ABSTRACT
Psychophysical studies on two subjects, each with a different lesion of the central visual pathways, are reviewed. One subject, G, suffers right homonymous hemianopia following accidental damage to the left optic radiation and/or visual cortex and the other, M, has a defect of a kind not previously described. The hemianope can detect and locate flashed or moving targets presented in his “blind” hemifield. but under the experimental conditions employed he exhibits no pattern or shape discrimination in response to such stimuli. G's threshold detection 30° off-axis in the “blind” hemifield is characterized by low sensitivity and large spatial summation and these features of his visual responses are examined in relation to a modified Fechner threshold function. The central feature of M's visual responses is a spreading inhibition elicited by chromatic, particularly red, stimuli, as a result of which his detection of high contrast targets located up to 12° from the inhibitory stimulus is suppressed. With the significant exception of fusion of two-color random dot stereograms, all of M's visual functions measured with colored lights are more or less abnormal, whereas those measured with white light are normal. Detailed incremental thresholds for the two subjects are presented and the factors which control their threshold functions are examined. The functional organization of the central visual pathways implied by the response characteristics of these two subjects is discussed.
