SPATIAL AND TEMPORAL RESPONSE PROPERTIES OF RESIDUAL VISION IN CASE OF HEMIANOPIA

Residual vision in subjects with damage of the primary visual cortex ( striate cortex) has been demonstrated in many previous studies and is taken to reflect the properties of known subcortical and extrastriate visual pathways. In this report we describe psychophysical experiments carried out on a subject clinically blind in half of his visual field (i.e. homonymous hemianopia) caused by striate cortex damage. They re veal the existence of two distinct channels mediating such vision. One channel responds to spatial structure and the other to light flux cha nges. The spatially tuned channel has a peak response at about 1.2 cyc les per degree and shows rapid loss of sensitivity at both high and lo w spatial frequencies. This channel does not respond to diffuse illumi nation. The light flux channel, however, responds only to sudden incre ments in light flux levels on the retina and shows extensive spatial s ummation. Both channels require transient inputs, with a peak sensitiv ity at about 10 cycles per second and show virtually complete attenuat ion at temporal frequencies below 2 cycles per second. The spatiotempo ral characteristics of these two channels account for much of the repo rted limits of visual performance attributed to subcortical or extrast riate pathways in some patients, and especially for their relatively g ood sensitivity for the detection of abrupt, transient stimuli or fast -moving targets. A new method is also applied to the measurement of th e amount of light scatter in the eye. The measurements show that light scatter into the sighted hemifield could not account for the results obtained with the stimuli used to characterize the residual vision of this subject.