Optimal contributions of head and eye positions to spatial accuracy in man tested by visually directed pointing

Encoding of visual target location in extrapersonal space requires convergence of at least three types of information: retinal signals, information about orbital eye positions, and the position of the head on the body. Since the position of gaze is the sum of the head position and the eye position, inaccuracy of spatial localization of the target may result from the sum of the corresponding three levels of errors: retina, ocular and head. In order to evaluate the possible errors evoked at each level, accuracy of target encoding was assessed through a motor response requiring subjects to point with the hand towards a target seen under foveal vision, eliminating the retinal source of error. Subjects had first to orient their head to one of three positions to the right (0, 40, 80°) and maintain this head position while orienting gaze and pointing to one of five target positions (0, 20, 40, 60, 80°). This resulted in 11 combinations of static head and eye positions, and corresponded to five different gaze eccentricities. The accuracy of target pointing was tested without vision of the moving hand. Six subjects were tested. No systematic bias in finger pointing was observed for eye positions ranging from 0 to 40° to the right or left within the orbit. However, the variability (as measured by a surface error) given by the scatter of hand pointing increased quadratically with eye eccentricity. A similar observation was made with the eye centreed and the head position ranging from 0 to 80°, although the surface error increased less steeply with eccentricity. Some interaction between eye and head eccentricity also contributed to the pointing error. These results suggest that pointing should be most accurate with a head displacement corresponding to 90% of the gaze eccentricity. These results explain the systematic hypometry of head orienting towards targets observed under natural conditions: thus the respective contribution of head and eye to gaze orientation might be determined in order to optimize accuracy of target encoding.