Insight to sight lines

May 1, 1998

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

A patient who is unable to recognise the shape of objects or their spatial orientation has given insights into the functioning of the brain's two known pathways for processing visual information, writes Kam Patel.

In Tucson David Milner, of the University of St Andrews, said that despite suffering from this very rare condition, visual form agnosia, the patient can reach out, grasp objects, examine them physically and then describe its shape and orientation in detail.

Dr Milner contrasts this condition with optic ataxia. It, too, is a rare condition that virtually mirrors agnosia, Dr Milner said. Those suffering from optic ataxia can recognise an object at a distance and describe its shape and orientation; however, they have difficulty guiding themselves to the object, and often miss it altogether. Once touching the object though, the patient is comfortable with its shape and orientation.

The brain's known visual pathways are the ventral pathway, which deals with conscious perception of the real world, and the dorsal pathway, associated with visual guidance and movement.

Dr Milner said: "What we think is happening with ataxia and agnosia is that one visual pathway has been damaged while the other one is intact." Research suggests that with optic ataxia, the dorsal pathway is damaged; while with agnosia the ventral pathway is malfunctioning.

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