Differential effects of blinks on horizontal saccade and smooth pursuit initiation in humans.

Blinks executed during eye movements affect kinetic eye movement parameters, e.g., peak velocity of saccades is decreased, their duration is increased, but their amplitude is not altered. This effect is mainly explained by the decreased activity of premotor neurons in the brainstem: omni-pause neurons (OPN) in the nucleus raphe interpositus. Previous studies examined the immediate effect of blinks directly on eye movements but not their effect when they are elicited several hundred milliseconds before the eye movements. In order to address this question we tested blinks elicited before the target onset of saccades and pursuit and compared the results to the gap effect: if a fixation light is extinguished for several hundred milliseconds, the reaction time (latency) for subsequent saccades or smooth pursuit eye movements is decreased. Monocular eye and lid movements were recorded in nine healthy subjects using the scleral search-coil system. Laser stimuli were front-projected onto a tangent screen in front of the subjects. Horizontal step-ramp smooth pursuit of 20 deg/s was elicited in one session, or 5 deg horizontal visually guided saccades in another experimental session. In one-third of the trials (smooth pursuit or saccades) the fixation light was extinguished for 200 ms before stimulus onset (gap condition), and in another third of the trials reflexive blinks were elicited by a short airpuff before the stimulus onset (blink condition). The last third of the trials served as controls (control condition). Stimulus direction and the three conditions were randomized for saccades and smooth pursuit separately. The latency of the step-ramp smooth pursuit in the blink condition was found to be decreased by 10 ms, which was less than in the gap condition (38 ms). However, the initial acceleration and steady-state velocity of smooth pursuit did not differ in the three conditions. In contrast, the latency of the saccades in the gap condition was decreased by 39 ms, but not in the blink condition. Saccade amplitude, peak velocity, and duration were not different in the three conditions. There was also no difference in blink amplitude and duration of pupil occlusion in the blink condition, neither in saccades nor in smooth pursuit. The latency reduction of smooth pursuit, but not of saccades, may neither be explained by the brief pupil occlusion nor by visual suppression, warning signals, or the startle response. Whether the effects are caused by the influence of blinks on OPNs or other premotor structures remains to be tested.