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.

Safety study of the Bio-10-SAD Bern strain of the rabies virus on the rhesus macaque monkey species

Based on a WHO recommendation, residual pathogenicity of the Bio-10-SAD Bern rabies virus strain (component of the Lysvulpen por. ad us. vet. vaccine) was tested on rhesus macaque monkeys. Each of the ten monkeys, females, two years old, was administered orally 2 ml × 109 TCID50 of the Bio-10-SAD Bern rabies strain. The animals were monitored for 90 days. Subsequently, the animals were sacrificed and their brains were examined for presence of the vaccination rabies virus by the immunofluorescence and PCR methods. The occurrence of anti-rabies antibodies prior to and following administration of the vaccination rabies virus was also evaluated. No clinical signs of rabies were observed nor did any of the animals die of rabies following application of the virus. No rabies was detected in the study animals by post mortem examination. All of the 10 animals developed anti-rabies antibodies during the 90 days following administration of the rabies virus. It can be concluded, that Bio-10-SAD Bern virus administered at a dose equal to the tenfold maximum dose specified for field uses is safe to monkeys of the rhesus macaque species. This study is the first of its type performed in rhesus macaque monkey species.