Diffuse consciousness

May 1, 1998

A news round-up from the third conference on consciousness in Tucson, Arizona

There may be no central part of the human brain receiving signals from specialist areas and integrating them to generate consciousness, according to research at University College London, writes Kam Patel.

Semir Zeki, a professor in UCL's department of cognitive neurology, argues that individual areas of neurons generate a unique consciousness and "correlate" without necessarily "talking" to others.

His psychophysical experiments have shown that different neural processing systems of the brain do not complete their tasks at the same time: different features of a visual scene are perceived at different times. Colour is seen before the orientation of an object, which is perceived before motion. The difference between perceptual times for colour and motion are typically 60 milliseconds to 80 milliseconds, a strong hint that the processing of colour and motion takes place in separate locations.

Professor Zeki says there is also strong evidence that damage to one processing system does not affect the other because they operate fairly autonomously. Specific damage to one system, say the colour processing centre, in an area of brain called V4, does not necessarily destroy a subject's vision. Typically, those with highly specific damage to V4 cannot perceive colours but are able to perceive motion normally.

Similarly, people with specific damage to V5, which deals with motion, cannot perceive movement but can perceive colour normally. The strong evidence for the processing of information by groups of neurons acting autonomously suggests that they might have consciousness without having to interact with other processing areas.

Professor Zeki cites victims of carbon monoxide poisoning. They lose their ability to see form, motion and depth but sometimes retain their ability to see colour. "They are 'blind' in many ways but can still see colours, are conscious of them and can name them."

Professor Zeki said the research suggests that perception and conscious experience of a visual event depends on the functioning of the entire brain or all the visual processing areas of the brain. "There may be many consciousnesses. Visual consciousness may itself be modular, reflecting the modularity of the processing system," he says.

On the question of which consciousness event dominates, Professor Zeki notes that when V4 is active, V5 is often suppressed. And although different processing areas can act independently, they do not necessarily do so, he said.

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