Microstimulation in visual area MT: effects on direction discrimination performance

Physiological and behavioral evidence suggests that the ac-tivity of direction selective neurons in visual cortex underlies the perception of moving visual stimuli. We tested this hy-pothesis by measuring the effects of cortical microstimu-lation on perceptual judgements of motion direction. To ac-complish this, rhesus monkeys were trained to discriminate the direction of motion in a near-threshold, stochastic motion display. For each experiment, we positioned a microelec-trode in the middle of a cluster of neurons that shared a common preferred direction of motion. The psychophysical task was then adjusted so that the visual display was pre-sented directly over the neurons ’ receptive field. The mon-keys were required to discriminate between motion shown either in the direction preferred by the neurons or in the opposite direction. On half the trials of an experiment, we applied electrical microstimulation while monkeys viewed the motion display. We hypothesized that enhancing the neurons ’ discharge rate would introduce a directionally spe-cific signal into the cortex and thereby influence the mon-keys ’ choices on the discrimination task. We compared the monkeys ’ performance on “stimulated ” and “nonstimulat-ed ” trials in 139 experiments; all trials within an experiment were presented in random order. Statistically significant ef-fects of microstimulation were obtained in 89 experiments. In 88 of the 89 experiments with significant effects (97%), the monkeys indicated that motion was in the neurons ’ pre-ferred direction more frequently on stimulated trials than on nonstimulated trials. The data demonstrate a functional link between the activity of direction selective neurons and per-ceptual judgements of motion direction. The primate visual system mediates the perception of complex stimuli that vary in shape, color, depth, and movement. On