The Role of Visual Feedback about Motion of the Ground on Postural Sway.

We asked whether body sway would be influenced by visual information about motion of the ground surface. On a ship at sea, standing participants performed a demanding visual search task or a simple visual inspection task. Display content was stationary relative to the ship or relative to the Earth. Participants faced the ship's bow or its port side. Performance on the visual search task was representative of terrestrial studies. Body sway was greater during viewing of the Earth Stationary displays than during viewing of the Ship Stationary displays. We discuss possible implications of these results for theoretical and applied issues.

Sensitivity to changes in dynamic affordances for walking on land and at sea.

We investigated the perception of affordances for walking along a narrow path. We asked whether participants could perceive changes in affordances brought about by manipulation of properties of the body, or of the environment, without direct practice of the to-be-perceived affordance, and without external feedback about the accuracy of perception. In Experiment 1, participants made a series of 8 judgments of how far they could walk along a narrow path either, 1) without added weight, 2) while wearing a weighted vest, or 3) while wearing weights on their ankles. Before walking, mean judgments were lower when wearing weights than in the no-weight condition. In addition, in both weight conditions judgments changed across the series of 8 judgments, in the direction of greater accuracy. Control of the body in walking also can be influenced by motion of the ground surface, as commonly happens in vehicles. In Experiment 2, on a ship at sea, we evaluated the effects of walking with or without weight added to the body at the ankles. We again asked participants (experienced maritime crewmembers) to judge how far they could walk along a narrow path, with versus without ankle weights. As in Experiment 1, judgments made before walking mirrored the observed differences in walking performance. In addition, we again found evidence that judgment improved (without walking practice, or feedback) over the series of judgments. We conclude that participants were sensitive to (and spontaneously learned about) how affordances for walking were influenced by changes in the dynamics of body and the environment.

Unstable coupling of body sway with imposed motion precedes visually induced motion sickness.

Motion sickness is preceded by differences in the quantitative kinematics of body sway between individuals who (later) become sick and those who do not. In existing research, this effect has been demonstrated only in measures of body sway, relative to the earth. However, body sway can become coupled with imposed oscillatory motion of the illuminated environment, and the nature of this coupling may differ between individuals who become sick and those who do not. We asked whether body sway would become coupled to complex oscillations of the illuminated environment, and whether individual differences in such coupling might be precursors of motion sickness. Standing participants (women) were exposed to complex oscillation of the illuminated environment. We examined the strength of coupling as a function of time during exposure. Following exposure, some participants reported motion sickness. The nature and temporal evolution of coupling differed between participants who later reported motion sickness and those who did not. Our results show that people can couple the complex dynamics of body sway with complex imposed motion, and that differences in the nature of this coupling are related to the risk of motion sickness.

Adaptive perception of changes in affordances for walking on a ship at sea.

Ocean waves cause oscillatory motion of ships. Oscillatory ship motion typically is greater in roll (i.e., the ship rolling from side to side) than in pitch (i.e., tipping from front to back). Affordances for walking on a ship at sea should be differentially influenced by ship motion in roll and pitch. When roll exceeds pitch, the maximum walkable distance within a defined path should be greater when walking along the ship's short, or athwart axis than when walking along its long, or fore-aft axis. When pitch exceeds roll, this relation should be reversed. We asked whether such changes in ship motion would be reflected in judgments of direction-specific affordances for walking. Participants (experienced maritime crewmembers) judged how far they could walk along a narrow path on the ship deck. On different days, sailing conditions were such that the relative magnitude of pitch and roll was reversed. Judgments of direction-specific affordances for walking mirrored these changes in ship motion. The accuracy of judgments was consistent across directions, and across changes in ship motion. We conclude that experienced maritime crewmembers were sensitive to dynamic variations in affordances for walking that were, themselves, a function of dynamic properties of the animal-environment system.

Real-time visual feedback about postural activity increases postural instability and visually induced motion sickness.

BACKGROUND: Several studies have shown that the kinematics of standing body sway can be influenced by the provision of real time feedback about postural activity through visual displays. RESEARCH QUESTION: We asked whether real time visual feedback about the position of the body's center of pressure (COP) might affect body sway and the occurrence of visually induced motion sickness. METHODS: Standing participants (women) were exposed to complex visual oscillation in a moving room, a device that nearly filled the field of view. During exposure to complex visual oscillations, we provided real time feedback about displacements of the body's center of pressure through a visual display presented on a tablet computer. RESULTS: The incidence of motion sickness was greater than in a closely related study that did not provide real time feedback. We monitored the kinematics of the body's center of pressure before and during exposure to visual motion stimuli. Body sway differed between participants who reported motion sickness and those who did not. These differences existed before any participants experienced subjective symptoms of motion sickness. SIGNIFICANCE: Real time visual feedback about COP displacement did not reduce visually induced motion sickness, and may have increased it. We identified postural precursors of motion sickness that may have been exacerbated by the COP display. The results indicate that visual feedback about postural activity can destabilize postural control, leading to negative side effects. We suggest possible alternative types of visual displays that might help to stabilize posture, and reduce motion sickness.