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Neural Correlates of Subjective Awareness and
Unconscious Processing: An ERP Study
Dominique Lamy, Moti Salti, and Yair Bar-Haim
Abstract
&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. &
INTRODUCTION
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.’’
METHODS
Participants
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.
Stimuli
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
filtering.
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).
RESULTS
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,
respectively
1
]. 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
Aware
Correct
Aware
Incorrect
Unaware
Correct
Unaware
Incorrect
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.
2
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
Region
Exposure
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
P3
a
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
P3
a
1.37 ns 3.75 <.003 <1 ns <1 ns
a
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
Chance-free
a
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
Chance-free
a
1.61 <.3 <1 ns 2.00 ns 7.04 <.02 3.26 <.09
a
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
average.Yet,onthoseindividualtrialsinwhich
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.
DISCUSSION
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
3
,
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.
Conclusion
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: domi@post.tau.ac.il.
Notes
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
Chance
), 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
Chance
trials. Thus, for each participant:
%Unaware TrialsChance ¼ð%Unaware Incorrect trialsÞ=0:75
%UCChance ¼%Unaware TrialsChance 0:25
¼ðð%Unaware Incorrect trialsÞ=0:75Þ0:25
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
Chance
, on which the correct response was pro-
duced by chance and a proportion of trials, %UC
Chance free
=
1%UC
Chance
, 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%UCChanceÞAðUCChance 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.
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