Seminar Series

Properties of residual vision in the cortically blind.

Some cortically blind patients are able to discriminate visual events presented within their field defect. This residual visual capacity may or may not accompany any conscious awareness of the event, termed blindsight type II and type I respectively. Spatial vision in blindsight has been examined in ten cortically blind patients using spatial gratings. Evidence for a narrowly tuned psychophysical spatial channel optimally responding to frequencies below 4 cycles/° will be discussed. After each trial, patients also reported whether they had any awareness of the visual events in order to determine whether mechanisms mediating type I and II blindsight have overlapping or separate frequency responses. Responses averaged across eight patients show that type I blindsight has a narrower frequency response tuning than that for type II. Only two patients did not show any evidence of blindsight. The performance of patients will be discussed in relation to the extent of their brain lesions.