The responses of visual cortical neurons encode differences across saccades

Primate vision consists mostly of periods of stable fixation separated by rapid saccadic eye movements. Each saccade brings a new scene onto the retina, and each new scene results in a burst of activity in the neurons of visual cortex. It might be expected that the activity of these neurons should only represent what is on the retina now, much as a video camera hooked up to a television only displays what the camera is currently pointed at. However, we show here that this is not the case. Recording from 25 primary visual cortical neurons in an awake primate demonstrated that the responses to the saccade-induced presentation of a stimulus within a neuron's receptive field (RF) are typically suppressed by the presence of a stimulus in the RF before the saccade. Flashing stimuli on with the eyes stationary showed, on average, suppressive effects of similar magnitude, suggesting that the mechanism is simple adaptation. However, while the mechanism may be simple, the implications for the operation of the visual system are not. The activity of visual cortical neurons does not represent just the current retinal image, but also the differences between the current retinal image and the previous one. These results suggest that the current approach of studying the visual system, which concentrates on determining the relationship between a single stimulus and a single response, may have to be modified to take into account the timing of retinal image changes that occurs in normal vision.