Subcortical projections of the cerebral nystagmogenic area

Cortical modulation of the nucleus of the optic tract in the rabbit

We analyzed in rabbits the relationships between the temporooccipital nystagmogenic cortex (NGC)--the region sited at the border between cortical areas 17, 21, and 22--and the nucleus of the optic tract (NOT). Two experimental approaches were used: (a) eye movement analysis before and after electrolytic lesion of the NOT region provided an indication of the importance of the NOT for the interaction between the ocular nystagmus elicited by natural optokinetic stimulation (OKN) and the nystagmus evoked by electrical stimulation of the nystagmogenic area; (b) NOT direction-selective and velocity-sensitive units were tested with single shock or repetitive electrical stimulation of the nystagmogenic region. Single-shock stimulation evoked single or multiple spikes in 50% of NOT units analyzed and repetitive stimuli induced prolonged facilitation and inhibitory rebounds in 70% of the units tested. Comparison of orthodromic activation latencies of the NOT cells (3.2 and 6.1 ms) after cortical stimulation and of antidromic activation latencies of cortical nystagmogenic units (2.6 ms) after NOT shocks, suggested monosynaptic as well as polysynaptic connections between the temporooccipital cortex and the NOT. The existence of such cortical-NOT linkage indicates that the NOT is intercalated between the cortex and the oculomotor centers and represents the most probable site of interaction of the cortical nystagmus pathway with the optokinetic reflex arc.