Abstract
Purpose. In patients with nystagmus the functional assessment with visual evoked potentials (VEPs) is severely impeded. Specifically, pattern-reversal responses are strongly reduced or absent in nystagmus patients, while pattern-onset responses can often still be obtained. To quantitatively assess the differential effect of retinal image slip on pattern-reversal and pattern-onset responses we simulated different degrees of nystagmus in normal subjects. Methods. In eight subjects we monitored eye-movements and recorded pattern-reversal and pattern-onset VEPs from occipital electrodes. Subjects viewed the stimulus via a mirror, which was placed close to the eye and driven by a scanner at four different amplitudes of a 4-Hz sawtooth waveform at 0, 1, 2, and 3 of amplitude. Results. Retinal image motion nearly abolished the pattern-reversal VEPs (by up to 85%; p<0.001), while there was no significant effect on pattern-onset VEPs. No oculomotor activity was induced by nystagmus-like retinal image slip at 4 Hz, in contrast to a control condition with slower motion (1 Hz) which induced marked eye movements. Conclusion. The strong differential effect of simulated nystagmus on pattern-reversal and pattern-onset VEPs indicates that the spatio-temporal properties of the pattern-reversal stimulus are sufficient to explain its low efficacy in patients with nystagmus. For clinical applications, it follows that pattern-onset is the preferred stimulation-mode in patients with oculomotor instabilities. Of the various possible explanations we quantitatively argue for a mechanism based on motion adaptation.
DFG HO-2002/3-1