Physiological and anatomical identification of the nucleus of the optic tract and dorsal terminal nucleus of the accessory optic tract in monkeys.

Physiological and anatomical criteria were used to clearly establish the existence of a pretectal relay of visual information to the ipsilateral inferior olive in the macaque monkey. After injection of horseradish peroxidase into the inferior olivary nucleus, retrogradely labelled neurons were found in the nucleus of the optic tract (NOT) and the dorsal terminal nucleus of the accessory optic tract (DTN). The labelled cells were distributed in a sparse band arching below the margin of the brachium of the superior colliculus between the dorsal and lateral borders of the brainstem at the caudal edge of the pulvinar. Various types of cells could be distinguished. More superficially the cells were extremely spindle shaped, cells deeper within the midbrain had more compact somata. NOT-DTN neurons in the same region were also found to respond with short latencies to electrical stimulation of both the inferior olive and the optic chiasm. All neurons in the NOT-DTN which were antidromically activated from the inferior olive were also found to have direction specific binocular visual responses. Such neurons were excited by ipsiversive motion and suppressed by contraversive motion, regardless of whether large area random dot stimuli moved across the visual field or small single dots moved across the fovea. Direct retinal input to these neurons was via slowly conducting fibers (3-9 m/s) from the monkey's optic tract conduction velocity spectrum. As shown previously for non-primates, NOT-DTN cells may also in the monkey carry a signal representing the velocity error between stimulus and retina (retinal slip), and relay this signal into the circuitry mediating the optokinetic reflex.