Control of the optokinetic reflex by the nucleus of the optic tract in primates.

Physiological and anatomical experiments clearly established the existence of a pretectal relay of visual information to the ipsilateral inferior olive in the macaque monkey. Horseradish peroxidase injected into the inferior olivary nucleus retrogradely labelled neurons in the nucleus of the optic tract (NOT) and the dorsal terminal nucleus of the accessory optic tract (DTN). The response characteristics of NOT-DTN neurones are described in this chapter. The visual receptive fields of neurones in NOT and DTN in anaesthetized and paralysed macaque monkeys prefer horizontal ipsiversive movements of single objects or whole field random dot patterns, i.e. neurones in the left NOT-DTN prefer leftward movements and vice versa. The directional tuning widths of NOT-DTN neurones are very broad. Directions withing a mean range of 127 +/- 25 degrees visual angle elicit response strengths greater than 50% of the maximal response. Visual latencies to reversals in directions of stimulus movement are in a range from 40 to 80 ms (mean 61 +/- 13 ms). Combining two visual stimuli by moving a random dot pattern and a single bar of light simultaneously but in opposite directions causes NOT-DTN neurones to respond to each stimulus as soon as it moves in the cell's preferred direction. The reduced overall response strengths indicate additional inhibitory interactions. All NOT-DTN neurones can be activated from each eye. Interactions between the two eyes are modest and unspecific. Optical speeds of stimulus movement vary for different NOT-DTN neurones (4-60 deg/s). The effective range of speeds is broad (0.1-400 deg/s for the total population). With oscillating horizontal stimulation NOT-DTN neurones follow repetition rates up to 4 Hz. Receptive fields are mostly large (20-40 degrees visual angle), include the fovea, and extend up to 20 degrees into the ipsilateral hemifield. The sensitivity to moving stimuli is highest near the fovea. Our results thus indicate that direction selective cells in the NOT and DTN have all the properties and connections which are necessary and sufficient to control the stability of the image on the retina by supplying retinal slip information to the velocity storage integrator in the brainstem (Raphan et al., 1979).