Horizontal optokinetic ocular nystagmus in wildtype (B6CBA+/+) and weaver mutant mice.

Horizontal optokinetic nystagmus (OKN) evoked by a random dot pattern moving at a constant speed around the animal was investigated in wildtype mice and "Weaver" mutants (cerebellar impairment) by means of chronically implanted EOG-electrodes. The shape of OKN in the homozygotic Weaver mouse was clearly different from that in normal mice. The OKN in the mutant showed inconstant velocity during the slow phase. Nystagmus frequency of the mutant was significantly below that of normal controls for velocities of 1.4 to 25 degrees.s-1. In one group of normals the mean slow-phase gain was relatively constant for stimulus angular velocities between 1.4 and 15 degrees.s-1 and declined thereafter. In a second group the mean slow-phase gain decreased gradually between stimulus angular velocities from 1.4 to 15 degrees.s-1 and thereafter with a steeper slope. In mutants gain decreases with increasing stimulus velocity over the entire range tested (1.4 to 42 degrees.s-1). Normals and mutants with one eye occluded exhibited strong OKN when the pattern was moved in a temporonasal direction; little response was obtained by stimuli moving in a naso-temporal direction.