ON THE SHORT-TERM ADAPTATION OF EYE SACCADES AND ITS TRANSFER TO HEADMOVEMENTS

During a sequence of eye saccades toward a target that is systematical ly displaced during initiation of the saccade, the oculomotor system a djusts saccadic amplitude and direction in less than 100 trials to dir ectly reach the second target position. The goal of the present work w as to test whether and under which conditions these short-term, adapti ve modifications in eye movements are transferred from horizontal eye saccades to horizontal head-pointing movements. In the first series of experiments subjects had to execute head yaw rotations to an extent d efined by verbal command (assessed movements). These head movements we re not part of visually elicited gaze shifts. They were recorded befor e and after a period of saccadic adaptation. Saccades were adapted to reduced amplitudes by using target displacements from 30 to 20 degrees and from 40 to 30 degrees. After 40-50 trials per target displacement , the amount of eye saccade adaptation was 79% (30-20 degrees) and 97% (40-30 degrees) of the displacement amplitude. In the second series o f experiments, visually triggered head movements to briefly illuminate d targets (100 ms) were measured before and after adaptation. The data obtained from both series did not reveal a functionally significant t ransfer of saccadic adaptation to head movements. The amount of possib le transfer given as a percentage of the amount of achieved adaptation was: assessed head movements, 40 degrees, 1.9%, 20 degrees, -8.6%; vi sually triggered movements, 40 degrees, 5.1%, 20 degrees, 10.0%. No va lues significantly deviated from zero.