Manual motor control during “virtual” self-motion: Implications for VR rehabilitation

The level of immersion that is induced in an individual can be measured by subjective report, but VE immersion can also affect automatic sensorimotor processes which function below perceptual thresholds. Such sub-threshold effects on central nervous system processing are important to understand for the purposes of shaping VE rehabilitation. This study investigates the effect of dynamic immersive VE on self-motion perception and automatic upper extremity motor response. Subjects viewed either horizontal or vertical sinusoidal linear translation plusmn1 m at 0.25 Hz via a head-mounted display (HMD) while sitting in a stationary motion apparatus. Subjects performed a perceptuomotor task of aligning a handheld object to perceived vertical using their unconstrained arm (i.e. free to move in 6 DOF). Two objects were tested, a joystick and a full glass of water, in counter-balanced order. Results show the majority of subjects perceive self-motion that spatially and temporally agrees with the visually depicted VE motion. This occurs despite the absence of sinusoidally varying changes to gravitoinertial forces, since subjects are not exposed to actual physical motion. Despite only being instructed to orient the handheld object, handheld object kinematics also show automatic motor responses involving object translation. These manual motor responses were dependent on the direction and phase of the visual motion depicted in the VE. Specifically, vertical visual motion induced vertical translation and pitch tilt of the handheld object, while horizontal visual motion induced horizontal translation and roll tilt of the object. Motor responses were significantly greater in subjects who reported compelling self-motion perception. These findings suggest that a representation of net gravitoinertial forces can be derived from the high-fidelity, pictorial and dynamic depth cues visually presented in a VE. Automatic upper extremity manual responses which are controlled by descending cent- ral systems and tracts dissociable from lower extremities can be affected by immersion in a VE much like automatic postural behavior has been shown to be. This new evidence supports current efforts to conduct upper extremity rehabilitation in the relative safe and controllable experimental environments that VE technology affords.