This is a study, in alert behaving rhesus macaques, of the relationship between the small involuntary eye movements that occur during voluntary eye fixation ("microsaccades"), and the firing of cells in the visual cortex. A cell is recorded from visual cortex in a macaque trained to fix its gaze on a small spot on a video- monitor. A line contour is positioned and orientated so as to match the cell's receptive field. One then determines the size and probability of occurrence of microsaccades just prior to impulses or to bursts containing various numbers of impulses. Comparisons will be made of the effectiveness of different directions of microsaccades. One will examine the relationship between saccades, stimulus location and exact receptive-field position, and the effects of saccades on responses to turning line stimuli on or off. One will study effects of diffuse-light stimuli on cell firing, especially on the incidence of bursts of impulses, and will look for any effects of microsaccades on this activity. The cells will be recorded from cortical areas VI, V2, V3 and MT, locations being verified using magnetic resonance imaging. At the outset, observations will be confined to one eye, but in the later stages search coils will be implanted in both eyes, so as to permit correlation of microsaccadic movements in the two eyes, and to make it possible to study the effect of microsaccades on cells sensitive to binocular disparity. These studies are considered important in understanding basic mechanisms of visual perception in humans, and to understand better the effect of abnormal eye movements on human vision.