Trends in Cognitive Sciences
Conscious, preconscious, and subliminal processing: a testable taxonomy
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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