Gaze direction as equilibrium: more evidence from spatial and temporal aspects of small-saccade triggering in the rhesus macaque monkey

Rigorous behavioral studies made in human subjects have shown that small-eccentricity target displacements are associated with increased saccadic reaction times, but the reasons for this remain unclear. Before characterizing the neurophysiological foundations underlying this relationship between the spatial and temporal aspects of saccades, we tested the triggering of small saccades in the male rhesus macaque monkey. We also compared our results to those obtained in human subjects, both from the existing literature and through our own additional measurements. Using a variety of behavioral tasks exercising visual and nonvisual guidance of small saccades, we found that small saccades consistently require more time than larger saccades to be triggered in the nonhuman primate, even in the absence of any visual guidance and when valid advance information about the saccade landing position is available. We also found a strong asymmetry in the reaction times of small upper versus lower visual field visually guided saccades, a phenomenon that has not been described before for small saccades, even in humans. Following the suggestion that an eye movement is not initiated as long as the visuo-oculomotor system is within a state of balance, in which opposing commands counterbalance each other, we propose that the longer reaction times are a signature of enhanced times needed to create the symmetry-breaking condition that puts downstream premotor neurons into a push-pull regime necessary for rotating the eyeballs. Our results provide an important catalog of nonhuman primate oculomotor capabilities on the miniature scale, allowing concrete predictions on underlying neurophysiological mechanisms. NEW & NOTEWORTHY Leveraging a multitude of neurophysiological investigations in the rhesus macaque monkey, we generated and tested hypotheses about small-saccade latencies in this animal model. We found that small saccades always take longer, on average, than larger saccades to trigger, regardless of visual and cognitive context. Moreover, small downward saccades have the longest latencies overall. Our results provide an important documentation of oculomotor capabilities of an indispensable animal model for neuroscientific research in vision, cognition, and action.

Gaze direction as equilibrium: more evidence from spatial and temporal aspects of small-saccade triggering in the rhesus macaque monkey

Rigorous behavioral studies made in human subjects have shown that small-eccentricity target displacements are associated with increased saccadic reaction times, but the reasons for this remain unclear. Before characterizing the neurophysiological foundations underlying this relationship between the spatial and temporal aspects of saccades, we tested the triggering of small saccades in the male rhesus macaque monkey. We also compared our results to those obtained in human subjects, both from the existing literature and through our own additional measurements. Using a variety of behavioral tasks exercising visual and non-visual guidance of small saccades, we found that small saccades consistently require more time than larger saccades to be triggered in the non-human primate, even in the absence of any visual guidance and when valid advance information about the saccade landing position is available. We also found a strong asymmetry in the reaction times of small upward versus downward visually-guided saccades, similar to larger saccades, a phenomenon that has not been described before for small saccades, even in humans. Following the suggestion that an eye movement is not initiated as long as the visuo-oculomotor system is within a state of balance, in which opposing commands counterbalance each other, we propose that the longer reaction times are a signature of enhanced times needed to create the symmetry-breaking condition that puts downstream premotor neurons into a push-pull regime necessary for rotating the eyeballs. Our results provide an important catalog of non-human primate oculomotor capabilities on the miniature scale, allowing concrete predictions on underlying neurophysiological mechanisms.

Saccadic Dyskinesia, Other Involuntary Eye Movements, and Gaze Deviations

■ A small saccade of 0.5–3° that takes the eye away from fixation, followed by a saccade that returns the eye back to fixation after about 200 msec (i.e., presence of intersaccadic interval during which visual feedback occurs) ■ So named because of its appearance in eye movement tracings ■ Normal subjects often have square wave jerks (SWJ), but the rate is only 4–6 per minute. ■ Pathologic SWJ occurs at a rate of >15 per minute. ■ Cerebellar diseases Square wave jerks result from damage of projections from the frontal eye field, rostral pole of the superior colliculus, and the central mesencephalic reticular formation to the omnipause cells in the pons. If symptomatic, SWJ may be treated with methylphenidate, diazepam, phenobarbital, or amphetamines. ■ Burst of saccades with defective steps of innervation (i.e., stepless saccades) ■ Conjugate or monocular Saccadic pulses are associated with multiple sclerosis. Saccadic pulses result from damage of omnipause cells or the neural integrator.