Effects of hand termination and accuracy constraint on eye-hand coordination during sequential two-segment movements.

The purpose of this study was to examine the effects of accuracy constraints and termination requirements of hand movement on eye-hand coordination. Healthy adults performed two-segment eye and hand aiming movements to predetermined stationary targets. While two-segment eye movements were made to the first and second targets for all conditions, hand movements were varied across conditions. The first segment had two target sizes to alter accuracy constraints. There were three hand movement types with different termination requirements: (1) stop both at the first and at the second targets, (2) stop at the first target and discontinue, and (3) move through the first target and discontinue. The results showed that the initiation of saccades was moderately correlated with the initiation of hand movements, and both initiations changed in a similar fashion depending on various hand termination requirements. Amplitude of primary saccades and frequency of corrective saccades during the first segment were affected by the combined effects of accuracy constraints and hand termination requirements. These results suggest that the planning and execution of saccades are based in part on global task constraints related to the accuracy and termination demands of hand movements over the two segments. During the transition from the first to the second segment, the gaze was held on the first target until shortly after the pointing to that target was terminated, showing gaze anchoring. The gaze anchoring was prolonged due to the increased accuracy constraint of that target or by including pointing to the second target. However, the gaze anchoring was broken prior to the completion of pointing when the accuracy constraint was reduced and pointing to the second target was excluded. The observed modifications of gaze anchoring imply that the oculomotor system is functionally obligated to fixate a gaze to a pointing target only to the extent that successful completion of a pointing task is ensured by the actual completion or by a predictive assessment of pointing termination.