| The central nervous system must resolve the ambiguity of inertial motion sensory cues in order to derive accurate spatial orientation awareness. Our general hypothesis is that the central nervous system utilizes both multi-sensory integration and frequency segregation as neural strategies to resolve the ambiguity of tilt and translation stimuli. Movement in an altered gravity environment, such as weightlessness without a stable gravity reference, results in new patterns of sensory cues. Adaptive changes in how inertial cues from the otolith system are integrated with other sensory information lead to perceptual and postural disturbances upon return to Earth's gravity. The primary goals of this ground-based research investigation were to explore physiological mechanisms and operational implications of disorientation and tilt-translation disturbances reported by crewmembers during and following re-entry, and to evaluate a tactile prosthesis as a countermeasure for improving control of whole-body orientation during passive tilt and translation motion paradigms.
Aim 1 was to examine the effects of stimulus frequency (0.01 - 0.6 Hz ) on adaptive changes in eye movements, motion perception and cognition during combined tilt and translation motion profiles. We hypothesized that adaptation of otolith-mediated responses will be greatest in the mid-frequency range where there is a tilt-translation crossover. Our findings emphasized differences in the neural processing to distinguish tilt and translation between eye movements and motion perception. Specifically, during dynamic linear stimuli in the absence of canal and visual input, a change in stimulus frequency alone elicits similar changes in the amplitude of both self motion perception and eye movements. However, in contrast to the eye movements, the phase of both perceived tilt and translation motion is not altered by stimulus frequency over this limited range. Our findings also suggest that the frequency at which there was a crossover of perceived tilt and translation gains appeared to vary across different motion paradigms (e.g., near 0.3 Hz during off-vertical axis rotation and near 0.15 Hz during sled translation).
Adaptation experiments conducted below this cross-over frequency using the 'vision-aligned' paradigm have resulted in modest changes to both eye movements and motion perception, consistent with our first hypothesis. Adaptation experiments conducted around this cross-over frequency range using the 'GIF-aligned' paradigm demonstrated a significant effect of stimulus frequency on both motion sickness and spatial cognitive performance.
Aim 2 was to examine changes in control errors during a closed-loop nulling task before and after tilt-translation adaptation. We hypothesized that the ability to control tilt orientation will be compromised following tilt-translation adaptation, with increased control errors corresponding to changes in self-motion perception. Roll tilt nulling was implemented using the both step and pseudorandom stimuli in darkness. Our findings suggest that these types of manual control tasks are sensitive to underlying changes in sensorimotor physiology, and specifically to changes in the brain's interpretation of linear acceleration stimuli.
Aim 3 was to evaluate how a tactile prosthesis might improve control performance. A simple 4 electromechanical tactor system was developed that provided 6 threshold levels of orientation information. We also examined the influence of vibrotactile feedback during computerized posturography. A significant reduction in RMS error (p<0.05) was observed using this simple tactile prosthesis, both during manual and balance control tasks. These results are promising in that a fairly simple device with as few as 4 tactors may prove useful to significantly improve landing performance.
Aim 4 was to examine how spatial awareness is impaired with changing gravitational cues during parabolic flight, and the extent to which vibrotactile feedback of orientation can be used to help improve spatial awareness. Our findings suggest that tactile cueing may improve navigation in operational environments, such as extravehicular activities on a lunar surface. This type of sensory feedback may also prove beneficial as a navigation aid in patient populations, providing non-visual, non-auditory feedback of orientation or desired direction heading.