Mechanisms of Sensitivity Loss due to Visual Cortex Lesions in Humans and Macaques

This study represents the first use of noise masking and signal detection theory to examine mechanisms of visual loss after lesions of visual cortex. Noise-masked contrast thresholds were increased in 2 macaques and 2 humans at lesion-affected, compared with control, regions of their visual fields. Experiments suggested by the organization of visual cortex examined possible mechanisms of the visual loss. Two experiments tested the hypothesis that damage to feedback connections might eliminate the benefit of comparing test stimuli with remembered representations but neither could account for the sensitivity loss. The third experiment found that extrastriate lesions did increase the trial-to-trial variability of sensory decisions, suggesting this as one mechanism of sensitivity loss. In addition to clarifying mechanisms of lesion-induced contrast sensitivity loss, this study also showed that elevated contrast thresholds, that are subtle in the absence of external noise, became dramatic when measured with masking noise.