Cross-axis adaptation of pursuit initiation in humans

The initial acceleration of pursuit in the open-loop period is under adaptive control and undergoes motor learning. The current study was undertaken to examine the hypothesis that the direction of pursuit initiation can also be adaptively modified.

Four neurologically and ophthalmologically normal subjects participated in the experiment. A modified step-ramp paradigm was used to induce cross-axis adaptation, in which a ramp target changed its direction orthogonally just after the target crossed the center. Four direction changes were tested in separate experiments: left to up, left to down, down to left, and up to left. During a 30-minute adaptation session, the target moved in one of two randomly chosen directions (right to left or up to down) at one of two randomly chosen speeds (15.6 or 22.3 deg/sec), but the target changed orthogonally in only one direction. A linear regression fit to the initial 100-msec segment of the pursuit trace was used to determine the direction of pursuit initiation.

In all cases, an adaptive change in pursuit initiation was gradually induced in the direction called for by the training paradigm. Adaptation was usually completed (90 degrees shift) within the 30-minute training session but declined quickly to an approximate 30 degrees -shift after training. The latency and vectorial amplitude of the initial acceleration remained unchanged. The adaptation was specific for the direction but not the velocity of the target.

This study showed that the direction of pursuit initiation is under adaptive control, as has been shown for saccadic eye movements and the vestibulo-ocular reflex.