awareness such as neglect (e. g., Driver & Vuilleumier,
2001) or blindsight (Lamme, 2001; Cowey & Stoerig,
1995). Yet, to our knowledge, the present study is the
first ERP experiment to apply this method to investigate
the neural correlates of visual consciousness.
We found that the amplitude of the P3 component
was larger in the aware relative to the unaware condi-
tion. This awareness-related difference was widely dis-
tributed over the scalp, as is clear from the highly
significant differences in P3 amplitudes across all scalp
regions. It specifically reflected only subjective aware-
ness of the target because seen and unseen target
displays were physically identical, appeared for the same
duration, and elicited the same correct localization
response. The amplitude of the P3 component was also
larger when a target that was not seen was correctly
localized vs. incorrectly localized. However, unlike the
wide scalp distribution of the activity related to subjec-
tive awareness, the difference in P3 amplitude associated
with correct objective performance was strictly limited
to parietal electrodes.
The amplitude of early ERP components (P1, N1, P2
and N2) was not affected by whether the observers
were subjectively aware of the target or missed it, or
by whether or not they localized it accurately. Preserva-
tion of early perceptual components in the absence of
subjective awareness suggests that although a stimulus
is more likely to be consciously seen if it undergoes
enhanced perceptual processing, perceptual processing
is not sufficient for reportable awareness (e.g., Super,
Spekreijse, & Lamme, 2001; Marcel, 1983).
Visual Awareness or Confidence Level?
It could be argued that the P3 differences observed here
might reflect variations in the participants’ confidence
level rather than variations in awareness. Such a pro-
posal was recently put forward by Eimer and Mazza
(2005). They suggested that the P3 amplitude modula-
tions observed in ERP studies of visual awareness (and
specifically of change detection) primarily ref lect varia-
tions in observers’ confidence with respect to the pres-
ence versus absence of the critical stimulus. To test this
claim, they used a change detection task in which
participants first indicated whether they detected a
change, and then rated how confident they were of
their decision. Thus, for instance, a high confidence
level, together with an ‘‘I did not see’’ response, indi-
cated that the participant was highly confident of not
seeing the change, whereas a high confidence level with
an ‘‘I saw’’ response indicated that the participant was
highly confident of seeing the target. Amplitude of the
P3 component was higher when participants reported
seeing the change than when they reported not seeing
it. However, this difference was modulated by the
participants’ confidence: It was significant when confi-
dence was high, but not when confidence was low.
Eimer and Mazza concluded that P3 modulations were
determined by participants’ confidence levels rather
than by variations in their awareness of the change.
However, this interpretation overlooks the fact that
significant effects of subjective awareness (detection vs.
1442 Journal of Cognitive Neuroscience Volume 21, Number 7
no detection) on P3 amplitude were found when confi-
dence was high, that is, for a constant confidence level.
Obviously then, variations in confidence cannot account
for this difference. In addition, P3 amplitude did not
differ between detection and no-detection trials when
confidence was low, which further supports the notion
that P3 is associated with variations in subjective aware-
ness. When confidence is low, participants are likely
to choose their response (‘‘I saw’’ or ‘‘I did not see’’)
at chance, and awareness level should be similar on
the two types of trials, hence, the null effect on low-
confidence trials. It follows that, although an alternative
account of our findings in terms of confidence level
cannot be rejected on the sole basis of the present data,
the results reported by Eimer and Mazza (2005) do not
favor an alternative account for P3 modulations by
awareness in terms of varying confidence levels.
Relation to Previous Findings
Our results are consistent with the conclusions from
previous reports involving a wide array of stimuli and
paradigms, according to which the amplitude of the P3
component is the primary ERP correlate of visual aware-
ness (Del Cul, Baillet, & Dehaene, 2007; Babiloni et al.,
2006; Sergent et al., 2005; Fernandez-Duque et al., 2003;
Kranczioch et al., 2003; Turatto et al., 2002; Niedeggen
et al., 2001). However, the present study goes beyond
previous ERP findings because it allows dissociating be-
tween subjective awareness and objective performance.
Previous studies compared the neural fate of seen and
unseen targets without considering potential differences
in objective performance relative to unseen targets. Two
ERP studies to date (see also Lau & Passingham’s, 2006
fMRI study for a similar rationale) have concomitantly
collected behavioral measures of subjective awareness
and objective performance but the specific procedures
they used did not allow them to dissociate between the
neural correlates of the two behavioral measures. In a
recent study by Babiloni et al. (2006), participants were
required first to localize a target preceded by a barely
visible cue that was spatially either congruent or incon-
gruent with the position of a subsequent target and then
to indicate whether or not they had seen the cue.
Subjective report of seeing the cue was the measure of
conscious processing and the effect of congruency on
response latencies to the target for unseen cues was the
measure of unconscious processing. Average reaction
times on incongruent-cue trials were faster than on
congruent-cue trials, which indicated that unconscious
processing occurred in that study. However, it was not
possible to distinguish between the individual trials in
which unconscious processing occurred and those trials
in which it did not occur because the congruency effect
could only be measured across the experiment as a
whole. In other words, one could not infer from the
reaction time obtained on a particular trial in which the
cue had not been seen whether this trial belonged to
the unconscious perception or to the no-perception
category. Accordingly, in Babiloni et al.’s study, the
neural correlate of subjective awareness was defined as
the difference between seen and unseen cues, and sub-
jective awareness and objective performance were there-
fore confounded.
Del Cul et al. (2007) varied target-to-mask SOA and
collected two responses with regard to a masked target,
namely, a forced-choice discrimination as to whether
the target digit was smaller or larger than the digit 5
(objective measure of perception) and a rating of how
visible the target was (subjective measure of percep-
tion). To investigate the neural correlate of subjective
awareness, they compared ERPs at the liminal exposure
of 50 msec between seen and unseen trials, irrespective
of objective performance. In addition, to assess the
neural correlates of unconscious processing (or correct
objective performance), these authors compared ERPs
on target-present correct-performance trials with ERPs
on no-target trials, acknowledging that there were not
enough trials in their experiment to analyze the more re-
vealing conditions of correct and incorrect performance
target-present trials with the same SOA. Thus, unlike in
the present experiment, the ERP correlate of subjective
awareness in Del Cul et al.’s study could also ref lect
differences in objective performance, and the ERP cor-
relate of objective performance could also ref lect phys-
ical stimulus differences (target plus mask vs. mask only).
Relation to Existing Models of Visual Awareness
Our results argue against the view that the differences
between conscious and unconscious processing arise
at early stages of perceptual processing (e.g., Pins &
ffytche, 2003). In our study, the ERP correlate of subjec-
tive visual awareness was reflected in an upsurge of
neural activation about 375 msec after stimulus onset
(P3 component). This awareness-related activation was
characterized by enhanced P3 amplitude with a widely
distributed topography (evident over all recorded elec-
trode sites). These topographical and temporal char-
acteristics are in line with recent findings supporting
the global workspace model (Block, 2001; Dehaene &
Naccache, 2001). For instance, Sergent et al. (2005) com-
pared the ERP waveforms associated with seen versus
unseen targets in an attentional blink task and conclud-
ed that the transition toward access to consciousness
is associated with a late P3 wave of activation that
spreads through a widely distributed network of cortical
association areas. In the same vein, using a backward
masking procedure, Del Cul et al. (2007) found that a
considerable amount of processing of unseen targets
occurs early on, whereas access to subjective awareness
relates to a late and highly distributed fronto-parieto-
temporal activation corresponding to the P3 component
of the ERP.
Lamy, Salti, and Bar-Haim 1443
The finding that subjective awareness is associated
with the late activation of a widely distributed cortical
network appears to stand in contrast with other views of
visual awareness, which associate it with brain activity
limited to specific regions of the prefrontal cortex (e.g.,
Lau & Passingham, 2006; Sahraie et al., 1997) or with
recurrent processing in posterior areas of the brain (e.g.,
Lamme, 2001). However, it is important to note that the
findings supporting these alternative views were ob-
tained using different methodologies and might there-
fore capture other aspects of the difference between the
aware and unaware conditions.
Specifically, the studies favoring the prefrontal cortex
as the seat of visual awareness used fMRI, the temporal
resolution of which may have been insufficient to re-
veal the relatively short-lived widespread activations
reported in the present and previous ERP studies. The
studies favoring the recurrent-processing hypothesis
typically used TMS in humans (e.g., Pascual-Leone &
Walsh, 2001), V1 lesions in monkeys (e.g., Cowey &
Stoerig, 1995) or blindsight patients (e.g., Lamme, 2001).
A common characteristic of the latter studies is that they
showed that recurrent processing in posterior cortical
areas is necessary for visual awareness. However, be-
cause they focused on early visual areas, they typically
did not investigate what other regions are activated
while or after reentrant processing occurs in the poste-
rior cortex. Thus, the findings from these studies are not
necessarily incompatible with the notion of a wide-
spread network of brain areas being activated in the
transition to visual awareness.
Is the Difference between Conscious
and Unconscious Processing Quantitative
or Qualitative?
The question of whether the difference between sub-
jective awareness and unconscious processing (ref lected
by above-chance objective performance) is quantitative
or qualitative has been hotly debated using behavioral
methods, yet remains unresolved (e.g., Debner & Jacoby,
1994 vs. Snodgrass, 2002). In previous ERP studies, this
question could not be addressed because conscious
processing was compared with nonconscious process-
ing, which included both unconscious processing (accu-
rate objective performance in the absence of subjective
awareness) and processing associated with failed percep-
tion (incorrect objective performance). In the present
study, the ERP modulations associated with subjective
awareness and unconscious processing were observed
on the same component, P3. Indeed, without subjective
awareness, P3 amplitude was larger when participants lo-
calized the target correctly than when they did not, and
P3 amplitude was even larger when correct localization
was accompanied by subjective awareness. This pattern
appears to be consistent with a quantitative modulation
of neural activity by different levels of awareness.
However, the scalp topography of the neural activity
associated with each awareness condition points to a
qualitative difference. Although the P3 difference related
to subjective awareness was observed in recordings over
the entire scalp, modulation by objective performance
was restricted to recordings over the parietal region
only. Because the P3 difference was maximal over the
parietal electrodes for both measures and the effect was
more pronounced for subjective awareness than for
unconscious perception, it could still be argued that
the apparent difference in topographic distribution be-
tween the two effects may be quantitative: that is, the
weaker effects of unconscious processing simply did not
reach significance over regions other than the parietal.
Yet, closer examination of the ERP waveforms indicates
that this is not the case. In particular, the amplitude of
the P3 component tended to be higher in the unaware-
incorrect relative to the unaware-correct condition over
the frontal electrodes, that is, showing a trend in the
direction opposite to the effect of subjective awareness.
It will be important to further refine this claim in future
research by exploring the neural underpinnings of con-
scious and unconscious perception using source estima-
tion techniques allowing for multiple source modeling
and converging fMRI evidence.
In the ever-expanding investigation of the neural corre-
lates of awareness, various ways of limiting subjective
awareness are used in a wide array of experimental
paradigms. However, the extent to which such proce-
dures affect objectively measured performance may vary
in ways that have not yet been fully described. Thus, the
dark side of awareness may conceal different shades of
gray. As electrophysiological and neural imaging proce-
dures rely on subtractive methods, the definition of the
‘‘unaware’’ baseline condition can dramatically affect
which neural processes or brain regions are labeled
‘‘neural correlates of consciousness.’’ In this perspec-
tive, the present study illustrates the potential benefits
of integrating fine-grained definitional distinctions de-
veloped by cognitive behavioral research into the study
of the neural correlates of awareness.
Reprint requests should be sent to Dominique Lamy, Depart-
ment of Psychology, Tel Aviv University, Ramat Aviv, POB
39040, Tel Aviv 69978 Israel, or via e-mail:
1. The analyses were conducted excluding one outlier.
2. We first calculated the actual proportion of unaware-
correct trials that were correctly responded to by chance
(henceforth, %UC
), separately for each participant. In
our task, because there are four possible responses, when an
observer performs at chance, that is, produces a response that
1444 Journal of Cognitive Neuroscience Volume 21, Number 7
is not based on successful perceptual processing, 75% of her or
his responses are expected to be incorrect and 25% of his or
her responses are expected to be correct. All unaware-
incorrect trials were chance trials and made up 75% of all
chance trials in the unaware condition, with the remaining 25%
being UC
trials. Thus, for each participant:
%Unaware TrialsChance ¼ð%Unaware Incorrect trialsÞ=0:75
%UCChance ¼%Unaware TrialsChance 0:25
¼ðð%Unaware Incorrect trialsÞ=0:75Þ0:25
Then, we mathematically derived the estimated waveform cor-
responding to chance-free unaware-correct trials. This esti-
mation rested on the premise that the observed waveform
corresponding to unaware-correct trials included a proportion
of trials, %UC
, on which the correct response was pro-
duced by chance and a proportion of trials, %UC
Chance free
, on which the correct response was based on
perceptual processing. Thus, for each time point the amplitude
of the hypothetical waveform corresponding to chance-free
unaware-correct trials—henceforth, A(UC
Chance free
)—was esti-
mated as follows:
AðUCobservedÞ¼%UCChance AðUCChance Þ
þð1%UCChanceÞAðUCChance freeÞ
Thus, as neural activity on chance trials is reflected by the
unaware-incorrect (UI) waveform:
AðUCChance freeÞ¼½AðUCobservedÞ%UCChance
AðUIobservedÞ=ð1%UCChance Þ
3. A trend for earlier modulation of neural activity by
subjective awareness emerged roughly 200 msec poststimulus
onset over the frontal electrodes (P2 component) for the
short-exposure condition (see Figure 2). A similar suppression
of the P2 component when participants were unaware of the
target was reported in an attentional blink task by Vogel et al.
(1998; see also Vogel & Luck, 2002). However, further research
is needed before firm conclusions can be drawn with regard to
P2 modulation by subjective awareness because, in our study,
the observed trends did not reach significance and occurred
only with one exposure duration.
Babiloni, C., Vecchio, F., Miriello, M., Romani, G., & Rossini, P.
(2006). Visuo-spatial consciousness and parieto-occipital
areas: A high-resolution EEG study. Cerebral Cortex, 16,
Block, N. (2001). Paradox and cross purposes in recent work
on consciousness. Cognition, 79, 197–219.
Cowey, A., & Stoerig, P. (1995). Blindsight in monkeys. Nature,
373, 247–249.
Crick, F., & Koch, C. (1998). Constraints on cortical and
thalamic projections: The no-strong-loops hypothesis.
Nature, 391, 245–250.
Debner, J., & Jacoby, L. (1994). Unconscious perception:
Attention, awareness, and control. Journal of Experimental
Psychology: Learning, Memory, and Cognition, 20,
Dehaene, S., & Naccache, L. (2001). Towards a cognitive
neuroscience of consciousness: Basic evidence and a
workspace framework. Cognition, 79, 1–37.
Del Cul, A., Baillet, S., & Dehaene, S. (2007). Brain dynamics
underlying the nonlinear threshold for access to
consciousness. PLoS Biology, 5, e260.
Draine, S., & Greenwald, A. (1998). Replicable unconscious
semantic priming. Journal of Experimental Psychology:
General, 127, 286–303.
Driver, J., & Vuilleumier, P. (2001). Perceptual awareness and
its loss in unilateral neglect and extinction. Cognition, 79,
Eimer, M., & Mazza, V. (2005). Electrophysiological correlates
of change detection. Psychobiology, 42, 328–342.
Fernandez-Duque, D., Grossi, G., Thornton, I., & Neville, H.
(2003). Representation of change: Separate
electrophysiological markers of attention, awareness,
and implicit processing. Journal of Cognitive
Neuroscience, 15, 491–507.
Green, D., & Swets, J. (1966). Signal detection theory and
psychophysics. New York: Wiley.
Holender, D. (1986). Semantic activation without conscious
identification in dichotic listening, parafoveal vision, and
visual masking: A survey and appraisal. Behavioral and
Brain Sciences, 9, 1–66.
Hunter, M., Turner, A., & Fulham, W. (2001). Visual signal
detection measured by event-related potentials. Brain and
Cognition, 46, 342–356.
Kaernbach, C., Schroger, E., Jacobsen, T., & Roeber, U. (1999).
Effects of consciousness on human brain waves following
binocular rivalry. NeuroReport, 10, 713–716.
Koivisto, M., & Revonsuo, A. (2003). An ERP study of change
detection, change blindness, and visual awareness.
Psychophysiology, 40, 423–429.
Koivisto, M., Revonsuo, A., & Lehtonen, M. (2006).
Independence of visual awareness from the scope of
attention: An electrophysiological study. Cerebral Cortex,
16, 415.
Kranczioch, C., Debener, S., & Engel, A. (2003). Event-related
potential correlates of the attentional blink phenomenon.
Cognitive Brain Research, 17, 177–187.
Lamme, V. (2001). Blindsight: The role of feedforward and
feedback corticocortical connections. Acta Psychologica,
107, 209–228.
Lau, H., & Passingham, R. (2006). Relative blindsight in normal
observers and the neural correlate of visual consciousness.
Proceedings of the National Academy of Sciences, U.S.A.,
102, 18763–18768.
Levitt, H. (1971). Transformed up–down methods in
psychoacoustics. Journal of the Acoustical Society of
America, 49, 467–477.
Lins, O., Picton, T., Berg, P., & Scherg, M. (1993). Ocular
artifacts in recording EEGs and event-related potentials: II.
Source dipoles and source components. Brain Topography,
6, 65–78.
Logothetis, N. (1998). Single units and conscious vision.
Philosophical Transactions of the Royal Society of London,
Series B, Biological Sciences, 353, 1801–1818.
Marcel, A. (1983). Conscious and unconscious perception: An
approach to the relations between phenomenal experience
and perceptual processes. Cognitive Psychology, 15,
Merikle, P., & Reingold, E. (1998). On demonstrating
unconscious perception: Comment on Draine and
Greenwald (1998). Journal of Experimental Psychology:
General, 127, 304–310.
Merikle, P., Smilek, D., & Eastwood, J. (2001). Perception
without awareness: Perspectives from cognitive psychology.
Cognition, 79, 115–134.
Lamy, Salti, and Bar-Haim 1445
Miller, A., & Tomarken, A. (2001). Task-dependent changes in
frontal brain asymmetry: Effects of incentive cues, outcome
expectancies, and motor responses. Psychophysiology, 38,
Niedeggen, M., Wichmann, P., & Stoerig, P. (2001). Change
blindness and time to consciousness. European Journal of
Neuroscience, 14, 1719–1726.
Ojanen, V., Revonsuo, A., & Sams, M. (2003). Visual awareness
of low-contrast stimuli is reflected in event-related brain
potentials. Psychophysiology, 40, 192–197.
Pascual-Leone, A., & Walsh, V. (2001). Fast backprojections
from the motion to the primary visual area necessary for
visual awareness. Science, 292, 510–512.
Pins, D., & ffytche, D. (2003). The neural correlates of
conscious vision. Cerebral Cortex, 13, 461–474.
Sahraie, A., Weiskrantz, L., Barbur, J., Simmons, A.,
Williams, S., & Brammer, M. (1997). Pattern of neuronal
activity associated with conscious and unconscious
processing of visual signals. Proceedings of the National
Academy of Sciences, U.S.A., 94, 9406–9411.
Sergent, C., Baillet, S., & Dehaene, S. (2005). Timing of
the brain events underlying access to consciousness
during the attentional blink. Nature Neuroscience, 8,
Sidis, B. (1898). The psychology of suggestion. New York:
D. Appleton & Co.
Snodgrass, M. (2002). Disambiguating conscious and
unconscious influences: Do exclusion paradigms
demonstrate unconscious perception? American Journal of
Psychology, 115, 545–580.
Super, H., Spekreijse, H., & Lamme, V. A. F. (2001). Two
distinct modes of sensory processing observed in monkey
primary visual cortex (V1). Nature Neuroscience, 4,
Tong, F., Nakayama, K., Vaughan, J., & Kanwisher, N. (1998).
Binocular rivalry and visual awareness in human extrastriate
cortex. Neuron, 21, 753–759.
Turatto, M., Angrilli, A., Mazza, V., Umilta, C., & Driver, J.
(2002). Looking without seeing the background change:
Electrophysiological correlates of change detection versus
change blindness. Cognition, 84, 1–10.
Vogel, E. K., & Luck, S. L. (2002). Delayed working memory
consolidation during the attentional blink. Psychonomic
Bulletin & Review, 9, 739–743.
Vogel, E. K., Luck, S. L., & Shapiro, K. L. (1998).
Electrophysiological evidence for a postperceptual locus of
suppression during the attentional blink. Journal of
Experimental Psychology: Human Perception and
Performance, 24, 1656–1674.
Wilenius-Emet, M., Revonsuo, A., & Ojanen, V. (2004). An
electrophysiological correlate of human visual awareness.
Neuroscience Letters, 354, 38–41.
1446 Journal of Cognitive Neuroscience Volume 21, Number 7
... One of the widely used methods for studying EEG biomarkers in a temporal domain is to record the event-related potentials (ERPs). This approach allows to study reaction and decision-making skills more thoroughly, as was shown in [5]. The authors suggested that early ERP components (P100, N100, P200, N200) provide information about unconscious reactions, while later components give insights into subjective awareness. ...
... Several studies dedicated to cognitive function analysis, in turn, took advantage of transforming neural data into the frequency domain. The study [15] describes how alpha (8)(9)(10)(11)(12) and theta (4)(5)(6)(7)(8) oscillations in EEG signals can be associated with cognitive and memory performance. Theta power was found to reflect the encoding of new information, while alpha frequency was connected to search and retrieval processes in long-term memory. ...
Full-text available
"Faster, higher, stronger" is the motto of any professional athlete. Does that apply to brain dynamics as well? In our paper, we performed a series of EEG experiments on Visually Evoked Potentials and a series of cognitive tests-reaction time and visual search, with professional eSport players in Counter-Strike: Global Offensive (CS:GO) and novices (control group) in order to find important differences between them. EEG data were studied in a temporal domain by Event-Related Potentials (ERPs) and in a frequency domain by Variational Mode Decomposition. The EEG analysis showed that the brain reaction of eSport players is faster (P300 latency is earlier on average by 20-70 ms, p < 0.005) and stronger (P300 peak amplitude is higher on average by 7-9 mkV, p < 0.01). Professional eSport players also exhibit stronger stimulus-locked alpha-band power. Besides, the Spearman correlation analysis showed a significant correlation between hours spend in CS:GO and mean amplitude of P200 and N200 for the professional players. The comparison of cognitive test results showed the superiority of the professional players to the novices in reaction time (faster) and choice reaction time-faster reaction, but similar correctness, while a significant difference in visual search skills was not detected. Thus, significant differences in EEG signals (in spectrograms and ERPs) and cognitive test results (reaction time) were detected between the professional players and the control group. Cognitive tests could be used to separate skilled players from novices, while EEG testing can help to understand the skilled player's level. The results can contribute to understanding the impact of eSport on a player's cognitive state and associating eSport with a real sport. Moreover, the presented results can be useful for evaluating eSport team members and making training plans.
... In contrast, global neu-ronal workspace theory argues that conscious experience is elicited only after activity from sensory modalities is propagated to areas in the global workspace that involve reporting, memory formation, and other cognitive processes (Dehaene & Changeux, 2011;Dehaene et al., 2006;Mashour et al., 2020). Because these ignition processes occur late, VAN is only a correlate of preconscious processes whereas LP is an NCC (Lamy et al., 2009;Salti et al., 2012). ...
Full-text available
Humans have conscious experiences of the events in their environment. Previous research from electroencephalography (EEG) has shown visual awareness negativity (VAN) at about 200 ms to be a neural correlate of consciousness (NCC). However, when considering VAN as an NCC, it is important to explore which particular experiences are associated with VAN. Recent research proposes that VAN is an NCC of lower-level experiences (detection) rather than higher-level experiences (identification). However, previous results are mixed and have several limitations. In the present study, the stimulus was a ring with a Gabor patch tilting either left or right. On each trial, subjects rated their awareness on a three-level perceptual awareness scale that captured both detection (something vs. nothing) and identification (identification vs. something). Separate staircases were used to adjust stimulus opacity to the detection threshold and the identification threshold. Bayesian linear mixed models provided extreme evidence (BF10 = 131) that VAN was stronger at the detection threshold than at the identification threshold. Mean VAN decreased from [Formula: see text]2.12 microV [[Formula: see text]2.86, [Formula: see text]1.42] at detection to [Formula: see text]0.46 microV [[Formula: see text]0.79, [Formula: see text]0.11] at identification. These results strongly support the claim that VAN is an NCC of lower-level experiences of seeing something rather than of higher-level experiences of specific properties of the stimuli. Thus, results are consistent with recurrent processing theory in that phenomenal visual consciousness is reflected by VAN. Further, results emphasize that it is important to consider the level of experience when searching for NCC.
... This indicates that interoceptive signals from the BRs attenuate the sensory processing of a visual stimulus. The modulation of the P1 with cyclic fluctuations in interoceptive input helps to solve the controversy whether it reflects the earliest marker of awareness (44) or not (45): it does but only when concomitant interoceptive input is minimal. ...
Full-text available
Conscious awareness for threshold-stimuli fluctuates stochastically with the pre-stimulus brain state and variations of interoceptive signals across the cardiac and respiratory cycle. It remains debated whether interoceptive signals are suppressed or not and at what stages of processing they affect awareness in different sensory modalities. Because cardiac and respiratory activity are coupled through respiratory sinus arrythmia (RSA), we investigated the influence of the cardiac (systole/diastole) and the respiratory (inhalation/exhalation) phase on awareness-related ERPs. Subjects discriminated visual threshold stimuli while their EEG, ECG and respiration were simultaneously recorded. We compared ERPs and their intracranial generators for stimuli identified correctly with and without awareness as a function of the cardiac and respiratory phase. Cyclic variations of interoceptive signals from the baroreceptors (BRs) modulated both the earliest electrophysiological markers and the trajectory of brain activity when subjects became aware of the stimuli: the P1 was the earliest marker of awareness for low (diastole/inhalation) and the VAN for high (systole/exhalation) BR activity, indicating that interoceptive signals interfere with sensory processing of the visual input. Likewise, activity spread from the anterior insula to prefrontal cortex during low and from the posterior insula to posterior parietal cortex during high BR activity, providing a new solution to the debate whether activity in prefrontal or posterior parietal cortex is crucial for awareness: these regions were differentially recruited as a function of cyclic variations of BR activity rather than task demands. Our results suggest that the cardiac and respiratory rhythms are important rhythms that shape awareness-related brain activity. Significance Statement The brain continuously processes stimuli from inside and outside the body, and interoceptive stimuli can modulate the perception of external stimuli. Cardiac and respiratory rhythms are important pacemakers of the organism and we show how they shape awareness-related brain activity for visual threshold stimuli in two ways. Variations of baroreceptor (BR) activity across the cardiac and respiratory cycle affect 1) the earliest electrophysiological marker (P1 for low (diastole/inhalation), VAN for high (systole/exhalation) BR activity) and 2) the brain areas activated (frontal cortex for low and parietal cortex for high BR activity) when subjects become aware of a stimulus. We propose to consider cardiac and respiratory activity as important modulators of brain activity rather than dismissing them as noise.
... The P300 is a posterior ERP observed 290-450 ms after a stimulus [55]. Historically, the P300, which includes components P3a (central maximum) and P3b (parietal maximum), has been thought to represent a gold-standard marker of conscious perception [56][57][58]. Recently, however, the P300 has been favored to specifically represent post perceptual processing, rather than phenomenal conscious awareness [See REF 59,60• for in-depth review]. ...
Full-text available
Purpose of Review In this review, we summarize the current understanding of consciousness including its neuroanatomic basis. We discuss major theories of consciousness, physical exam-based and electroencephalographic metrics used to stratify levels of consciousness, and tools used to shed light on the neural correlates of the conscious experience. Lastly, we review an expanded category of ‘disorders of consciousness,’ which includes disorders that impact either the level or experience of consciousness. Recent Findings Recent studies have revealed many of the requisite EEG, ERP, and fMRI signals to predict aspects of the conscious experience. Neurological disorders that disrupt the reticular activating system can affect the level of consciousness, whereas cortical disorders from seizures and migraines to strokes and dementia may disrupt phenomenal consciousness. The recently introduced memory theory of consciousness provides a new explanation of phenomenal consciousness that may explain better than prior theories both experimental studies and the neurologist’s clinical experience. Summary Although the complete neurobiological basis of consciousness remains a mystery, recent advances have improved our understanding of the physiology underlying level of consciousness and phenomenal consciousness.
... ERP studies have found that the visual awareness negativity (VAN) (Lamy et al., 2009;Jimenez et al., 2018) in the occipital region and the late positivity (LP) (Pitts et al., 2014; in the parietal region co-occur in awareness detection experiments,. VAN typically precedes LP in the time course of detection. ...
The psychological effects of long-term exposure to high-altitude environments have attracted great attention. These effects are usually attributed to the diminished cognitive resources due to high-altitude exposure. This study employed electroencephalography (EEG) to investigate the effects of exposure duration on awareness detection tasks. Neither reaction time nor accuracy showed the direct effects of the exposure duration, so did the model indexes obtained from drift diffusion model analysis. However, event-related potentials (ERP) analysis revealed that exposure duration was associated with changes in the visual awareness negativity (VAN) and the late positivity (LP) components, which in turn affected reaction time. Specifically, longer exposure durations were associated with lower VAN and higher LP, resulting in shorter reaction times and greater drift rate. In contrast to previous studies, the reverse relationship between VAN and LP may reflect a compensatory response to the reduced cognitive resources caused by high-altitude exposure. Additionally, increased LP and shorter reaction times with exposure duration may reflect a resistance to the high-altitude environment. We also conducted time-frequency analysis and found that theta power did not vary with exposure duration, suggesting that the reduction in cognitive resources remains stable in these individuals over time. Overall, our study provides new insights into the dynamic effects of high-altitude environments on awareness detection in the presence of reduced cognitive resources.
... For instance, some correct detections of the stimulus in unaware trials could occur due to lucky guesses rather than to perceptual processing. One could attempt more sophisticated corrections to guesses in unaware trials by taking into account subjects' guessing rate (Lamy, Salti, & Bar-Haim, 2009). But this approach is insufficient for matching the underlying perceptual capacity and the corresponding neural activity that Downloaded from by guest on 02 May 2023 ...
Full-text available
Philosophers and neuroscientists address central issues in both fields, including morality, action, mental illness, consciousness, perception, and memory. Philosophers and neuroscientists grapple with the same profound questions involving consciousness, perception, behavior, and moral judgment, but only recently have the two disciplines begun to work together. This volume offers fourteen original chapters that address these issues, each written by a team that includes at least one philosopher and one neuroscientist, who integrate disciplinary perspectives and reflect the latest research in both fields. Topics include morality, empathy, agency, the self, mental illness, neuroprediction, optogenetics, pain, vision, consciousness, memory, concepts, mind wandering, and the neural basis of psychological categories. The chapters first address basic issues about our social and moral lives: how we decide to act and ought to act toward each other, how we understand each other's mental states and selves, and how we deal with pressing social problems regarding crime and mental or brain health. The following chapters consider basic issues about our mental lives: how we classify and recall what we experience, how we see and feel objects in the world, how we ponder plans and alternatives, and how our brains make us conscious and create specific mental states. Contributors Sara Abdulla, Eyal Aharoni, Corey H. Allen, Sara Aronowitz, Jenny Blumenthal-Barby, Ned Block, Allison J. Brager, Antonio Cataldo, Tony Cheng, Felipe De Brigard, Rachel N. Denison, Jim A. C. Everett, Gidon Felsen, Julia Haas, Hyemin Han, Zac Irving, Kristina Krasich, Enoch Lambert, Cristina Leon, Anna Leshinskaya, Jordan L. Livingston, Brian Maniscalco, Joshua May, Joseph McCaffrey, Jorge Morales, Samuel Murray, Thomas Nadelhoffer, Laura Niemi, Brian Odegaard, Hannah Read, Robyn Repko Waller, Sarah Robins, Jason Samaha, Walter Sinnott-Armstrong, Joshua August Skorburg, Shannon Spaulding, Arjen Stolk, Rita Svetlova, Natalia Washington, Clifford Workman, Jessey Wright
... They represent the minimal observable sets of neuronal events that are necessary for specific conscious percept Koch 2003, Koch et al. 2016) and for the distinction between conscious and unconscious processing. Thorough theoretical and philosophical work on NCCs has been performed (Metzinger 2000), and many NCCs were proposed, starting first from specific brain areas, such as the thalamus (Penfield 1938) primary visual cortex, or specific local activations of parietal brain areas (Koch et al. 2016), through specific brain rhythms and their synchronization, such as P300 (Babiloni et al. 2006, Del Cul et al. 2007, Lamy et al. 2009, Dehaene and Changeux 2011, Salti et al. 2012, King et al. 2014 or gamma synchrony (Crick and Koch 1990, Llinas et al. 1994, Engel et al. 1999, connectivity, such as thalamocortical re-entrant loops (Edelman 1989(Edelman , 2004, or connectivity of frontoparietal brain areas through parts of the thalamus (Dehaene 2014), to their various combinations in theories of consciousness, such as Attended Intermediate Representation Theory (Prinz 2012), Posterior Cortical Hot Zone Theory (Koch et al. 2016), Global Neuronal Workspace Theory (Dehaene 2014), and many others. To date, no specific correlate or theory has been accepted as universal, sufficient, or satisfactory. ...
Full-text available
Conscious experience represents one of the most elusive problems of empirical science, namely neuroscience. The main objective of empirical studies of consciousness has been to describe the minimal sets of neural events necessary for a specific neuronal state to become consciously experienced. The current state of the art still does not meet this objective but rather consists of highly speculative theories based on correlates of consciousness and an ever-growing list of knowledge gaps. The current state of the art is defined by the limitations of past stimulation techniques and the emphasis on the observational approach. However, looking at the current stimulation technologies that are becoming more accurate, it is time to consider an alternative approach to studying consciousness, which builds on the methodology of causal explanations via causal alterations. The aim of this methodology is to move beyond the correlates of consciousness and focus directly on the mechanisms of consciousness with the help of the currently focused brain stimulation techniques, such as geodesic transcranial electric neuromodulation. This approach not only overcomes the limitations of the correlational methodology but will also become another firm step in the following science of consciousness.
... It is generally considered that reactions that occur within the first 300 ms of decision-making are unconscious, while those occurring after 300 ms are related to conscious inclinations [165,193]. The consensus regarding whether the P300 and N400 components reflect conscious or unconscious reactions remains contentious, and these components may reflect a critical phase in the transition from unconscious to conscious mental processes [57,156,250]. ...
Full-text available
Introduction The present paper discusses the findings of a systematic review of EEG measures in neuromarketing, identifying which EEG measures are the most robust predictor of customer preference in neuromarketing. The review investigated which TF effect (e.g., theta-band power), and ERP component (e.g., N400) was most consistently reflective of self-reported preference. Machine-learning prediction also investigated, along with the use of EEG when combined with physiological measures such as eye-tracking. Methods Search terms ‘neuromarketing’ and ‘consumer neuroscience’ identified papers that used EEG measures. Publications were excluded if they were primarily written in a language other than English or were not published as journal articles (e.g., book chapters). 174 papers were included in the present review. Results Frontal alpha asymmetry (FAA) was the most reliable TF signal of preference and was able to differentiate positive from negative consumer responses. Similarly, the late positive potential (LPP) was the most reliable ERP component, reflecting conscious emotional evaluation of products and advertising. However, there was limited consistency across papers, with each measure showing mixed results when related to preference and purchase behaviour. Conclusions and implications FAA and the LPP were the most consistent markers of emotional responses to marketing stimuli, consumer preference and purchase intention. Predictive accuracy of FAA and the LPP was greatly improved through the use of machine-learning prediction, especially when combined with eye-tracking or facial expression analyses.
La scoperta che una parte straordinaria della nostra vita mentale avviene al di fuori della nostra consapevolezza viene generalmente accreditata a Sigmund Freud, anche se storicamente è di molto antecedente. La mente inconscia è gene-ralmente considerata in psicologia generale come l'ombra della "reale" mente con-scia, ma vi è una significativa evidenza che l'inconscio non sia meno flessibile, complesso, intenzionale della sua controparte conscia. Numerosi sistemi inconsci regolano processi attentivi, percettivi, emozionali, mnestici, valutativi e motivazio-nali. Tali processi cognitivi si manifestano a livello inconscio e influenzano com-portamento ed emozioni (affetti). Le neuroscienze soltanto di recente hanno co-minciato a comprendere i correlati neurali di tali processi e le loro interazioni di-namiche con i processi consci. Ad esempio, come impulsi, pensieri e desideri consci diventino inconsci (e.g., repressione, dissociazione) e, d'altra parte, come impulsi, motivazioni e desideri inconsci diventino consci (e.g., lapsus e/o atti mancati freu-diani). Sulla base dell'evidenza disponibile, non esistono strutture e circuiti cerebra-li specificamente deputati all'elaborazione del pensiero inconscio, ma tutte le re-gioni del cervello partecipano al pensiero cosciente e non cosciente. Qual è il rap-porto tra inconscio e coscienza? Si tende a ritenere che le radici profonde della co-scienza e del senso di sé risiedano in un grande insieme di strutture cerebrali fina-lizzate alla mappatura neurale del corpo e all'omeostasi in modo non consapevo-le. Sarebbe questo il "Proto-Sé" Inconscio (Damasio, 2003), dal quale emergerebbe l'espe¬rienza cosciente. In questa prospettiva, si può arguire che le azioni della men-te inconscia precedono e influenzano l'emergenza della mente conscia. Non sap-piamo se queste attività implicite e automatiche possano essere concettualizzate e organizzate in una mente auto-inconscia che assomigli alla mente auto-cosciente. Anche se percezioni, sentimenti, motivazioni e alcuni processi decisionali possono verificarsi al di fuori della consapevolezza, ciò che sembra mancare è la capacità e l'intensità di queste attività implicite di auto-organizzarsi, possibilmente attraverso un meccanismo bottom-up di coalizioni neurali, in una rappresentazione coerente, significativa e mirata della realtà.
Full-text available
Objective The objective of this study was to investigate neural responses during subliminal oddball tasks concerning reward-directed motivation to distinguish the P3a and P3b components of evoked P300 potentials. Methods The subliminal oddball task included congruent/incongruent stimuli and masked prime subliminal stimuli. The task was to push the button when an incongruent stimulus appeared. Participants underwent two pre- and post-break electroencephalogram (EEG) recordings. During the break, the thirst scores of the participants were evaluated. Then, participants consumed three different salty foods in the same amount and completed the remaining two post-break EEG recordings, followed by the second thirst scores evaluation. Finally, participants physically selected lettered cards (A, B, C, and D) to receive a reward and quench their thirst. Results Thirty participants were enrolled, 28 of whom were included for data analysis. Ten participants selected lettered cards as the given subliminal stimulus (sub ⁽⁺⁾ group), and 18 participants selected different lettered cards from the given subliminal stimulus (sub (–) group). We found a significant increase in post-P3a and post-P3b amplitudes in the sub ⁽⁺⁾ group at the Pz/Oz electrodes. Changes in P3b amplitude were significantly higher in the sub ⁽⁺⁾ group (2.83 ± 1.14 μV) than in the sub (–) group (0.62 ± 2.29 μV) at the Pz/Oz electrodes. Correlation analysis revealed that higher thirst scores resulted in higher P3b amplitudes in the sub ⁽⁺⁾ group. Conclusion These findings suggest that reward-directed motivation increases parietal-posterior P3b amplitudes, signifying the involvement of cognitive processes to achieve a reward.
Full-text available
Following striate cortex damage in monkeys and humans there can be residual function mediated by parallel visual pathways. In humans this can sometimes be associated with a “feeling” that something has happened, especially with rapid movement or abrupt onset. For less transient events, discriminative performance may still be well above chance even when the subject reports no conscious awareness of the stimulus. In a previous study we examined parameters that yield good residual visual performance in the “blind” hemifield of a subject with unilateral damage to the primary visual cortex. With appropriate parameters we demonstrated good discriminative performance, both with and without conscious awareness of a visual event. These observations raise the possibility of imaging the brain activity generated in the “aware” and the “unaware” modes, with matched levels of discrimination performance, and hence of revealing patterns of brain activation associated with visual awareness. The intact hemifield also allows a comparison with normal vision. Here we report the results of a functional magnetic resonance imaging study on the same subject carried out under aware and unaware stimulus conditions. The results point to a shift in the pattern of activity from neocortex in the aware mode, to subcortical structures in the unaware mode. In the aware mode prestriate and dorsolateral prefrontal cortices (area 46) are active. In the unaware mode the superior colliculus is active, together with medial and orbital prefrontal cortical sites.
Full-text available
During the past decade a number of variations in the simple up‐down procedure have been used in psychoacoustic testing. A broad class of these methods is described with due emphasis on the related problems of parameter estimation and the efficient placing of observations. The advantages of up‐down methods are many, including simplicity, high efficiency, robustness, small‐sample reliability, and relative freedom from restrictive assumptions. Several applications of these procedures in psychoacoustics are described, including examples where conventional techniques are inapplicable.
Full-text available
Blindsight, the visually evoked voluntary responses of patients with striate cortical destruction that are demonstrated despite a phenomenal blindness, has attracted attention from neuroscientists and philosophers interested in problems of perceptual consciousness and its neuronal basis. It is assumed to be mediated by the numerous extra-geniculostriate cortical retinofugal pathways whose properties are studied primarily in monkeys. Like patients with blindsight, monkeys with lesions of the primary visual cortex can learn to detect, localize and distinguish between visual stimuli presented within their visual field defects. Although the patients deny seeing the stimuli they can nevertheless respond to (by forced-choice guessing) in their phenomenally blind fields, it is not known whether the monkeys experience the same absence of phenomenal vision. To determine whether they too have blindsight, or whether they actually see the stimuli in their field defects, monkeys who showed excellent detection in tasks where a visual stimulus was presented on every trial, albeit at different positions, were tested in a signal-detection task in which half the trials were blank trials, with no visual stimulus. They classified the visual stimuli presented in the field defect as blank trials, demonstrating, like patients, blindsight rather than degraded real vision.
Full-text available
Conscious perception is substantially overestimated when standard measurement techniques are used. That overestimation has contributed to the controversial nature of studies of unconscious perception. A process-dissociation procedure (L. L. Jacoby, 1991) was used for separately estimating the contribution of conscious and unconscious perception to performance of a stem-completion task. Unambiguous evidence for unconscious perception was obtained in 4 experiments. In Experiment 1, decreasing the duration of a briefly presented word diminished the contribution of both conscious and unconscious perception. In Experiments 2-4, dividing attention reduced the contribution of conscious perception while leaving that of unconscious perception unchanged. Discussion focuses on the measurement of awareness and the relation between perception and memory.
When the stored representation of the meaning of a stimulus is accessed through the processing of a sensory input it is maintained in an activated state for a certain amount of time that allows for further processing. This semantic activation is generally accompanied by conscious identification, which can be demonstrated by the ability of a person to perform discriminations on the basis of the meaning of the stimulus. The idea that a sensory input can give rise to semantic activation without concomitant conscious identification was the central thesis of the controversial research in subliminal perception. Recently, new claims for the existence of such phenomena have arisen from studies in dichotic listening, parafoveal vision, and visual pattern masking. Because of the fundamental role played by these types of experiments in cognitive psychology, the new assertions have raised widespread interest. The purpose of this paper is to show that this enthusiasm may be premature. Analysis of the three new lines of evidence for semantic activation without conscious identification leads to the following conclusions. (1) Dichotic listening cannot provide the conditions needed to demonstrate the phenomenon. These conditions are better fulfilled in parafoveal vision and are realized ideally in pattern masking. (2) Evidence for the phenomenon is very scanty for parafoveal vision, but several tentative demonstrations have been reported for pattern masking. It can be shown, however, that none of these studies has included the requisite controls to ensure that semantic activation was not accompanied by conscious identification of the stimulus at the time of presentation. (3) On the basis of current evidence it is most likely that these stimuli were indeed consciously identified.
Detection of changes in a visual scene can be substantially delayed when the original and the modified image are separated by a brief screen flicker. We used this phenomenon of ‘change blindness’ to find when the brain detects the mismatch in relation to when the observer reports it, and whether changes in identity and position are processed similarly. Event-related brain potentials (ERPs) recorded while the subjects searched for the change in alternating series of images showed that the epoch during which they indicated detection was characterized by a marked positivity from 300 to 700 ms. Analysis of data from image presentations preceding the subjects' response revealed a similar but smaller ERP positivity one (identity) or even two (position) epochs before detection. As each epoch lasted 1500 ms, the brain may register a change as early as 3000 ms before the observer.
An approach to the relationships between conscious perception and nonconscious perceptual processes is outlined. Its basis is the rejection of the assumption that phenomenal experience is identical to or is a direct reflection of representations yielded by perceptual processes. Nonconscious perceptual processes automatically redescribe sensory data into every representational form and to the highest levels of description available to the organism. Such processes (a) provide records of each resultant representation, (b) produce perceptual hypotheses in different domains, (c) activate related structures, and (d) affect analog aspects of actions. Conscious perception requires a constructive act whereby perceptual hypotheses are matched against information recovered from records, and serves to structure and synthesize that information recovered from different domains. These processes are related to three aspects of phenomenal experience: awareness, unity of percepts, and selectivity. Consciousness is seen as an attempt to make sense of as much data as possible at the most functionally useful level. Explication of the approach consists of (a) discussion of differences between conscious and nonconscious representations and processes; (b) exposition of the characteristics of the process of recovery; (c) a theory of central visual masking as a consequence of temporal and spatial parsing involved in recovery, wherein masking is seen as an aspect of the structural nature of consciousness whose goal is event perception, and does not affect nonconscious perceptual processing; (d) an interpretation of various clinical neuropsychological and normal phenomena in terms of limitations and impairments in the processes of recovery and synthesis; (e) reinterpretation of several perceptual phenomena in terms of the recovery of information and of how nonconscious processes precede and affect consciousness.