Microsaccadic response to visual events that are invisible to the superior colliculus

Even when people think their eyes are still, tiny fixational eye movements, called microsaccades, occur at a rate of -1 Hz. Whenever a new (and potentially dangerous) event takes place in the visual field, the microsaccadic frequency is at first inhibited and then is followed by a rebound before the frequency returns to baseline. It has been suggested that this inhibition-rebound response is a type of oculomotor reflex mediated by the superior colliculus (SC), a midbrain structure involved in saccade programming. The present study investigated microsaccadic responses to visual events that were invisible to the SC; the authors recorded microsaccadic responses to visual oddballs when the latter were equiluminant with respect to the standard stimuli and when both oddballs and standards were equiluminant with respect to the background. Results showed that microsaccadic responses to oddballs and to standards were virtually identical both when the stimuli were visible to the SC and when they were invisible to it. Although the SC may be the generator of microsaccades, this research suggests that the specific fixational oculomotor activity in response to visual events can be controlled by other brain centers.