On the short-term adaptation of eye saccades and its transfer to head movements.

During a sequence of eye saccades toward a target that is systematically displaced during initiation of the saccade, the oculomotor system adjusts saccadic amplitude and direction in less than 100 trials to directly reach the second target position. The goal of the present work was to test whether and under which conditions these short-term, adaptive 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 defined by verbal command (assessed movements). These head movements were not part of visually elicited gaze shifts. They were recorded before 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 of experiments, visually triggered head movements to briefly illuminated targets (100 ms) were measured before and after adaptation. The data obtained from both series did not reveal a functionally significant transfer of saccadic adaptation to head movements. The amount of possible transfer given as a percentage of the amount of achieved adaptation was: assessed head movements, 40 degrees, 1.9%, 20 degrees, -8.6%; visually triggered movements, 40 degrees, 5.1%, 20 degrees, 10.0%. No values significantly deviated from zero.