RESPONSE VARIABILITY OF NEURONS IN PRIMARY VISUAL-CORTEX (V1) OF ALERT MONKEYS

Response variability of neurons limits the reliability and resolution of sensory systems. It is generally thought that response variability in the visual system increases at cortical levels, but the causes of t he variability have not been identified. We have measured the response variability of neurons in primary visual cortex (VI) of alert monkeys . We recorded from 80 single cells distributed over all V1 layers and from 8 parvocellular cells of the lateral geniculate nucleus. All cell s were stimulated with a bar of near-optimal orientation, color, and d imensions while continuously monitoring the eye movements of fixation. To minimize the effects of eye movements, responses that occurred whi le the eye was relatively steady were selected for analysis. The impul ses elicited by each stimulus presentation were counted, and the varia nce and coefficient of variation were computed. Both measures of respo nse variability were much lower than reported previously for V1 cells of both alert and anesthetized monkeys. Our data show that fixational eye movements cause a large component of response variance in alert mo nkeys. Moreover, the reliability of V1 neurons is not obviously degrad ed compared with lateral geniculate nucleus cells. The high reliabilit y of neurons in alert monkeys is consistent with expectations from con ventional biophysical models, and it suggests that activity in a modes t number of neurons may suffice to form a perceptual decision.