Properties of horizontal saccades accompanied by blinks.

Using the magnetic search coil technique to record eye and lid movements, we investigated the effect of voluntary blinks on horizontal saccades in five normal human subjects. The main goal of the study was to determine whether changes in the dynamics of saccades with blinks could be accounted for by a superposition of the eye movements induced by blinks as subjects fixated a stationary target and saccadic movements made without a blink. First, subjects made voluntary blinks as they fixed on stationary targets located straight ahead or 20 degrees to the right or left. They then made saccades between two continuously visible targets 20 or 40 degrees apart, while either attempting not to blink, or voluntarily blinking, with each saccade. During fixation of a target located straight ahead, blinks induced brief downward and nasalward deflections of eye position. When subjects looked at targets located at right or left 20 degrees, similar initial movements were made by four of the subjects, but the amplitude of the adducted eye was reduced by 65% and was followed by a larger temporalward movement. Blinks caused substantial changes in the dynamic properties of saccades. For 20 degrees saccades made with blinks, peak velocity and peak acceleration were decreased by approximately 20% in all subjects compared with saccades made without blinks. Blinks caused the duration of 20 degrees saccades to increase, on average, by 36%. On the other hand, blinks had only small effects on the gain of saccades. Blinks had little influence on the relative velocities of centrifugal versus centripetal saccades, and abducting versus adducting saccades. Three of five subjects showed a significantly increased incidence of dynamic overshoot in saccades accompanied by blinks, especially for 20 degrees movements. Taken with other evidence, this finding suggests that saccadic omnipause neurons are inhibited by blinks, which have longer duration than the saccades that company them. In conclusion, the changes in dynamic properties of saccades brought about by blinks cannot be accounted for simply by a summation of gaze perturbations produced by blinks during fixation and saccadic eye movements made without blinks. Our findings, especially the appearance of dynamic overshoots, suggest that blinks affect the central programming of saccades. These effects of blinks need to be taken into account during studies of the dynamic properties of saccades.