Visual perception and phenomenal consciousness.

Summary

Consciousness is exclusive to living organisms that can distinguish self from non-self and voluntarily modify their behavior, requiring an intermediary neuronal net between sensory input and behavioral output. The visual system reveals two distinct aspects of consciousness: phenomenal vision (subjective experience) and conscious access (ability to retrieve and manipulate information). Blindsight patients, who process visual information without phenomenal vision, demonstrate this dissociation. Monkeys with striate cortex removal show similar absence of phenomenal vision, enabling further study of its neural basis. Conscious access likely requires higher cortical structures and depends on phenomenal representations, which may function to allow conscious thinking and planning.

Study at a glance

Abstract

In the (re-)animated debate on consciousness we focus on three questions: Who has consciousness? What is its neuronal basis? What is its function? Regarding the first, we suggest that consciousness is exclusive to living organisms able to distinguish self from non-self. It may be restricted further to organisms who possess a repertoire of overt and covert behaviour which can be voluntarily modified and suppressed. This requires an intermediary neuronal net mediating between sensory input and behavioural output. What are the properties of this net which distinguish unalloyed information processing per se from conscious representation? To tackle this second question, we use the visual system and the functional losses that result from lesions at its different levels, and differentiate a reflexive, a phenomenal, and a consciously accessible stage of visual processing. We suggest that the latter two represent two distinct aspects of consciousness. Blindsight, a neurological example of visual processing in the absence of phenomenal vision, could help to elucidate the neuronal basis of phenomenality, and the special role of striate cortex. Like the patients, our monkeys with unilateral striate cortical removal show evidence not just of residual visual processing, but of the same absence of phenomenal vision, opening routes to further exploring the details of its neuronal implementation. The second aspect, conscious access to presently or previously processed information, is likely to require higher cortical structures, and may depend on the stage of phenomenal representations. In patients with blindsight, both aspects are lost, and it is conceivable that a loss of phenomenality generally causes a loss of conscious accessibility. One important function of phenomenal representations, our third question, would then be to allow conscious retrieval and manipulation of currently processed or formerly stored information, enabling organisms to consciously think and plan.