Attention and consciousness: two
distinct brain processes
Christof Koch1and Naotsugu Tsuchiya2
1Division of Biology 216–76, California Institute of Technology, Pasadena, CA 91125, USA
2Division of the Humanities and Social Sciences, 228–77, California Institute of Technology, Pasadena, CA 91125, USA
consciousness has led many scholars to conflate these
processes. This article summarizes psychophysical
evidence, arguing that top-down attention and con-
sciousness are distinct phenomena that need not occur
together and that can be manipulated using distinct
paradigms. Subjects can become conscious of an iso-
lated object or the gist of a scene despite the near
absence of top-down attention; conversely, subjects
can attend to perceptually invisible objects. Further-
more, top-down attention and consciousness can have
opposing effects. Such dissociations are easier to
understand when the different functions of these two
processes are considered. Untangling their tight rela-
tionship is necessary for the scientific elucidation of
consciousness and its material substrate.
close relationshipbetweenattention and
Commonly used in everyday speech and in the scholarly
literature, ‘attention’ and ‘consciousness’ have resisted
clear and compelling definition. As argued elsewhere
[1,2], this unfortunate state of affairs will remain until
the mechanistic basis of these phenomena has been enun-
ciated thoroughly at the neuronal and molecular levels.
Few would dispute that the relationship between
selective attention and consciousness is an intimate one.
When we attend to an object, we become conscious of its
attributes; when we shift attention away, the object fades
from consciousness. This has prompted many to posit that
these two processes are inextricably interwoven, if not
identical [3–6]. Others, however, going back to the 19th
century , have argued that attention and consciousness
are distinct phenomena that have distinct functions and
neuronal mechanisms [2,8–17]. If this proposition were
true, what would be the nature of their causal interaction?
Is paying attention necessary and sufficient for conscious-
ness? Or can conscious perception occur outside the spot-
light of attention? Of course, this presupposes that
consciousness is a unitary concept, which is not the case;
consciousness has been dissected on conceptual grounds
(access versus phenomenal consciousness), ontological
grounds (hard problem versus easy problem) and psycho-
logical grounds (explicit versus implicit processes).
Here, we summarize recent psychophysical evidence
in support of a dissociation between attention and
consciousness, and provide functional justifications for this
viewpoint. We argue that events or objects can be attended
to without being consciously perceived. Furthermore, an
event or object can be consciously perceived in the near
absence of top-down attentional processing. Note that our
usage of ‘attention’ always implies selective attention,
rather than the processes that control the overall level
of arousal and alertness.
Functional roles of attention and consciousness
informational overload. In primates, about one million
per second of raw information. One way to deal with this
deluge is to select a small fraction and process this reduced
input in real time, while the non-attended portion of the
input is processed at a reduced bandwidth. In this view,
attention ‘selects’ information of current relevance to the
organism, while the non-attended data are neglected.
Since the time of Williams James, it has been known
that selection is based on either bottom-up exogenous or
top-down endogenous factors. Exogenous cues are image-
immanent features that transiently attract attention or
eye gaze, independent of a particular task. Thus, if an
object attribute (e.g. flicker, motion, color, orientation,
depth or texture) differs significantly from a neighboring
attribute, the object will be salient. This definition of
bottom-up saliency has been implemented into a suite of
neuromorphic vision algorithms that have at their core a
topographic saliency map that encodes the saliency or
conspicuity of locations in the visual field, independent
of the task . [See iLab Neuromorphic Vision C++
Toolkit (http://iLab.usc.edu) for a C++ implementation,
and Saliency Toolbox (http://www.saliencytoolbox.net) for
a Matlab toolbox.] Such algorithms account for a signifi-
cant proportion of scanning eye movements [19,20].
However, under many conditions, subjects disregard
salient, bottom-up cues when searching for particular
objects in a scene, by dint of top-down, task-dependent
control of attention. Bringing top-down, sustained atten-
tion to bear on an object or event in a scene takes
time. Top-down attention selects input defined by a
circumscribed region in space (focal attention), by a
particular feature (feature-based attention) or by an
object (object-based attention). It is on the relationship
between these volitionally controlled forms of selective,
endogenous attention and consciousness that this article
TRENDS in Cognitive Sciences Vol.11 No.1
Corresponding author: Koch, C. (email@example.com).
Available online 28 November 2006.
www.sciencedirect.com1364-6613/$ – see front matter. Published by Elsevier Ltd. doi:10.1016/j.tics.2006.10.012
functions from attention. These include summarizing all
information that pertains to the current state of the organ-
ism and its environment and ensuring this compact sum-
mary is accessible to the planning areas of the brain, and
also detecting anomalies and errors, decision making, lan-
guage, inferring the internal state of other animals, setting
long-term goals, making recursive models and rational
To the extent that one accepts that attention and
consciousness have different functions, one must also
accept that they cannot be the same process. It follows,
then, that any conscious or unconscious percept or beha-
vior can be classified in one of four ways, depending on
whether top-down attention is required and whether it
necessarily gives rise to consciousness.
The four ways of processing visual events and
consciousness are distinct, they insist that attention is
necessary for consciousness, and that non-attended events
remain hidden. For example, Dehaene et al.  state that
considerable evidence indicates that, without attention,
conscious perception cannot occur. We now review
evidence that argues otherwise.
agree that attentionand
Attention with consciousness
When we attend to a face or to an object within a cluttered
scene, we usually become conscious of its attributes and
have access to all of the attendant privileges of conscious-
ness (e.g. working memory and verbal reportability).
Although the minimal neuronal mechanisms that are
jointly sufficient for any one conscious percept remain
elusive, evidence from models agrees that these mechan-
isms must involve a reciprocal relationship between cel-
lular populations in extrastriate visual cortices and
neurons in the premotor and prefrontal cortices, mediated
by long-range corticocortical feedforward and feedback
More than a century of research has quantified the
ample benefits afforded by attention and conscious percep-
tion. For example, Mack and Rock  demonstrated
compellingly that subjects must attend to novel or unex-
pected stimuli to become conscious of them. These percepts
consciousness design matrix (Table 1).
No attention, no consciousness
At the other end of the spectrum are objects or events that
do not benefit from a top-down attentional bias. Under
these conditions, the associated net wave of spiking activ-
ity moving from the retina into primary visual cortex and
beyond will not trigger a conscious percept (see ‘Processing
without top-down attention and consciousness’). However,
such non-attended or minimally attended, non-conscious
activity can still be causally effective and leave traces that
can be detected using sensitive behavioral techniques. For
instance, we surmise that negative afterimages [25–27]
top-down attention. These occupy the upper-left quadrant
of Table 1.
What about the two remaining quadrants, which cover
events and behaviors that require top-down attention but
do not give rise to conscious perception, and events and
behaviors that give rise to consciousness without top-down
attention? These can be studied using techniques that
independently manipulate top-down attention and visual
consciousness (Box 1).
Attention without consciousness
Subjects can attend to a location for many seconds and
yet fail to see one or more attributes of an object at that
location (lower-left quadrant in Table 1). In lateral
masking (visual crowding), the orientation of a periph-
erally presented grating is hidden from conscious sight
but remains sufficiently potent to induce an orientation-
dependent aftereffect . An aftereffect induced by an
invisible illusory contour can require focal attention even
when the object at the center of attention is invisible
. Furthermore, priming
for invisible words (suppressed by a combination of for-
ward and backward masking), but only if the subject was
attending to the invisible prime–target pair; without
Male and female nudes attract attention when they
are rendered completely invisible by continuous flash
suppression . Interestingly, in heterosexuals, these
effects are apparent only for nudes of the opposite sex.
Note that without the mask, these stimuli are clearly
visible. Likewise, the blindsight patient GY has the
usual reaction-time advantages for the detection of tar-
gets in his blind visual field when attentionally cued,
even when the cues are located in his blind field  (see
also Refs [31,32] for further evidence).
hasbeen elicited 
Table 1. A fourfold classification of conscious and unconscious percepts and behaviorsa
Might not give rise to consciousness
Top-down attention is not required
Formation of afterimages
Rapid vision (<120 ms)
Gives rise to consciousness
Pop-out in search
Animal and gender detection in dual tasks
Detection and discrimination of unexpected and
Top-down attention is required
aThis classification organizes conscious and unconscious percepts and behaviors into four, psychophysically defined, categories, depending on whether top-down attention
is necessary and whether they necessarily give rise to phenomenal consciousness.
TRENDS in Cognitive SciencesVol.11 No.1 17
Finally, feature-based attention can spread to invisible
stimuli [33,34]. Indeed, when searching for an object in a
cluttered scene (e.g. keys in a messy room), attention is
paid to an invisible object and its associated features. This
research shows that attentional selection does not neces-
sarily engender conscious sensation.
Consciousness in the near absence of attention
Conversely, when focusing intensely on one event, the
world is not perceived as a tunnel, with everything outside
the focus of attention gone (upper-right quadrant in
Table 1). We are always aware of some aspects of the world
that surrounds us, such as its gist. Indeed, gist is immune
from inattentional blindness : when a photograph was
briefly flashed unexpectedly onto a screen, subjects could
accurately report a summary of the photograph. In a mere
30 ms presentation time, the gist of a scene can be appre-
hended. This is insufficient time for top-down attention to
associated with the entire image, any process that locally
enhances features, such as focal attention, will be of
Box 1. Psychophysical methods for independent manipulation of visual consciousness and top-down attention
Top-down attention and consciousness can be tightly coupled. To
dissociate them, experimental methods are required that manipulate
either attention or consciousness, independently or semi-indepen-
dently, in a specific manner with few side effects. The dual-tasks
paradigm [36,37] manipulates top-down, focal attention without
affecting bottom-up saliency; a central, attentional-demanding discri-
mination task is presented at the center of gaze, while a secondary
complex peripheral tasks can be performed equally well under either
the single-task or dual-tasks condition [38–40], whereas computation-
ally simpler tasks deteriorate when performed simultaneously with the
central task (Figure Ib,c). The dual-tasks paradigm quantifies what type
of stimulus attributes can be signaled and consciously perceived in the
near absence of top-down spatial attention .
Visual consciousness is manipulated using a multitude of techni-
ques, such as backward masking, standing wave of invisibility,
crowding, bistable figures (Figure Id), binocular rivalry, flash suppres-
sion, continuous flash suppression [26,52], motion-induced blindness
and attentional blink (for a review, see Ref. ). These techniques
control the visibility of an object or part of an object in space and time.
The dual-tasks paradigm can be combined with these methods,
enabling the independent manipulation of top-down attention and
consciousness (Figure Ie), although we await a full factorial analysis
for many popular experiments (Box 2).
Figure I. The dual-tasks paradigm. (a) The dual-tasks paradigm studies how performance of a secondary task in the periphery (empty red circle) is affected when a
centrally presented attention-demanding task (centered red circle) is performed simultaneously. (b) Participants can decide whether a natural scene includes an animal
at the same time as performing a central task. Scene categorization task performance (y-axis) remains at the same level under single-task (normalized to 100% on the y-
axis) and dual-tasks (depicted as circles with standard error of the mean) conditions. Note that the central task performance (x-axis) also remains the same in single-task
(normalized to 100% on the x-axis) and dual-tasks (shown as circles) situations. (c) However, participants are unable to distinguish between a red–green disk and a
green–red one when attention is engaged at the center. (d) An example of a bistable conscious percept (Rubin’s vase: two silhouettes versus a vase). (e) The effect of
withdrawing top-down attention could be characterized by embedding Rubin’s vase, a bistable percept, into a dual-tasks experiment  (Box 2). Figure Ib,c modified
from Ref. .
TRENDS in Cognitive SciencesVol.11 No.1
A common arrangement in many experiments involves
studying the perception of a single object (e.g. a bar) in
an otherwise empty display. In this situation, what func-
tion would top-down, selective attention need to perform
when there is no competing object in or around fixation?
Indeed, the most popular neuronal model of attention,
biased competition , predicts that in the absence of
competition, no or little attentional enhancement occurs
(see also Ref. ).
In a dual-tasks paradigm, the subject’s attention is
drawn to an attentionally demanding central task, while
at the same time a secondary stimulus is flashed in
the periphery [36,37] (Box 1). With focal attention busy
at the center, subjects can determine whether a scene
contains an animal (or a vehicle) but are unable to distin-
guish between a red–green and a green–red disk .
Likewise, subjects can distinguish between male and
female faces in the periphery or even between famous
and non-famous faces [39,40]. However, remarkably, sub-
jects are frustrated by tasks that are computationally
much simpler (e.g. discriminating between a rotated ‘L’
and a rotated ‘T’). Thus, although it cannot be said with
certainty that observers do not deploy some top-down
attention to the peripheral target in dual-tasks experi-
ments that require training and concentration (i.e. high
arousal), it seems that subjects can perform certain dis-
criminations in the near absence of top-down attention.
And they are not guessing: they can be confident of their
discrimination choices and ‘see’, albeit often indistinctly,
the peripheral stimuli.
Processing without top-down attention and
Visual input can be classified rapidly. As demonstrated by
Kirchner and Thorpe , at ?120 ms, the brain can
begin to determine whether a briefly flashed image
contains animals or not. At this speed, it is no surprise
that subjects often respond without having consciously
seen the image; consciousness of the image might come
later or not at all. Dual-tasks and dual-presentation para-
digms support the idea that such discriminations can
occur in the near absence of focal, spatial attention
[38,42], implying that purely feedforward networks can
support complex visual decision making [43,44]. Support-
ing this conclusion are neurophysiologically grounded
computer models of the feedforward stages of the
visual-processing hierarchy  that achieve human-like
performance levels in such categorization tasks (T. Serre
et al., unpublished; see http://cbcl.mit.edu/publications/
Animalexperiments couldprove thisassertion. Imagine
that all the corticocortical pathways from prefrontal cortex
back to visual cortex could be transiently knocked out
using a molecular silencing tool (without compromising
feedforward processing). That is, for a couple of hours, the
brain of a monkey would support only feedforward path-
ways. It is likely that such an animal could still perform a
previously learnt rapid discrimination task with the same
level of performance as before the intervention (upper-left
quadrant in Table 1), without top-down attention and
without conscious perception.
Attention and consciousness can oppose each other
Withdrawing top-down attention from a stimulus and
cloaking it from consciousness can have opposing effects.
When observers try to find two embedded targets within a
rapidly flashed stream of images, they often fail to see the
second target, the attentional blink . Counterintui-
tively, Olivers and Nieuwenhuis  found that observers
can see both the first and the second targets better when
when they are encouraged to think about task-irrelevant
events. Recent work on afterimages, stabilization of bis-
table figures and complex decision making hint at striking
and consciousness independently (Box 2). Such findings
are difficult to understand within a framework that aligns
top-down attention closely with consciousness.
Relationship to other conceptual distinctions
Dehaene and colleagues  propose a tripartite ontology,
based on the global workspace hypothesis of Baars (for an
updated view, see Ref. ) and Dehaene et al. ,
whereby any physical stimulus triggers subliminal, pre-
conscious or conscious processing. What decides the fate of
a stimulus is its strength and whether or not top-down
attention is deployed. This threefold distinction maps onto
our fourfold one if subliminal processing is equated with
the two left quadrants and preconscious processing with
the upper-right quadrant (Table 1). One important differ-
ence is our assumption that consciousness can occur with-
out top-down attention (upper-right quadrant). A priori,
there is no fundamental reason why global workspace
theory requires actively paying attention to a stimulus
in order to be conscious of it. There might be many routes
by which the global workspace could be accessed, in
addition to that of top-down attention.
Do these conclusions hold for real life?
It could be contested that top-down attention without
consciousness and consciousness with little or no top-down
attention are arcane laboratory curiosities that have little
relevanceto the real world. Webelieve otherwise. Alasting
insight into human behavior – eloquently articulated by
Friedrich Nietzsche – is that much action bypasses con-
scious perception and introspection. In particular, Goodale
and Milner  isolated highly trained, automatic, stereo-
typed and fluid visuomotor behaviors that work in the
absence of phenomenal experience. As anybody who runs
mountain trails, climbs, plays soccer or drives home on
automatic pilot knows, these sensorimotor skills – dubbed
zombie behaviors  – require rapid and sophisticated
sensory processing. Confirming a long-held belief among
trainers, athletes perform better at their highly tuned skill
when they are distracted by a skill-irrelevant dual task
(e.g. paying attention to tones) than when they pay
attention to their exhaustively trained behaviors .
The history of any scientific concept (e.g. energy, atoms
or genes) is one of increasing differentiation and sophisti-
cation until its action can be explained in a quantitative
and mechanistic manner at a lower, more elemental level.
We are far from this ideal in the inchoate science of
consciousness. Yet functional considerations and the
TRENDS in Cognitive SciencesVol.11 No.1 19
Box 2. Can top-down attention and awareness have opposite effects?
Attention and its neuronal correlate can be understood in the context
of selection and biased competition ; attention acts as a winner-
takes-all, enhancing one coalition of neurons (representing the
attended object) at the expense of others (non-attended stimuli).
Paradoxically, reducing attention can enhance awareness  and
certain behaviors .
Consider the formation of afterimages (Figure Ia of this box). If an
item is attended to during adaptation, the intensity of the subsequent
afterimage becomes weaker and its duration shorter compared with
an unattended item [55,56] (Figure Ib). However, if the image is
perceptually suppressed during adaptation, the afterimage is sub-
stantially weakened [26,27]. Thus, focal attention and consciousness
have opposing effects.
Next, consider freezing in bistable perception  (Figure Ic).
During continuous viewing of an ambiguous stimulus, the percept
flips stochastically. Yet if the bistable figure is briefly removed
(leaving the display empty), the dominant percept at the start of the
new display is the same as when the percept disappeared. This
freezing is disrupted if spatial attention is distracted from the empty
display , most probably by disrupting memory buildup. This can
be thought of as speeding up perceptual switching. Yet distracting
focal attention during bistable perception slows down the switching
rate  (Figure Id). In other words, withdrawing focal attention when
the stimulus is invisible (i.e. not consciously seen) disrupts perceptual
freezing, whereas withdrawing attention when the stimulus is visible
slows down switching.
Finally, consider complex decision making (Figure Ie). The study by
Dijksterhuis et al.  consisted of three phases: examination of items,
deliberation and decision. Either four or 12 properties for each of four
cars were shown one at a time during the examination phase. Subjects
then deliberated forseveral minutes withouttheattributes beingvisible
(i.e. subjects had to remember them; this can be thought of as an
‘invisible’ condition) before making a purchasing decision. Dijksterhuis
et al. manipulated whether or not subjects were kept busy by
performing a cognitively engaging task during the deliberation period.
They concluded that when faced with working-memory overload, an
explicit strategy based on deliberate and rational thought leads to poor
decision making, whereas distracting subjects while they decide which
car to buy greatly increases the probability of a good decision (Figure
If). We surmise that if the list of items were present throughout the
decision-making period, thereby reducing working-memory load, an
attention-distracting task would degrade purchasing performance.
Note that a complete independent manipulation of attention and
consciousness has not been performed in any of these examples.
Figure I. Three examples of dissociation of the effects of attention and awareness. (a,b) When an adaptor is attended to, the associated afterimage becomes weaker and
appears later (blue gradation) [55,56]; note that the afterimages are fainter than graphically indicated. Yet, paradoxically, when the adaptor is rendered invisible, its
afterimage is substantially weakened (pink gradation) [26,27]. (c,d) When attention is withdrawn from a visible bistable rivalry target (e.g. a Necker cube), the rate of
perceptual flips slows down (blue gradation) . When the target stimulus is intermittently presented (stabilization), the opposite occurs: withdrawing attention from
the target causes less stabilization and the perceptual flip rate speeds up (pink gradation) . (e,f) When confronted with a complex decision for which many items
must be remembered (i.e. the list is invisible), distracting subjects from the decision-making process improves performance (pink gradation) . ‘(?)’ indicates that data
are not available at this point. Figure If modified from Ref. .
TRENDS in Cognitive SciencesVol.11 No.1
empirical and conceptual work of many scholars over the
past decademake itclearthatthesepsychologicallydefined
conflated, are not the same. This empirical and functional
distinction clears the deck for a concerted neurobiological
attack on the core problem – that of identifying the neces-
sary and sufficient neural causes of a conscious percept (see
Box 3 for Questions for future research).
Owing to space limitations, we could not discuss all the relevant
literature and apologize to those whose work was not cited here. We
thank R. Blake, N. Block, A. Cleeremans, S. He, Y. Jiang, R. Kanai, V.
Lamme, C. Paffen, and M. Snodgrass for discussion. We thank the
participants of our tutorial at ASSC10 Oxford for their feedback. This
research was supported by the NIMH, the NSF, the Keck Foundation, the
Moore Foundation, and the Tom Slick Research Awards from the Mind
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Box 3. Questions for future research
? When studying the neuronal correlates of consciousness (NCC),
great care must be taken to untangle the effects of top-down
attention from those of consciousness [60–62]. Have the sug-
gested NCC been confounded by attentional effects?
? Does perception of gist, a high-level semantic description of a
scene (e.g. two people drinking; a man walking a dog), depend on
focal, top-down attention? How good are people at describing the
gist of novel, natural scenes under dual-tasks conditions?
? What are the neuronal mechanisms that lead to improved zombie
behaviors in the near absence of top-down attention ? Do
those aspects of reasoning, language processing and thinking that
proceed in the absence of consciousness function better without
? The arguments outlined in this article also apply for other
modalities (i.e. hearing and touch), although it might be difficult
to find ways to manipulate awareness of stimuli. Can robust
illusions be found to manipulate awareness in other modalities?
Can attention be shown without awareness and awareness be
shown without attention, and what are the opposing effects of the
two in these modalities?
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