SINGLE-UNIT RESPONSES TO WHOLE-FIELD VISUAL-STIMULATION IN THE PRETECTUM OF RANA-PIPIENS

Visually responsive single units were recorded from the pretectal regi on that includes the large-celled nucleus lentiformis mesencephali (nL M) in the leopard frog, Rana pipiens. During monocular stimulation of the contralateral eye, 60 single units responding to movement of a lar ge-field, random-dot pattern were quantitatively analyzed using horizo ntal and vertical directions at each of four pattern velocities (0.4-4 0 degrees/s). All units were spontaneously active, motion sensitive, a nd the majority showed 'on'-'off' responses. Several different respons e profiles were observed, including velocity-sensitive units with peak response at 10 degrees/s, most of which showed directional selectivit y, and speed-sensitive units that showed increasing spike frequencies as pattern velocity increased, but little or no directional selectivit y. About one-third of all units analyzed were direction-selective, and 55% of those responded optimally to the temporal-to-nasal (T-N) direc tion of motion. T-N units were recorded primarily from an area that li es dorsolaterally between nLM and the optic tectum, in the 'peri-nLM' region. The pronounced monocular optokinetic nystagmus (OKN) response asymmetry that occurs in anurans appears to be reflected in the respon se profiles of the T-N direction selective units.