The vestibulo-ocular response during transient arousal shifts in man

Horizontal vestibulo-ocular response (VOR) evoked by continuous sinusoidal rotation (0.1 Hz, +/- 90 degrees) was recorded in 20 young and healthy volunteers by DC oculography (EOG). Arousal was assessed by EEG and by reaction time measurement. While alert subject showed the characteristic VOR with vestibular nystagmus, the quick repositioning flicks disappeared during light sleep and changed to largely compensatory smooth eye deviations. This state of reduced arousal was also characterized by EEG attenuation and slow reaction times. Furthermore, we observed brief states with complete extinction of the vestibular response but without significant EEG change. The results demonstrate the high variability of VOR with shifting arousal. The polysynaptic system in the reticular formation which generates the fast phase of the nystagmus beat is far more modifiable than the three-neuronal reflex arc of the slow nystagmus component.