Visual following during stimulation of an immobile eye (the open loop condition).

We had the rare opportunity to examine a patient with a unilateral complete ophthalmoplegia in an eye with normal visual acuity. We recorded the movements of the covered, sound eye of the patient during full-field optokinetic stimulation of his seeing, immobile eye (open loop condition). The open loop gain for low velocity stimuli (less than 1 deg/sec) was high (greater than 50) but progressively fell with faster stimuli. This finding is inconsistent with the representation of the optokinetic system as a linear velocity servo. In a second experiment, the patient foveally fixated, with the immobile eye, a head-fixed target during sinusoidal, en block rotation. The vestibular movements of the sound eye were suppressed, even though no retinal image motion occurred in the seeing eye. This suggests that visual attention to a stable retinal image can be used to suppress the vestibulo-ocular reflex. Our results were from one patient only and any generalizations must be made cautiously. Nevertheless, our findings are comparable with previous "open loop" studies in normal human beings and patients.