Brain, Vol 121, Issue 1 25-45, Copyright © 1998 by Oxford University Press
S Zeki and DH Ffytche
We have studied a patient, G.Y., who was rendered hemianopic following a
lesion affecting the primary visual cortex (area VI), sustained 31 years
ago, with the hope of characterizing his ability to discriminate visual
stimuli presented in his blind field, both psychophysically and in terms of
the brain activity revealed by imaging methods. Our results show that (i)
there is a correlation between G.Y.'s capacity to discriminate stimuli
presented in his blind field and his conscious awareness of the same
stimuli and (ii) that G.Y.'s performance on some tasks is characterized by
a marked variability, both in terms of his awareness for a given level of
discrimination and in his discrimination for a given level of awareness.
The observations on G.Y., and a comparison of his capacities with those of
normal subjects, leads us to propose a simple model of the relationship
between visual discrimination and awareness. This supposes that the two
independent capacities are very tightly coupled in normal subjects
(gnosopsia) and that the effect of a VI lesion is to uncouple them, but
only slightly. This uncoupling leads to two symmetrical departures, on the
one hand to gnosanopsia (awareness without discrimination) and on the other
to agnosopsia (discrimination without awareness). Our functional MRI
studies show that V5 is always active when moving stimuli, whether slow or
fast, are presented to his blind field and that the activity in V5
co-varies with less intense activity in other cortical areas. The
difference in cerebral activity between gnosopsia and agnosopsia is that,
in the latter, the activity in V5 is less intense and lower statistical
thresholds are required to demonstrate it. Direct comparison of the brain
activity during individual 'aware' and 'unaware' trials, corrected for the
confounding effects of motion, has also allowed us, for the first time, to
titrate conscious awareness against brain activity and show that there is a
straightforward relationship between awareness and activity, both in
individual cortical areas, in this case area V5, and in the reticular
activating system. The imaging evidence, together with the variability in
his levels of awareness and discrimination, manifested in his capacity to
discriminate consciously on some occasions and unconsciously on others,
leads us to conclude that agnosopsia, gnosopsia and gnosanopsia are all
manifestations of a single condition which we call the Riddoch syndrome, in
deference to the British neurologist who, in 1917, first characterized the
major aspect of this disability. We discuss the significance of these
results in relation to historical views about the organization of the
visual brain.
REVIEWS
The Riddoch syndrome: insights into the neurobiology of conscious vision
Wellcome Department of Cognitive Neurology, University College, London, UK.
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