Compensatory manual motor responses while object wielding during combined linear visual and physical roll tilt stimulation.

Dynamic signals from multiple sensory channels must be integrated by the central nervous system to create a unified perception of self-motion and spatial orientation. Using immersive virtual environments, we altered the relative contribution of visual and inertial inputs and evaluated the effects on perceptuomotor outputs. Subjects seated in a tilting chair were exposed to a combined 0.25 Hz sinusoidal roll-tilt (+/-7.5 degrees) about the naso-occipital axis while viewing one of four visual conditions. One visual condition was in darkness, and the other three depicted 2 m of sinusoidal horizontal or vertical linear motion either synchronous or asynchronous with the roll-tilt. Subjects performed a perceptuomotor task of aligning a handheld object to gravitational vertical (GV) with the entire arm being free to move in six degrees of freedom. Subjects were tested with two objects, a joystick and glass of water, in counter-balanced order. Specific visual effects were as follows: (1) the phase leads of object tilt relative to chair/subject roll-tilt were affected by visual condition, (2) horizontal translation of the object was entrained with visual velocity, rather than with visual acceleration or maximum roll-tilt, and (3) when vertical visual motion was viewed during chair/subject roll-tilt, vertical object translation increased. Although the head-fixed scene meant visual vertical cues were always aligned with the subject's median sagittal plane, object tilt showed sensitivity to inertial roll-tilt (Gain > 0.5) which was not significantly different from the dark condition. Two object effects were found: (1) tilt deviation from GV was greater when wielding a joystick compared to a full glass of water, and (2) the phase of horizontal visual motion relative to subject roll tilt affected the joystick amplitude of horizontal translation but not the glass of water. In conclusion, an attentional shift driven by postural assumptions can account for the two object effects, however, the visual effects suggest that a process for deriving the net gravitoinertial force from visual and inertial cues is involved. Inertial signals dominated the perception of verticality, but visual linear translation affected the spatiotemporal dynamics of the manual motor responses during object wielding.