Effects of head orientation and lateral body tilt on egocentric coding: cognitive and sensory-motor accuracy.

A major issue in motor control studies is to determine whether and how we use spatial frames of reference to organize our spatially oriented behaviors. In previous experiments we showed that simulated body tilt during off-axis rotation affected the performance in verbal localization and manual pointing tasks. It was hypothesized that the observed alterations were at least partly due to a change in the orientation of the egocentric frame of reference, which was indeed centered on the body but aligned with the gravitational vector. The present experiments were designed to test this hypothesis in a situation where no inertial constraints (except the usual gravitational one) exist and where the orientation of the body longitudinal z-axis was not aligned with the direction of the gravity. Eleven subjects were exposed to real static body tilt and were required to verbally localize (experiment 1) and to point as accurately as possible towards (experiment 2) memorized visual targets, in two conditions, Head-Free and Head-Fixed conditions. Results show that the performance was only affected by real body tilt in the localization task performed when the subject's head was tilted relative to the body. Thus, dissociation between gravity and body longitudinal z-axis alone is not responsible for localization nor for pointing errors. Therefore, the egocentric frame of reference seems independent from the orientation of the gravity with regard to body z-axis as expected from our previous studies. Moreover, the use of spatial referentials appears to be less mandatory than expected for pointing movements (motor task) than for localization task (cognitive task).

Accuracy of spatial localization depending on head posture in a perturbed gravitoinertial force field.

Spatial orientation is crucial when subjects have to accurately reach memorized visual targets. In previous studies modified gravitoinertial force fields were used to affect the accuracy of pointing movements in complete darkness without visual feedback of the moving limb. Target mislocalization was put forward as one hypothesis to explain this decrease in accuracy of pointing movements. The aim of this study was to test this hypothesis by determining the accuracy of spatial localization of memorized visual targets in a perturbed gravitoinertial force field. As head orientation is involved in localization tasks and carrying relevant sensory systems (visual, vestibular and neck muscle proprioceptive), we also tested the effect of head posture on the accuracy of localization. Subjects (n=10) were seated off-axis on a rotating platform (120 degrees s(-1)) in complete darkness with the head fixed (head-fixed session) or free to move (head-free session). They were required to report verbally the egocentric spatial localization of visual memorized targets. They gave the perceived target location in direction (i.e. left or right) and in amplitude (in centimeters) relative to the direction they thought to be straight ahead. Results showed that the accuracy of visual localization decreased when subjects were exposed to inertial forces. Moreover, subjects localized the memorized visual targets more to the right than their actual position, that was in the direction of the inertial forces. With further analysis, it appeared that this shift of localization was concomitant with a shift of the visual straight ahead (VSA) in the opposite direction. Thus, the modified gravitoinertial force field led to a modification in the orientation of the egocentric reference frame. Furthermore, this shift of localization increased when the head was free to move while the head was tilted in roll toward the center of rotation of the platform and turned in yaw in the same direction. It is concluded that the orientation of the egocentric reference frame was influenced by the gravitoinertial vector.