SHORT WAVELENGTH CONE ACTIVITY TO AND THROUGH THE SUPERIOR COLLICULUS

A key structure for directing saccadic eye movements is the superior colliculus (SC). The SC is thought to be unresponsive to stimuli that activate only short wavelength sensitive cones (S-cones) in the retina. The apparent lack of S-cone input to the SC was recognized as an opportunity to test SC function. The assumption that S-cone stimuli are invisible to the SC has been used in numerous human clinical and psychophysical studies. The idea is that visually guided behavior dependent on the SC should be impaired when S-cone stimuli are used. Behavioral impairment to S-cone stimuli is used to infer the role of the SC in a given behavior. The goal here was to directly test this assumption by recording from single neurons in macaque SC. This hypothesis was tested in three steps. First, the perceptual reports of monkeys were used to psychophysically calibrate stimuli that activate only S-cones. Second, individual SC neurons were tested for visual sensitivity to S-cone stimuli. Finally, express saccades, a behavior known to depend on the SC, were examined physiologically and behaviorally. Both neural activity and SC-dependent behavior are present to S-cone stimuli and depend on the amount of S-cone contrast. These results demonstrate that the SC receives S-cone input, and outputs S-cone dependent behavioral commands. Together the data indicate that S-cone stimuli cannot be used to diagnose SC involvement in behavior