LUMINANCE NEURONS IN THE PRETECTAL OLIVARY NUCLEUS MEDIATE THE PUPILLARY LIGHT REFLEX IN THE RHESUS-MONKEY

In humans and other primates, an increase in luminance in either eye e licits bilateral pupilloconstriction that is essentially equal in both eyes. Current models of the neural substrate for this clinically impo rtant light reflex propose that a retinorecipient pretectal nucleus pr ojects bilaterally to the Edinger-Westphal nucleus (EW), which contain s the parasympathetic, preganglionic neurons controlling pupilloconstr iction. Based on single-unit recording studies in anesthetized cats an d rats, it has been further suggested that luminance neurons in only o ne pretectal nucleus, the pretectal olivary nucleus, mediate this refl ex. However, to our knowledge, there have been no comparable electroph ysiological studies in primates of the pupillary light reflex or the p retectal luminance neurons that mediate this reflex. To address this i ssue, single-unit recording and electrical microstimulation studies we re carried out in the pretectum of alert, trained, rhesus monkeys. The se studies demonstrated that the primate pretectum contains luminance neurons with the characteristics appropriate for mediating the pupilla ry light reflex and that these neurons are located in one retinorecipi ent pretectal nucleus, the prerectal olivary nucleus. Electrical micro stimulation at the site of these neurons often elicited pupilloconstri ction. Our results provide clear evidence for the involvement of the p retectum, and more specifically the pretectal olivary nucleus, in medi ating the pupillary light reflex in primates.