ASGSB 1999 Annual Meeting Abstracts

EFFICACY OF FIXATION AND SACCADES IN HUMANS IS INFLUENCED BY DIRECTION OF GRAVITY. S.A. Rude, Univ of Washington, Dept of Physiology and Biophysics, Seattle WA.

Primate voluntary saccade accuracy is known to be slightly hypometric for initial horizontal saccades made to a visual target (Becker & Fuchs, 1969). The primary saccade is usually followed by one corrective saccade but may involve more or fewer saccades dependent on thedifficulty of the task, and mental state and skill of the subject. Correction saccades may be influenced "on the fly" by short latency visual feedback, as well as be preprogrammed along with the primary saccade, as during saccades to remembered or unexpectedly blanked targets. In the vertical direction, upward primary saccades are asymmetrically larger than downward saccades, and in the case of remembered visual targets, an upward bias predominates over all directions of eye movement, resulting in hypermetric upward saccades.

It is well known that many species of primates exhibit an upward drift in the dark. It is also reported in cat (Rude & Baker, 1996) that this drift increases with increasing angles of tilt. Horizontal eye movements in frontal-eyed animals generally do not exhibit this kind of drift.

To determine whether these effects might be attributable to a static otolith influence on the oculomotor system, eye movements in three subjects in six experiments were assessed using search coil techniques. Subjects were inverted and both horizontal and vertical eye movements were monitored. Fixation and saccade metrics were quantified, both with and without visual feedback, in a light-tight and sound attenuated chamber. Responses were compared to upright conditions.

In the upright condition, no subject displayed a systematic drift in the dark. When inverted, two subjects exhibited a persistent vertical drift which was downward in the orbit (upward in space) and a third subject exhibited an upward drift (downward in space), ranging from 2-8/s. All subjects displayed decreased accuracy in visually guided saccades when inverted, with increased occurrences of correction saccades compared to the upright condition. Supported by EY621030.

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