Effects of visually simulated roll motion on vection and postural stabilization

Abstract

Background: Visual motion often provokes vection (the induced perception of self-motion) and postural movement. Postural movement is known to increase during vection, suggesting the same visual motion signal underlies vection and postural control. However, self-motion does not need to be consciously perceived to influence postural control. Therefore, visual motion itself may affect postural control mechanisms. The purpose of the present study was to investigate the effects of visual motion and vection on postural movements during and after exposure to a visual stimulus motion.

Methods: Eighteen observers completed four experimental conditions, the order of which was counterbalanced across observers. Conditions corresponded to the four possible combinations of rotation direction of the visually simulated roll motion stimulus and the two different visual stimulus patterns. The velocity of the roll motion was held constant in all conditions at 60 deg/s. Observers assumed the standard Romberg stance, and postural movements were measured using a force platform and a head position sensor affixed to a helmet they wore. Observers pressed a button when they perceived vection. Postural responses and psychophysical parameters related to vection were analyzed.

Results: During exposure to the moving stimulus, body sway and head position of all observers moved in the same direction as the stimulus. Moreover, they deviated more during vection perception than no-vection-perception, and during no-vection-perception than no-visual-stimulus-motion. The postural movements also fluctuated more during vection-perception than no-vection-perception, and during no-vection-perception than no-visual-stimulus-motion, both in the left/right and anterior/posterior directions. There was no clear habituation for vection and posture, and no effect of stimulus type.

Conclusion: Our results suggested that visual stimulus motion itself affects postural control, and supported the idea that the same visual motion signal is used for vection and postural control. We speculated that the mechanisms underlying the processing of visual motion signals for postural control and vection perception operate using different thresholds, and that a frame of reference for body orientation perception changed along with vection perception induced further increment of postural sway.

Figure 1

Schematic illustration of the virtual environment. Observers stood at the center of the rectangular space whose wall was textured with one of the two different patterns shown in Figure 2.

Figure 2

The two different visual contexts. Textures presented on a wall of the rectangular solid were either (a) a random-dot texture, or (b) a CG-image that simulated an ordinary room.

Figure 3

Sample data for head position, COP, and vection responses in a typical trial. The data labeled as motion indicates the period of visual-stimulus-motion, while the data labeled as no-motion indicates the periods of no-visual-stimulus-motion. The positive vertical values indicate that head position and COP changes were in the direction of the visual-stimulus-motion. The value zero in the ordinate represents the average value during no-visual-stimulus-motion, prior to any visual-stimulus-motion.

Figure 4

Mean postural responses for the L/R and A/P directions. (a) Mean of COP or head position during vection and no-vection in both the L/R and A/P directions. Also shown are the continuous mean values of mean of either (b) COP or (c) head position after the visual stimulus motion ceased. The two data points in the left-most part of (b) and (c) represent averaged values in the L/R and A/P directions during visual-stimulus-motion.

Figure 5

Averaged-SD of postural responses for L/R and A/P directions. (a) Averaged-SD of COP or head position during vection and no-vection in both the L/R and A/P directions. Also shown are the continuous values of averaged-SD of either (b) COP or (c) head position after the visual stimulus motion ceased. The two data points in the left-most part of (b) and (c) represent averaged-SD in the L/R and A/P directions during visual-stimulus-motion.

Figure 6

Averaged head position and COP for latter part of experimental trial. The data labeled as motion indicates the period of visual-stimulus-motion, while the data labeled as no-motion indicates the periods of no-visual-stimulus-motion.

Figure 7

Vection parameters across different trials. Means of the psychophysical parameters of circular vection measured during the perception of vection for each trial.

Figure 8

Postural sway in both L/R and A/P directions across different trials. Means of the postural responses measured during visual-stimulus-motion.

Figure 9

A model of the relationship between postural control and vection. Visual and non-visual signals are used for both vection and postural control mechanisms.