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Several recent neuroimaging studies have contrasted
conscious and non-conscious visual processing, but their
results appear inconsistent. Some support a correlation
in the brainstem and thalamus, then in the cortex with a
particularly important increase in prefrontal-cingulate
activity during vigilant rest [12] and encompasses
Opinion TRENDS in Cognitive Sciences Vol.10 No.5 May 2006conscious processing. In particular, within non-consciousdifferent experimental conditions, one of which leads to
conscious perception while the other does not. Surpris-
ingly, however no coherent picture has emerged from
those experiments. On the contrary, a controversy has
arisen, as some studies suggest that consciousness
depends mostly on the thalamus and brain stem [1],
others on early visual areas [2,3], and yet others on higher
prefrontal and parietal association areas [4–9].
Here, we propose that those apparent contradictions
can be resolved by a relevant theorizing of the physiologi-
cal conditions for conscious processing of sensory stimuli.
Based on the recent proposal of a large-scale thalamo-
cortical formal network and its simulations [4,5], we
tentatively propose a plausible and testable taxonomy of
brain activity states associated with conscious and non-
prefrontal, cingulate and inferior parietal nodes.
These observations may be captured by a recent
implementation of the neural workspace model [4] in
which ascending brain stem nuclei (e.g. cholinergic among
others) send globally depolarizing neuromodulatory sig-
nals to a thalamic and cortical hierarchy. Simulations
show a progressive increase in spontaneous firing as a
function of neuromodulator release, which evolves into
what is known in dynamical systems theory as a Hopf
bifurcation: spontaneous firing increases continuously in
intensity, but high-frequency oscillations appear suddenly
in the gamma band (20–80 Hz). By increasing spon-
taneous activity, and thus bringing a broad thalamo-
cortical network closer to firing threshold, vigilance lowers
the threshold for external sensory inputs.brain activation images obtained during minimally
Recently, great progress has been achieved by contrasting thalamocortical network which also shows high baselineof conscious perception with early occipital events,
others with late parieto-frontal activity. Here we attempt
to make sense of these dissenting results. On the basis of
the global neuronal workspace hypothesis, we propose a
taxonomy that distinguishes between vigilance and
access to conscious report, as well as between sub-
liminal, preconscious and conscious processing. We
suggest that these distinctions map onto different neural
mechanisms, and that conscious perception is system-
atically associated with surges of parieto-frontal activity
causing top-down amplification.
Introduction
Understanding the neuronal mechanisms of conscious-
ness is a major challenge for cognitive neuroscience.Conscious, precons
subliminal processi
taxonomy
Stanislas Dehaene1,2, Jean-Pierre Change
and Claire Sergent1
1INSERM-CEA Cognitive Neuroimaging Unit, Service Hospitalie
2Colle`ge de France, Paris, France
3CNRS Unit, Receptors and Cognition, Institut Pasteur, Paris, Fra
Of the many brain events evoked by a visual stimulus,
which are specifically associated with conscious percep-
tion, andwhichmerely reflectnon-conscious processing?activation and functional connectivity [10]. Anesthesia,
sleep, vegetative state and coma [1,11] are all associated
with modulations of the activity of this large-scaleious, and
g: a testable
2,3, Lionel Naccache1, Je´roˆme Sackur1
e´de´ric Joliot, Orsay, France
e
processing, we distinguish a transient ‘preconscious’ state
of activity in which information is potentially accessible,
yet not accessed.
An enabling condition: vigilance
The term ‘consciousness’ has multiple meanings, one of
them intransitive (e.g. ‘the patient regained conscious-
ness’), and the other transitive (e.g. ‘consciousness of
color’). To avoid further confusion, we abandon the term
and use ‘states of vigilance’ to refer to the non-transitive
meaning, i.e. a continuum of states which encompasses
wakefulness, sleep, coma, anesthesia, etc.
Being in an appropriate state of vigilance (e.g. awake
rather than asleep) is an obvious enabling condition for
conscious processing of sensory stimuli. Empirically,
awakening into the vigilant state correlates with a
progressive increase in regional cerebral blood flow, firstminimum level is essential for placing thalamo-cortical
systems into a receptive state.
Corresponding author: Dehaene, S. (dehaene@shfj.cea.fr).
www.sciencedirect.com 1364-6613/$ - see front matter Q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tics.2006.03.007In summary, vigilance is a graded variable, and a

visibility are found solely in occipital areas, not in higher
associative regions, and therefore argue that the mech-
Opinion TRENDS in Cognitive Sciences Vol.10 No.5 May 2006 205Early visual activation is not sufficient
for conscious report
We now consider the neural bases of the second, transitive
meaning of consciousness, which we term ‘access to
conscious report’. How do we consciously perceive a visual
stimulus? Many neuroimaging experiments have demon-
strated a tight correlation between the conscious visual
perception and the activation of striate and extrastriate
visual areas [13–18]. For instance, unmasking of a visual
stimulus increases activity in extrastriate areas in tight
correlation with subjective reports of stimulus visibility
[18]. Furthermore, extrastriate regions clearly play a
causal role in conscious visual perception, because their
selective lesioning eliminates the corresponding contents
from experience – for instance a lesion of area V4 can
destroy color perception in the contralateral
hemifield [19].
On the basis of such data, Zeki [2] has proposed that the
conscious perception of a given visual attribute resides in
the extrastriate area specialized for that attribute (e.g.
area MT/V5 for motion, or area V4 for color). A ‘micro-
consciousness’ would be involved whenever that area
receives a sufficient amount of activation.
We argue, however, that early sensory activation is
necessary but not sufficient for conscious access, because
activity in extrastriate visual areas is frequently observed
while participants deny having seen any stimulus [14,20–
23]. When invisibility is caused by masking [20] or by
dichoptic stimulation [14] this stimulus-evoked activity
remains weak, and one might argue that its small
amplitude alone could explain the absence of conscious
perception [2,14]. However, the visual activation evoked
by invisible stimuli can also be very strong, for instance
when invisibility is caused by neglect [21] or inattention
[22,23]. In a recent study of the attentional blink, we
observed that up to about 180 ms after stimulus presen-
tation, the occipito-temporal event-related potentials
evoked by a invisible word were large and essentially
indistinguishable from those evoked by a visible word [23].
Yet on invisible trials, the participants’ visibility ratings
did not deviate from the lowest value, used when no word
was physically present. Thus, intense occipito-temporal
activation can be accompanied by a complete lack of
conscious report.
Top-down amplification, long-distance reverberation,
and reportability
We [4–6] and others [7,8,24] have suggested that, in
addition to vigilance and bottom-up activation, a third
factor underlying conscious access is the extension of
brain activation to higher association cortices intercon-
nected by long-distance connections and forming a
reverberating neuronal assembly with distant perceptual
areas. Why would this brain state correspond to conscious
access? Neurocomputational simulations show that once
stimulus-evoked activation has reached highly intercon-
nected associative areas, two important changes occur: (1)
The activation can reverberate, thus holding information
on-line for a long duration essentially unrelated to the
initial stimulus duration; (2) Stimulus information can be
rapidly propagated to many brain systems. We argue that
www.sciencedirect.comanisms of conscious visual perception lie in
extrastriate cortex.
We obviously agree on one point: it is important to
design paradigms in which conscious perception is not
confounded with massive changes in overt or covert
behaviour. However, this goal has been achieved in several
studies. In our recent study of the attentional blink [23],
for instance, subjects viewed a fixed stimulus and made
similar motor gestures on seen and not-seen trials, yet
those were still distinguished by strong parieto-
frontal activation.
We question, however, the proposal that inattention is
an appropriate control. Under conditions of diverted
attention, such as those studied by Tse et al. [18], even
an unmasked stimulus is not guaranteed to be consciously
perceived. On the contrary, considerable evidence indi-
cates that without attention, conscious perception cannot
occur. In the inattentional blindness paradigm, even a
700-ms stimulus presented in the fovea, when unat-
tended, might fail to be seen [33]. During the attentional
blink, a mildly masked stimulus, normally quite visible,both properties are characteristic of conscious information
processing which in our view is associated with a distinct
internal space, buffered from fast fluctuations in sensory
inputs, where information can be shared across a broad
variety of processes including evaluation, verbal report,
planning and long-term memory [25].
Empirically, access of sensory stimuli to conscious
report correlates with the activation of higher associative
cortices, particularly parietal, prefrontal and anterior
cingulate areas. In fMRI, the activation of those regions
systematically separates masked versus unmasked pre-
sentations of words [20] or images [26]; undetected versus
detected changes during change blindness [27,28]; extin-
guished versus seen visual stimuli in neglect patients [29];
or missed versus reported stimuli during the attentional
blink [9,22,23,30–32]. In many of these paradigms,
anterior activation is accompanied by an amplification
and an increase in functional correlation with posterior
stimulus-specific areas [20,26,30]. Sudden parieto-frontal
activation and top-down amplification are two frequent
signatures of conscious perception.
Is attention a confound or a necessity for conscious
access?
Some have argued that many of the above neuroimaging
paradigms are inappropriately controlled because con-
scious perception is confounded with increased attention
and more extended stimulus processing. For instance, a
conscious word can be attended, repeated or memorized
while a non-conscious word cannot. Such confounds would
suffice to explain the greater parieto-prefrontal activity to
unmasked words [20]. For this reason, Tse et al. [18] have
argued that one should prefer experimental designs in
which attention is drawn away from the stimulus. They
show that, in such a situation, correlates of stimulusbecomes invisible when attention is diverted to another
task [23,34].

The relations between stimulus strength, attention,
and conscious perception are complex because attention
mechanisms can also be activated automatically in a
bottom-up manner. When the stimuli have strong energy,
sharp onsets or strong emotional content, they might
trigger an activation of frontal eye fields or amygdala
pathways, thus causing an amplification that can lower
their threshold for conscious perception [35]. Thus, both
bottom-up stimulus strength and top-down attentional
amplification (whether triggered voluntarily or by auto-
matic attraction) are jointly needed for conscious percep-
tion, but they might not always be sufficient for a
stimulus to cross the threshold for conscious perception.
Conscious perception must therefore be evaluated by
subjective report, preferably on a trial-by-trial basis.
Verifying that the stimuli can be consciously perceived in
a separate experimental block where they are attended,
as done by Tse et al. [18], does not suffice to guarantee
conscious perception in a different block where attention
is diverted. One cannot simply assume that, by
unmasking stimuli, one is studying the neural correlates
of conscious processing.
Distinguishing accessibility from access
The above distinctions lead us to proposal a formal
definition of two types of non-conscious processes
(Figure 1):
(1) Subliminal processing. We define subliminal proces-
sing (etymologically ‘below the threshold’) as a
condition of information inaccessibility where bot-
tom-up activation is insufficient to trigger a large-scale
reverberating state in a global network of neurons
with long range axons. Simulations of a minimal
thalamo-cortical network [4] indicates that such a non-
linear self-amplifying system possesses a well-defined
dynamical threshold. A processing stream that
exceeds a minimal activation level quickly grows
until a full-scale ignition is seen, while a slightly
Top-down attention
Absent PresentBottom-up
stimulus
strength
Weak
or
interrupted
Subliminal (unattended) Subliminal (attended)
• Very little activation
• Activation is already weak in
early extrastriate areas
• Little or no priming
• No reportability
• Strong feedforward activation
• Activation decreases with depth
• Depth of processing depends on attention
and task set
• Activation can reach semantic level
• Short-lived priming
• No durable fronto-
parietal activity
• No reportability
cess
zon
tation of top-down attention to the stimulus, or away from it (‘task-unrelated attention’).
a sharp transition between states. During subliminal processing, activation propagates
con
an b
ge b
des
Opinion TRENDS in Cognitive Sciences Vol.10 No.5 May 2006206Sufficiently
strong
Preconscious
• Intense activation, yet confined to
sensori-motor processors
• Occipito-temporal loops and local
synchrony
• Priming at multiple levels
• No reportability
while attention is
occupied
elsewhere
Figure 1. Proposed distinction between subliminal, preconscious, and conscious pro
up stimulus strength (on the vertical axis at left) and top-down attention (on the hori
arrows the interactions among them. Large arrows schematically illustrate the orien
Dashed curves indicate a continuum of states, and thick lines with separators indicate
but remains weak and quickly dissipating (decaying to zero after 1–2 seconds). A
attention, and instructions (see Box 1). During preconscious processing, activation c
frontal eye fields). However, when attention is oriented away from the stimulus (lar
establishing long-distance synchrony. During conscious processing, activation inva
becomes capable of guiding intentional actions including verbal reports. The transition
self-amplified non-linear system [4].
www.sciencedirect.comtinuum of subliminal states can exist, depending on masking strength, top-down
e strong, durable, and can spread to multiple specialized sensori-motor areas (e.g.
lack arrows), activation is blocked from accessing higher parieto-frontal areas and
a parieto-frontal system, can be maintained ad libidum in working memory, andTRENDS in Cognitive Sciences
Conscious
• Orientation of top-down attention
• Amplification of sensori-motor activity
• Intense activation spreading to parieto-
frontal network
• Long-distance loops and global synchrony
• Durable activation, maintained at will
• Conscious reportability
ing. Three types of brain states are schematically shown, jointly defined by bottom-
tal axis). Shades of color illustrate the amount of activation in local areas, and smallbetween preconscious and conscious is sharp, as expected from the dynamics of a

weaker activation quickly dies out. Subliminal proces-
sing corresponds to the latter type.
Note that, under our hypothesis, subliminal processing
is not confined to a passive spreading of activation,
independent of the subject’s attention and strategies, as
previously envisaged. On the contrary, whichever task
and attentional set are prepared consciously can orient
and amplify the processing of a subliminal stimulus, even
if its bottom-up strength remains insufficient for global
ignition. In agreement with this analysis, many top-down
influences on subliminal processing have now been
experimentally observed (Box 1).
(2) Preconscious processing. Freud [36] noted that ‘some
processes [.] may cease to be conscious, but can
become conscious once more without any trouble’, and
he proposed that ‘everything unconscious that behaves
in this way, that can easily exchange the unconscious
condition for the conscious one, is therefore better
described as “capable of entering consciousness” or as
preconscious.’
Here we further specify the latter term. We propose to
call preconscious (or potentially conscious, or P-conscious)
Box 1. Why attention and consciousness are different: top-
down influences on subliminal processing
Subliminal processing is frequently thought to be automatic and
independent of attention. However, the present framework implies
that top-down attention and task set can have an effect on subliminal
processing (see Figure 1 in main text, top row). This prediction has
been verified in several recent reports.
Modulation of subliminal priming by temporal attention
In a numerical masked priming paradigm, Naccache et al. [43] first
showed that subliminal priming was present when subjects could
allocate attention to the prime-target pair, but vanished when
Opinion TRENDS in Cognitive Sciences Vol.10 No.5 May 2006 207stimuli could not be temporally attended. Kiefer and Brendel [44]
observed a similar effect in an experiment investigating the N400
potential elicited by masked words. Unseen masked words elicited
a much larger N400 when they were temporally attended than when
they were not.
Modulation by spatial attention
Kentridge et al. [45,46] first reported that blindsight patient GY could
use consciously perceived cues to enhance unconscious processing
of visual targets. When a target was presented in his blind visual
field, GY responded faster and more accurately when it was validly
cued by a consciously perceptible arrow pointing to it, than when it
was invalidly cued. In both cases, he still claimed that he could not
see the target. Modulation of subliminal priming by spatial attention
was also observed in normal subjects [47].
Modulation by strategies
Task instructions also alter the fate of subliminal stimuli. For
instance, masked primes can elicit instruction-dependent activation
in motor cortex [48,49], suggesting that arbitrary stimulus–response
mappings conveyed by conscious instructions can also apply to non-
conscious stimuli. The influence is always unidirectional: once a
strategy or response mapping is consciously adopted, it extends to
non-conscious primes [50,51]. Kunde et al. [51] studied the ‘Gratton
effect’, a strategic increase in executive control that follows Stroop
interference trials. They observed this effect following conscious
conflict trials, but not following subliminal conflict trials. Once
established, however, the increase in control applied to both
subliminal and supraliminal trials – another instance of a top-down
effect on subliminal processing.
www.sciencedirect.coma neural process that potentially carries enough activation
for conscious access, but is temporarily buffered in a non-
conscious store because of a lack of top-down attentional
amplification (for example, owing to transient occupancy
of the central workspace system). As shown by the
attentional blink and inattentional blindness paradigms,
even strong visual stimuli can remain temporarily
preconscious. They are potentially accessible (they could
quickly gain access to conscious report if they were
attended), but they are not consciously accessed at
the moment.
At the neurocomputational level, preconscious proces-
sing is proposed to involve resonant loops within medium
range connections which maintain the representation of
the stimulus temporarily active in a sensory buffer for a
few hundred milliseconds. A preconscious stimulus might
ultimately achieve conscious access once the central
workspace is freed (as exemplified by the psychological
refractory period paradigm [37,38], in which one task is
put on hold while another task is being processed). It
might never gain access to conscious processing if the
preconscious buffer is erased before the orienting of top-
down attention (as achieved by masking in the attentional
blink paradigm).
Accounting for conflicting neuroimaging data
In experimental studies of conscious perception, precon-
scious processing, as an intermediate category, has some-
times been confounded with subliminal processing, and
sometimes with conscious processing. We now show how
this distinction can provide a simple account of conflicting
neuroimaging results (Figure 2).
(1) Masking when stimuli are attended. Some exper-
iments require participants to attend to masked
stimuli which are made visible or invisible by
changing the masking strength. In our taxonomy,
those experiments contrast subliminal versus con-
scious stimuli – a major contrast which should reveal
both early stimulus processing regions and a dis-
tributed parieto-frontal workspace system. Indeed,
empirically, both early extrastriate and late parietal
and prefrontal differences have been observed [20,26].
(2) Stimuli presented at threshold. Even when attended,
stimuli presented at sensory threshold may or may not
be perceived. In our theory this is again a contrast
between subliminal and conscious stimuli. As pre-
dicted, neuroimaging experiments relying on this
contrast have yielded both early (e.g. P100) and late
(e.g. P300) correlates of conscious perception
[15,16,39]. The theory can also explain why conscious
access fluctuates even though the stimulus remains
constant. Simulations show that the threshold for
global ignition can vary both with vigilance and with
the amount of spontaneous activity before stimulus
presentation [4]. Several experiments confirm that the
perception of near-threshold stimuli can be predicted
by the prestimulus state, in both humans and
monkeys [40,41].(3) Masking when stimuli are not attended. If stimuli are
not attended, then the comparison of masked and

1 ve
T2
Opinion TRENDS in Cognitive Sciences Vol.10 No.5 May 2006208Global
workspace
T1
Conscious
high strength
and attention
TT2
Preconscious
high strength,
no attention
T3
Subliminal
weak strength(a) (b)unmasked stimuli becomes a contrast between sub-
liminal and preconscious processing. As predicted,
only the early components of occipito-temporal acti-
vation are seen [18]. According to our terminology,
these are the correlates of preconscious visual proces-
sing (potential visibility, yet no conscious access).
(4) Stimuli made invisible by inattention. Some exper-
iments have contrasted consciously perceived stimuli
with stimuli made invisible by diverting top-down
attention (attentional blink, change blindness, inat-
tentional blindness). This is a contrast between
preconscious and conscious processing. As expected,
the difference appears late (200–300 ms after the
stimulus) and involves parieto-prefrontal activation
as well as late amplification of posterior activity
[22,23,26–30,32].
Conclusion
Instead of the classical binary separation between non-
conscious and conscious processing, we introduce here a
tripartite distinction between subliminal, preconscious,
and conscious processing. The key idea is that, within non-
conscious states, it makes a major difference whether
stimuli invisibility is achieved by a limitation in bottom-up
stimulus strength, or by the temporary withdrawal of
Seen stim
Figure 2. Resolving contradictions in neuroimaging studies. (a) Schematic representation
accessed when it activates, in a synchronized, reciprocal and long-lasting manner, a set o
cortices, and whose long-distance connections enable broadcasting to many distant are
enough bottom-up strength, for example, owing to low-level masking or presentation clo
to be visible, but still fail to be seen by losing the competition for central access relat
(b) Reinterpretation of neuroimaging experiments in this framework. When masked and
difference in brain activation is seen, with both early sensory and late parieto-frontal enh
also [16,26,39]). When masked and unmasked stimuli are contrasted while attention is
temporal cortices (fMRI data by Kouider and Dehaene; see also [18]). When stimuli ar
attention to or away from the stimuli (bottom right), the difference in activation is late an
(illustrations from the attentional blink paradigm reproduced with permission from [23]
www.sciencedirect.comT1 versus T3: unmasked or masked stimuli
(both attended)
rsus T2: accessed versus non-accessed stimuli
Unmasked words (T1) Masked words (T3)
Unmasked words (T2)
>
masked words (T3)
(both used as unattended primes)
versus T3: unmasked versus masked stimuli
(both unattended)top-down attention. The first case corresponds to sub-
liminal processing, the second to preconscious processing.
We have shown how this distinction is theoretically
motivated and helps make sense of neuroimaging data.
Is our taxonomy complete? Box 2 briefly discusses three
other types of non-conscious knowledge in the nervous
system: latent connectivity patterns, distributed firing
patterns, and functionally disconnected brain systems.
Altogether, these distinctions might suffice to capture the
known experimental conditions in which information
escapes conscious reportability. The proposed taxonomy
is testable, not only with neuroimaging tools, but also
using electrophysiological techniques in the awake
monkey, provided that tasks similar to the attentional
blink and psychological refractory period can be developed
in these species (see Box 3).
Our proposal could also lead to a reconciliation of several
major theories of conscious perception. The distinction
between preconscious and conscious processing is consist-
ent with Lamme’s proposal of a progressive build-up of
recurrent interactions, first locally within the visual
system, and second more globally into parieto-frontal
regions [3]. It is also consistent with Zeki’s hypothesis of
an asynchronous construction of visual perception
in multiple distributed sites before binding into a
uli (T1) > missed stimuli (T2) during the attentional blink
of the global neuronal workspace model. A visual target T1 (in green) is consciously
f ‘central workspace’ neurons particularly dense in parietal, prefrontal and cingulate
as. A stimulus can fail to become conscious for two reasons: (1) it might not have
se to threshold (subliminal stimulus T3, in red); or (2) it might have enough strength
ive to other concurrent stimuli or task sets (preconscious stimulus T2, in orange).
unmasked stimuli are contrasted while subjects are attending (top right), a major
ancements for seen stimuli (illustration reproduced with permission from [20]; see
drawn elsewhere (middle right), the effect of masking is confined to early occipito-
e above the masking threshold, and conscious access is manipulated by drawing
d confined to higher association cortices, particular parietal and prefrontal regions
– left image, and [30] – right image); see also [21,22,31].

ssible? A hypothetical taxonomy
workspace hypothesis states that it must be represented by small
groups of neurons whose firing provides an unambiguous index of the
relevant attribute, and which would be amplified by top-down
attention. For instance, although the collective firing of V1 neurons
encodes all aspects of the visual scene, including the presence of faces
or color, those attributes would not be consciously seen unless the
extrastriate areas involved in their extraction are intact. At a higher
cognitive level, when we gain conscious access to a previously
subliminal distinction (e.g. development of ‘phonemic awareness’ in
children), neuronal populations selective for this learned distinction
should be found.
(iii) Information is coded by neurons functionally discon-
nected from the workspace
Even information in explicit firing form can remain non-conscious if
the relevant neurons lack the bidirectional projections appropriate
to establish a reverberant assembly with parietal and prefrontal
cortices. This functional disconnection hypothesis might explain
why we have no conscious access to the state of activity of
subcortical systems sustaining basic maintenance processes (res-
piration, ingestion, posture, etc). Patients with white matter lesions,
including callosal lesions, can also lose conscious access to word,
color or object information that is still extracted, yet
functionally disconnected.
Opinion TRENDS in Cognitive Sciences Vol.10 No.5 May 2006 209Box 2. Why does some knowledge remain permanently inacce
This article discusses a model of how visual processing can remain
non-conscious for dynamical reasons of insufficient strength or
concurrent attentional load. A complete taxonomy such as that
proposed in Table I, however, should also capture the many types of
permanentlynon-conscious knowledge stored in the nervous system.
The global neuronal workspace hypothesis stipulates that infor-
mation is consciously accessible if it is explicitly coded in the firing of
groups of excitatory neurons with bidirectional links to a distributed
network of workspace neurons. Accordingly, information might
remain permanently non-conscious for at least three reasons [6]:
(i) Information is not encoded in neuronal firing
Knowledge stored in a latent form as synaptic efficacies remains
inaccessible until it is used to recreate evoked patterns of neural firing.
This constraint may explain instances of implicit learning, and why we
do not have conscious access to most of our mental algorithms. In the
few cases where we do (e.g. when we describe the steps needed for
long division), the model predicts that each step should be explicitly
coded in the firing of workspace neurons. Indeed, experimentally,
prefrontal neurons coding for intentions, plans, ordinal steps,
evaluations, intermediate decisions, and errors have been identified.
(ii) Information is not represented in explicit firing form [52]
For an aspect of the visual scene to be consciously accessible, the‘macro-consciousness’ [2]. Our only source of disagreement
– but an important one – resides in their attribution of
‘phenomenal consciousness’ or ‘micro-consciousness’ to
what we have termed pre-conscious processing. Remember
that, in such a state, only a few hundreds of milliseconds
after a stimulus was presented and yielding strong visual
activity, participants deny perceiving anything [34].
Whether they actually had a conscious phenomenal
experience but no possibility of reporting it, does not seem
to be, at this stage, a scientifically addressable question.
The only rationale for attributing phenomenal conscious-
ness to preconscious processing seems to be the intuition
that visual experience involves a richness of content that
goes beyond what we can report [42]. However, this
intuition itself can be explained as a kind of illusion – we
think that we see more than we actually do (Box 4).
To further explore these difficult issues in the future, it
will be crucial to develop better methods for the formal
collection and quantification of introspective reports
[23,34], as well as for the study of the spontaneous flow
of conscious processes [4,12]. Meanwhile, the proposed
distinction between subliminal, preconscious and con-
scious processing, and the identification of conscious
Box 3. Questions for further research
† Can one design attentional blink, psychological refractory period,
and partial report paradigms for non-human primates? Can they be
used to dissect the neural mechanisms of the ‘preconscious buffer’?
Does this preconscious state engage solely local occipito-temporal
loops?
† Do all demonstrations of non-conscious information processing in
humans fall into one of the categories of the proposed taxonomy? In
particular, can one identify model cases where the non-conscious
information is demonstrably encoded in synaptic weights, or in
neural systems functionally disconnected from parieto-frontal
areas?
† Can transcranial magnetic stimulation (TMS) be used to disrupt
parieto-frontal circuits and probe their causal involvement in
conscious visual perception? Would occipito-temporal TMS simi-
larly disrupt the preconscious buffer during the psychological
refractory period?
† Can one find experimental means of testing whether any
subjective content is associated with preconscious states? Or is
the existence of non-reportable conscious states untestable by
definition?
† Can better non-verbal methods be developed for the quantification
of introspective reports, both in humans and in non-human
primates?
† What type of neural activity patterns underlies introspective
reports, as opposed to other more direct sensory-motor decisions?
Table I. A theoretical taxonomy of conscious and non-conscious information encoding in the brain
Information encoding Main features
Non-conscious Latent connectivity
patterns
Information is encoded in latent form as matrices of synaptic weights
Distributed firing patterns Information is encoded in the distributed firing of many neurons, not condensed
in small specialized groups of neurons
Functionally disconnected
systems
Information is encoded in the firing of neurons functionally disconnected from
the workspace
Subliminal processing Processing is confined to a brief travelling pulse of firing
Preconscious processing Processing involves local resonant firing loops, but top-down attention is
focussed on another stimulus or task set.
Conscious processing Processing receives top-down amplification and expands into a global parieto-
frontal reverberant state.
www.sciencedirect.com

attention or occur at an attended location are immediately detected.
Thus, the illusion of seeing might arise because viewers know that
Opinion TRENDS in Cognitive Sciences Vol.10 No.5 May 2006210they can, at will, orient attention to any location and obtain
conscious information from it.
In summary, according to the present terminology, the wholeBox 4. ‘Phenomenal consciousness’ without reportability?
Following Weiskrantz [53], we consider that subjective reports are the
primary criterion that can establish whether a percept is conscious or
not. Such reports need not be verbal. Many neuroimaging exper-
iments rely on manual reports of conscious perception, which can be
made more precise by the use of a continuous visibility measure
[23,34].
The philosopher Ned Block, however, has suggested that the
reportability criterion underestimates conscious contents [42]. When
we view a complex visual scene, we experience a richness of content
that seems to go beyond what we can report. This intuition led Block
to propose a distinct state of ‘phenomenal consciousness’ prior to
global access. This proposal receives an apparent confirmation in
Sperling’s iconic memory paradigm. When an array of letters is
flashed, viewers claim to see the whole array, although they can later
report only one subsequently cued row or column. One might
conclude that the initial processing of the array, before attentional
selection of a row or column, is already phenomenally conscious
[3,42].
However, these intuitions are questionable, because viewers are
known to be over-confident and to suffer from an ‘illusion of seeing’
[54]. The change-blindness paradigm demonstrates this ‘discre-
pancy between what we see and what we think we see’ [55]. In this
paradigm, viewers who claim to perceive an entire visual scene fail
to notice when an important element of the scene changes. This
suggests that, at any given moment, very little of the scene is
actually consciously processed. Interestingly, changes that attractcontents with globally accessible ones, appear to be
productive avenues for scientific research.
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