Conscious, preconscious, and subliminal processing: a testable taxonomy

doi:10.1016/j.tics.2006.03.007

Trends in Cognitive Sciences

Volume 10, Issue 5, May 2006, Pages 204-211

https://doi.org/10.1016/j.tics.2006.03.007 Get rights and content

Of the many brain events evoked by a visual stimulus, which are specifically associated with conscious perception, and which merely reflect non-conscious processing? Several recent neuroimaging studies have contrasted conscious and non-conscious visual processing, but their results appear inconsistent. Some support a correlation of 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 subliminal, preconscious and conscious processing. We suggest that these distinctions map onto different neural mechanisms, and that conscious perception is systematically associated with surges of parieto-frontal activity causing top-down amplification.

Introduction

Understanding the neuronal mechanisms of consciousness is a major challenge for cognitive neuroscience. Recently, great progress has been achieved by contrasting brain activation images obtained during minimally different experimental conditions, one of which leads to conscious perception while the other does not. Surprisingly, 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, 5, 6, 7, 8, 9.

Here, we propose that those apparent contradictions can be resolved by a relevant theorizing of the physiological 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-conscious processing. In particular, within non-conscious processing, we distinguish a transient ‘preconscious’ state of activity in which information is potentially accessible, yet not accessed.

Section snippets

An enabling condition: vigilance

The term ‘consciousness’ has multiple meanings, one of them intransitive (e.g. ‘the patient regained consciousness’), 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

Early 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 demonstrated a tight correlation between the conscious visual perception and the activation of striate and extrastriate visual areas 13, 14, 15, 16, 17, 18. For instance, unmasking of a visual stimulus increases activity in extrastriate areas in tight correlation with subjective

Top-down amplification, long-distance reverberation, and reportability

We 4, 5, 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 interconnected 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

Is attention a confound or a necessity for conscious access?

Some have argued that many of the above neuroimaging paradigms are inappropriately controlled because conscious 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

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 processing (etymologically ‘below the threshold’) as a condition of information inaccessibility where bottom-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

Accounting for conflicting neuroimaging data

In experimental studies of conscious perception, preconscious processing, as an intermediate category, has sometimes 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 experiments require participants to attend to masked stimuli which are made visible or invisible by changing the masking strength. In our taxonomy,

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 top-down attention. The first case corresponds to subliminal processing, the second to preconscious

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Trends in Cognitive Sciences

Conscious, preconscious, and subliminal processing: a testable taxonomy

Introduction

Section snippets

An enabling condition: vigilance

Early visual activation is not sufficient for conscious report

Top-down amplification, long-distance reverberation, and reportability

Is attention a confound or a necessity for conscious access?

Distinguishing accessibility from access

Accounting for conflicting neuroimaging data

Conclusion

Prog. Brain Res.

Trends Cogn. Sci.

Trends Cogn. Sci.

Cognition

Trends Cogn. Sci.

Trends Cogn. Sci.

Neuron

Trends Neurosci.

Neuron

Neuroimage

Trends Cogn. Sci.

Neuropsychologia

Cognition

Ongoing spontaneous activity controls access to consciousness: a neuronal model for inattentional blindness

PLoS Biol.

A neuronal network model linking subjective reports and objective physiological data during conscious perception

Proc. Natl. Acad. Sci. U. S. A.

Neural correlates of consciousness in humans

Nat. Rev. Neurosci.

Are we aware of neural activity in primary visual cortex?

Nature

The process of awakening: a PET study of regional brain activity patterns mediating the re-establishment of alertness and consciousness

Brain

Searching for a baseline: functional imaging and the resting human brain

Nat. Rev. Neurosci.

Localizing the cortical region mediating visual awareness of object identity

Proc. Natl. Acad. Sci. U. S. A.

The relationship between cortical activation and perception investigated with invisible stimuli

Proc. Natl. Acad. Sci. U. S. A.

Neuronal correlates of perception in early visual cortex

Nat. Neurosci.

The neural correlates of conscious vision

Cereb. Cortex

The dynamics of object-selective activation correlate with recognition performance in humans

Nat. Neurosci.

Visibility, visual awareness, and visual masking of simple unattended targets are confined to areas in the occipital cortex beyond human V1/V2

Proc. Natl. Acad. Sci. U. S. A.

A Vision of the Brain

Cerebral mechanisms of word masking and unconscious repetition priming

Nat. Neurosci.