Effects of microsaccades on contrast detection and V1 responses in macaques.

Microsaccades can elevate contrast detection thresholds of human observers and modulate the activity of neurons in monkey visual cortex. Whether microsaccades elevate contrast detection thresholds in monkey observers is not known and bears on the interpretation of neurophysiological experiments. To answer this question, we trained two monkeys to perform a 2AFC contrast detection task. Performance was worse on trials in which a microsaccade occurred during the stimulus presentation. The magnitude of the effect was modest (threshold changes of <0.2 log unit) and color specific: achromatic sensitivity was impaired, but red-green sensitivity was not. To explore the neural basis of this effect, we recorded the responses of individual V1 neurons to a white noise stimulus. Microsaccades produced a suppression of spiking activity followed by an excitatory rebound that was similar for L - M cone-opponent and L + M nonopponent V1 neurons. We conclude that microsaccades in the monkey increase luminance contrast detection thresholds and modulate the spiking activity of V1 neurons, but the luminance specificity of the behavioral suppression is likely implemented downstream of V1.