Head stabilization shows visual and inertial dependence during passive stimulation: implications for virtual rehabilitation.

Sensorimotor coordination relies on the fine calibration and integration of visual, vestibular, and somatosensory input. Using virtual environments (VE) allows for the dissociation of visual and inertial inputs to manipulate human behavioral outputs. Our goal was to employ VE technology in a novel manner to investigate how head stabilization is affected by spatiotemporal properties of dynamic visual input when combined with passive motion on a linear sled. Healthy adults (n = 12) wore a head-mounted display during naso-occipital sinusoidal horizontal whole body translations while seated. Subjects were secured in a seat with a five-point harness, with the head free to move. Frequency and amplitude of sinusoidal input (i.e., inertial conditions) were set to create overlapping conditions of maximum acceleration (amax) or velocity (vmax). Four inertial conditions were combined with four visual conditions (VIS). VIS were created so that direction of optic flow either matched direction of passive motion or did not. The effect of near and far fixation distance within the VE was also tested. Head kinematics were collected with a three-axis gyro. Head stability showed a complex interaction dependent on changes in weighting of visual and inertial inputs that changed with the sled driving frequency. Inertial condition affected amplitude (p < 0.0000) and phase (p < 0.0000) of head pitch angular velocity. In the absence of visual input, head pitch velocity amplitude increased (p < 0.01). An interaction effect between inertial and VIS conditions on head yaw occurred in SW (p < 0.05). There was also a significant interaction of depth of field and inertial condition on amplitude (p < 0.001) and phase (p < 0.05) of head yaw velocity in SW, especially during high vmax conditions. We conclude visual flow can organize lateral cervical responses despite being discordant with inertial input. When using VE for rehabilitation, possible unintended, involuntary or reflexive motor responses that may not be present in traditional training environments should be taken into consideration.