The horizontal optokinetic nystagmus in the cat

Horizontal optokinetic eye nystagmus (OKN) and after nystagmus (OKAN) were recorded in the alert cat (head restrained) in response to velocity steps and sinusoidal optokinetic stimuli. A strong dependency of OKN performance on stimulus pattern was found: responses were most regular and gain was high over a large range of stimulus velocities when the stimulus consisted of a high-contrast random dot pattern. Following velocity steps, OKN showed a small amplitude fast rise in slow phase velocity (SPV) which was followed by a slow build-up to steady state. The amplitude of the initial jump in SPV increased with stimulus amplitude up to 30 degrees/s and saturated afterwards. The plateau level of initial SPV ranged from 5 to 15 degrees/s. The slow build-up of SPV showed non-linearities, i.e. the time to steady state increased with stimulus amplitude and the slow rise of SPV was irregular. In most animals steady state SPV showed no signs of response saturation for step amplitudes up to 60-80 degrees/s or more. The open-loop gain (steady state SPV/retinal slip velocity) depended on retinal slip velocity and decreased from 46 at 0.5 degrees/s to 0.4 at about 60 degrees/s. OKAN I and II were consistently observed and occasionally OKAN III was noted. OKAN I durations (mean 13.8 +/- 5.1 s) and OKAN II amplitudes were independent of stimulus magnitude. Initial SPV of OKAN I was typically the same as that of OKN, i.e. no fast fall was observed. Cessation of pattern rotation in light, however, produced a fast initial decay of SPV. A least square fitting of OKAN time course was performed with various time functions. The SPV of OKAN I and II was best fitted with a damped sine wave, indicating that cat optokinetic system behaves like a second order underdamped system. Sinusoidal stimuli produced strong response non-linearities. At a given frequency gain decreased with increasing stimulus amplitudes. Gain correlated best with stimulus acceleration. In addition, strong stimuli produced characteristic response distortions. In the visual-vestibular conflict situation vectorial summation of VOR and OKN was observed only with small stimuli.