Neural correlates of optokinetic nystagmus in the mesencephalon of Rana pipiens: a functional analysis.

The effects of lesions of the anuran mesencephalic retinal terminal fields on horizontal optokinetic nystagmus (OKN) were examined. Lesion sites which produced effects upon OKN responses were as follows: BOR, nBOR, peri-nBOR, the large-celled pretectal nucleus, and the dorsal tegmental gray and deep tectal layers. Transection of BOR generally resulted in an increase in saccadic frequently at the lower stimulus velocities. Lesions of nBOR produced a decrease in the frequency of both head and eye saccades in the middle to high range of stimulus velocities. The only lesions which totally abolished horizontal OKN were those located medical to nBOR, in the peri-nBOR region. Lesions of the large-celled pretectal nucleus and dorsal tegmental gray substantially reduced both head and eye saccades at all stimulus velocities. Small lesions in the deep tectal layers also depressed OKN frequency. These studies indicate that horizontal OKN may be mediated by a number of structures within the anuran mesencephalon.