Most studies and experiment to find Neural Correlates of Consciousness (NCCs) are carried out on the visual system. So far, there have been some interesting finds:
All vision does not follow the same path. There are separate streams of vision, cortical (via the cortex) and sub-cortical ('beneath' the cortex). Cortical pathways; the Ventral Stream, are involved in 'higher processing' such as finer details of visual information and the generation of awareness of vision. This is the area of interest here, rather than the Dorsal Stream (sub-cortical pathway) which is inolved in visual processing that occurs without conscious awareness. Yes, we are not consciouslly aware of everything we see, and furthermore, we are not consciouslly aware of everything we sense, but clever experiments can show that unconscious elements of the brain are at work, and pick up things that go unnoticed. Although damage to the primary visual cortex (V1) can cause an indivudal to lose the ability to 'see' objects at a conscious level, the objects do not go unnoticed and this is known as blindsight.
But this does not mean that the primary visual cortex alone constitutes the Ventral stream and gives rise to visual perception. Experiments on monkeys show that the higher degree of neural correlation to conscious perception (the experimenter devised a way to confirm when a monkey was consciouslly perceiving) occurs in the inferotemporal cortex, and that there isn't much correlation in the primary vsual cortex at all. The degree of neural correlation becomes stronger as we move anteriorally along the Ventral stream:
In the inferotemporal cortex we have what are known as essential nodes. These refer to areas of the inferotemporal cortex that are separated by function. In other words, there are different areas for finer detailed visual processing like colour or face recognition. Damage to any one of these specific areas will cause neurological deficeits like prospagnosia (the inability to recognize faces - one famous patient of visual agnosia who showed prospagnosia symptoms is the subject of the book; The Man Who Mistook his Wife for a Hat). Achromatopsia may result from damage to the region that deals with colour.
When these areas are active, we are consciouslly aware of the information they process. It is thus suggested that essential nodes are rather like micro-consciousness'.
The inferotemporal cortex is not the only suspect for NCC in vision.
Damage to the parietal cortex can cause spatial neglect, and this is because spatial awareness is processed in the parietal cortex. There are also prefrontal correlates to visual conscious awareness.
A problem appears to exist. Where as we have subjective experience of unified visual perception, that is, we see the "whole", our visual system is modular. There is no evidence for a space in the brain where everything comes together to generate the unified subjective experience we apparently... experience.
Personally, I don't think we have a unified consciousness. I think consciousness is built of modular, micro-units. Evidence suggest we are aware of colour 80 milliseconds befoe motion. This leads me to believe that the appearance of visual consciousness is just a result of VERY closely related neuronal processes in the visual systems. Breaks occur, but they are so slim we could never notice them. When neurons fire they are not firing continuously- they take very short breaks - so logically, we could never expect a neural correlation of consciousness to show true unification. The location of neurons should be unimportant, just as long as they each serve their own specified and detailed purpose which generates awareness, which I now see as something analogous to disco lights that flicker on and off in short sucession.
