Trends in Cognitive Sciences
OpinionTowards a true neural stance on consciousness
Introduction
Consciousness is a multifaceted and complex phenomenon, encompassing functions such as language, attention and control. Its most enigmatic aspect, however, is that of conscious experience [1]. Why do some processes in the brain evoke conscious experiences, but others do not? The question is still far beyond our reach, but neuroscience is expected to provide an answer by finding the neural correlate of consciousness (NCC) [2]. Experiments to find the NCC invariably involve some manipulation of consciousness, induced experimentally [3] or accidentally, as in lesion patients. Choices are plentiful (Table 1). However, the intended presence or absence of conscious experience always has to be probed behaviorally, and that is where trouble begins.
This is best illustrated with a classic example. Split-brain patients (Table 1) can always report objects presented in the right half field of vision. But when objects are presented to the left, the patients say not to see them, simply because these are processed by the isolated right hemisphere, which has no capacity for language and speech. However, many patients can draw these unseen objects, select them from a row of other objects, match them to words, or perform other (simple) cognitive operations, as long as the behavior is executed by the left hand, which is connected to the right hemisphere 4, 5.
Here is the crucial question: Are these patients seeing the objects in the left half field? They say not, and they know best, don’t they? Consequently, we might conclude that the right hemisphere is not part of the NCC for visual experience. But can you draw something without seeing it? And isn’t selection an expression of recognition and attention? Isn’t it unlikely that the right hemisphere cannot sustain conscious experience, simply because it cannot ‘talk’? This would argue for not excluding the right hemisphere from the NCC. Note that nothing intrinsic to the behavioral experiments provides arguments for or against either conclusion. It all depends on preconceived notions about the role of language in consciousness, about which much has been written but little concluded.
The example highlights two notorious difficulties in gauging conscious experience from the third- (or even first-) person perspective. The first is that some choice has to be made as to what behavioral measures ‘count’ as evidence for the subject having conscious experience (e.g. drawing versus talk) (Figure 1). The second is that in such heterophenomenological observations conscious experience is easily conflated with cognitive functions that are necessary for the report (in this case language).
This problem has grave consequences for the search for the NCC. Many similar examples exist, in which various, if not all, parts of the brain have been included or excluded from the NCC, entirely depending on the measure of conscious experience, or the notion of consciousness that is started with 6, 7 (Table 1). This not only poses a problem for finding the ‘true’ NCC; more serious is that, in this way, neuroscience will hardly fulfill its promise to get rid of the ‘tedium of philosophers perpetually disagreeing with each other’ [8].
To find a way out, I think we have to go beyond finding ‘neural correlates of’, and let the arguments from neuroscience have a true say in the matter. To illustrate this, I first identify the problems of treating visual experience as an entirely mental or psychological concept. I then show how a more coherent scientific definition of conscious visual experience emerges, when both behavioral and neural findings, combined with theoretical concepts, are put on an equal footing. This will, however, entail a view on consciousness that is moving away from our traditional or introspective notions. But this is the only way towards progress.
Section snippets
From neural activation to visual experience
A basic assumption of consciousness research is that when a subject is presented with a visual stimulus it is either seen or not seen. What happens in the brain when a stimulus is shown, and can we establish when conscious experience emerges from the neural activity it causes?
When a new image hits the retina, it is processed through successive levels of visual cortex, by means of feedforward connections, working at an astonishing speed. Each level takes only 10 ms of processing, so that in
An impossible question?
Much more challenging is another question: is recurrency in itself sufficient for conscious experience? Do we see a face as soon as face-selective and lower-level cells engage in recurrent interactions (Figure 2b)? At first sight, this seems easy to answer. Any instance of RP in the absence of reportable awareness would falsify the idea. Super et al. [15] showed that recurrent interactions, recorded in V1 of the awake monkey, are necessary for the animal reporting the presence of texture
Taking the neuroscience argument seriously
This is where I diverge. The question seems not addressable because we do not let all scientific arguments have their way, and treat conscious experience as something that can only be observed behaviorally or introspectively. We could find a solution, when we let neuroscience go beyond ‘finding neural correlates of’. Consciousness should not only be inferred from behavioral measures, to which neural measurements are subordinate. Both measures should be on an equal footing.
There is no need to
What do we lose, what do we gain…
Letting arguments from neuroscience override our intuitive and introspective notion of consciousness seems strange. After all, consciousness is almost synonymous to private access and personal feeling. So what is the benefit of adopting a more neural stance on consciousness? At first sight, we seem to lose explanatory power. There would be instances of (neurally defined) conscious processing that are not evident from behavioral measures or introspection (Figure 3). It is not unusual in science,
…by moving our notion of mind towards that of brain?
I have tried to point out that, by adopting a partly neural stance on consciousness, seemingly unsolvable issues in consciousness research can be solved. I do not claim that the neural definition of consciousness that I propose 10, 33 is ultimately correct. It is the approach that I advocate. Alternative neural definitions might be proposed 7, 35. Our task should be to evaluate each of these on its scientific merits and capability to deal with all the phenomena concerning consciousness and
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