Horizontal optokinetic ocular nystagmus in the pigmented rat

Horizontal optokinetic nystagmus was elicited in rats by rotation of a pattern of bright dots projected onto a cylinder surrounding the animal. Eye position was measured with the electromagnetic search coil technique. Optokinetic stimuli consisted either of velocity steps of pattern rotation or sinusoidal oscillations. Closed-loop gain (slow phase eye velocity/pattern velocity) of steady-stage step responses in binocular vision ranged between 0.8 and 1.0 for pattern velocities up to 20-40 degrees/s and decreased thereafter. Open-loop gain (steady-state slow phase velocity/retinal slip velocity) was dependent on retinal slip velocity and decreased linearly in double logarithmic plot from about 30 (at 0.5 degree/s) to about 9 (at 5 degrees/s). For retinal slip velocities larger than 5 degrees/s open-loop gain decayed faster and reached about 1 at 30 degrees/s. Step response profiles showed a gradual increase in slow phase eye velocity reaching steady-state after a time period roughly proportional to stimulus velocity. Initial slow phase velocity measured within 500 ms after stimulus onset reached between 2 and 4 degrees/s and was largely independent of stimulus amplitudes above 10 degrees/s. Occasionally rats showed fast rises in slow phase eye velocity at the onset of the step response profiles. Primary and secondary optokinetic afternystagmus were present. Duration of primary afternystagmus was largely independent of stimulus amplitude and lasted 8.0 +/- 4 s. Closed-loop gain of steady-state step responses in monocular vision was, for temporonasal stimuli, similar to that measured in binocular condition while for nasotemporal stimulation gain was much smaller even at low stimulus velocities. Sinusoidal modulation of slow phase velocity was linearly dependent on stimulus velocity; the linear range decreased as frequency of stimulation increased. Slow phase velocity gain was relatively constant (ca 0.8) between 0.05 and 0.3 Hz and showed only a small tendency to decrease at larger stimulus frequencies. Phase-lag increased strongly with stimulus frequency and could be fitted by assuming a response time delay of 100 ms. The results show that the rat's optokinetic system is qualitatively similar to that found in another lateral-eyed species, namely the rabbit. At a quantitative level, however, both fast and slow optokinetic response dynamics appear to be better developed in the rat than in the rabbit.(ABSTRACT TRUNCATED AT 400 WORDS)