Human Visual Responses in the Absence of the Geniculo-Calcarine Projection

This paper describes an investigation of responses elicited by flashed and moving light stimuli presented to the 'blind'half-field of a hemianopic subject. The detection mechanism is significantly less sensitive than that for vision in the normal half-field, but the increment threshold varies with the background illumination level in a manner similar to that for normal vision. The responses of the 'blind' hemifield are characterized by low critical fusion frequency (less than or equal to 8 Hz), large spatial summation and high sensitivity to fast moving targets. Velocity discrimination for targets located in the 'blind' hemifield is normal, but neither the shape nor the size of the target is discriminated. It is shown that the experimental data cannot be attributed to light scattered from the 'blind' into the normal hemifield, and other mechanisms which may be responsible for the observed responses are examined.