The variable gain element of the vestibulo-ocular reflex is common to the optokinetic system of the cat.

The gain (slow-phase eye velocity/head velocity) of the vestibulo-ocular reflex (VOR) of 6 alert cats was sequentially adapted to values between 0.2 and 1.66 by the chronic wearing of visual reversing or 2 X magnifying spectacles, combined with forced rotation in the light. Gain was measured during sinusoidal oscillation in darkness at 0.05 Hz at a peak velocity of about 30 degrees/s. In each state of VOR gain adaptation, optokinetic nystagmus (OKN) and optokinetic afternystagmus (OKAN) were measured in a full-field optokinetic drum at velocities of 20-80 degrees/s. Steady-state, slow-phase, optokinetic eye velocity nearly equaled low drum velocities, but saturated at higher velocities and declined when drum velocity further increased. The saturation velocity varied in relation to VOR gain, ranging from 10-20 degrees/s at a VOR gain of 0.2-0.4, to 65 degrees/s at a VOR gain of 1.66. The means that the variable gain element of the VOR is shared by the optokinetic system (OKS). OKAN, measured in darkness, had a roughly exponential decay. The time constant of OKAN (Tokan) also varied with VOR gain, ranging form about 2 s at a VOR gain of 0.2, to 10 s at a VOR gain of 1.66. This is a novel finding which suggests that the velocity-storage mechanism was also affected by gain changes. A model is proposed in which a neural, variable-gain element is located in a positive-feedback, velocity-storage loop common to both the VOR and the OKS. Computer simulation showed that this hypothesis could account for most of the observed changes in OKN saturation and Tokan with changes in VOR gain. The model also predicts that low frequency VOR phase lead in darkness should increase with decreasing VOR gain. Experimental VOR phase lead at 0.05 Hz varied from about 10 degrees for VOR gains above 1.1 to about 50 degrees for VOR gains below 0.3. Such phase-lead data agree with the trend predicted by the model.