Allocentric cues do not always improve whole body reaching performance

Abstract

The aim of this investigation was to gain further insight into control strategies used for whole body reaching tasks. Subjects were requested to step and reach to remembered target locations in normal room lighting (LIGHT) and complete darkness (DARK) with their gaze directed toward or eccentric to the remembered target location. Targets were located centrally at three different heights. Eccentric anchors for gaze direction were located at target height and initial target distance, either 30 degrees to the right or 20 degrees to the left of target location. Control trials, where targets remained in place, and remembered target trials were randomly presented. We recorded movements of the hand, eye and head, while subjects stepped and reached to real or remembered target locations. Lateral, vertical and anterior-posterior (AP) hand errors and eye location, and gaze direction deviations were determined relative to control trials. Final hand location errors varied by target height, lighting condition and gaze eccentricity. Lower reaches in the DARK compared to the LIGHT condition were common, and when matched with a tendency to reach above the low target, help explain more accurate reaches for this target in darkness. Anchoring the gaze eccentrically reduced hand errors in the AP direction and increased errors in the lateral direction. These results could be explained by deviations in eye locations and gaze directions, which were deemed significant predictors of final reach errors, accounting for a 17-47% of final hand error variance. Results also confirmed a link between gaze deviations and hand and head displacements, suggesting that gaze direction is used as a common input for movement of the hand and body. Additional links between constant and variable eye deviations and hand errors were common for the AP direction but not for lateral or vertical directions. When combined with data regarding hand error predictions, we found that subjects' alterations in body movement in the AP direction were associated with AP adjustments in their reach, but final hand position adjustments were associated with gaze direction alterations for movements in the vertical and horizontal directions. These results support the hypothesis that gaze direction provides a control signal for hand and body movement and that this control signal is used for movement direction and not amplitude.