Postural and spatial orientation driven by virtual reality

Abstract

Orientation in space is a perceptual variable intimately related to postural orientation that relies on visual and vestibular signals to correctly identify our position relative to vertical. We have combined a virtual environment with motion of a posture platform to produce visual-vestibular conditions that allow us to explore how motion of the visual environment may affect perception of vertical and, consequently, affect postural stabilizing responses. In order to involve a higher level perceptual process, we needed to create a visual environment that was immersive. We did this by developing visual scenes that possess contextual information using color, texture, and 3-dimensional structures. Update latency of the visual scene was close to physiological latencies of the vestibulo-ocular reflex. Using this system we found that even when healthy young adults stand and walk on a stable support surface, they are unable to ignore wide field of view visual motion and they adapt their postural orientation to the parameters of the visual motion. Balance training within our environment elicited measurable rehabilitation outcomes. Thus we believe that virtual environments can serve as a clinical tool for evaluation and training of movement in situations that closely reflect conditions found in the physical world.

Figure 1

The Virtual Environment and Postural Orientation Laboratory currently at Temple University is a three-wall virtual environment. Each wall measures 2.4 m × 1.7 m. The visual experience is that of being immersed in a realistic scene with textural content and optic flow. Built into the floor is a 3 degree of freedom posture platform (NeuroCom Inc., Clackamas, OR) with two integrated force plates (AMTI, Watertown, MA) on which sit reflective markers from the Motion Analysis (Santa Rosa, CA) infrared camera system.

Figure 2

Power of head, trunk, and shank center of mass for four subjects is normalized to the largest response of each subject during 0.1 Hz motion of a visual scene with dioptic (2D) and stereo (3D) images while on a full (100%) and reduced (35%) base of support.

Figure 3

(Left) A subject standing within a field of random dots projected in the VE. The subject is tethered to three flock-of-birds sensors that are recording 6 axes of motion of the head, trunk, and lower limb. (Right) Graphs of two subjects (A and B) showing the relationship of the head, trunk, and left ankle during locomotion. The two gait patterns produced by the subjects walking from the rear of the CAVE (bottom of the y-axis) to the front wall (top of the y-axis) are shown. (A) The subject takes one step forward and then walks in the direction of the counterclockwise scene by crossing one limb over the other. (B) The subject crouches down and stamps his feet to progress forward in the CAVE.

Figure 4

Amplitudes of head, trunk, and ankle to pitch, roll, and A–P motion of the VE. For pitch and roll, both constant velocity at 5°/s (A) and sinusoidal motion of the VE at 0.1Hz (B) and 0.5Hz (C) were used. (A) Vertical dashed lines indicate the start and termination of constant velocity visual scene motion. (B and C) Sinusoidal motion of the visual scene is illustrated by the light grey lines in each plot. (D) Sinusoidal motion of the visual scene at 0.1 Hz is shown in the bottom trace. Time scale shows responses from early and late portions of the experiment. In all A–P plots, upward peaks represent anterior motion relative to the room; downward peaks represent posterior motion relative to the room.

Figure 5

Orientation of the hand held wand, the head, and the center of pressure (COP) while viewing counterclockwise (CCW) roll motion of the visual scene (bold line) and a stationary visual scene (broken line) in three subjects demonstrates a fluctuating response (top row), a bi-directional response (middle row), and a constant response (bottom row) that is consistent across all three variables. Dashed vertical lines mark the start and end of the scene motion.

Figure 6

Schematic illustration of the vection phenomenon. Gravitational and visual signals stimulate the otoliths and the visual system, respectively, which, when combined, produce the perception of tilt. Thus, as seen on the right, when the visual scene is rotating counterclockwise there is a mismatch with the vertically directed otolith vector. The CNS determines that it doesn’t make sense for the world to be moving, thereby resolving this conflict with a perception of tilt. The response is to correct for the perceived tilt (in the irection opposite that of the visual world) by tilting the body in the same direction as the motion of the visual world.

Figure 7

(Top) Power of the relative angles between head, trunk, shank and the moving platform (sled) over the period of the trial at the relevant frequencies of platform motion (0.25 Hz) and visual scene motion (0.1 Hz) are shown for each protocol for one young adult, one elderly adult, and one labyrinthine deficient adult. The power at each segment is portrayed as the percentage of the maximum response power (observed in the trunk) across segments for that subject. (Bottom) Mean area under the power curve ± standard error of the mean across all young adult subjects at the relevant frequency for platform (sled) motion only (0.25 Hz), visual scene motion only (0.1 Hz), and both frequencies of combined platform and visual scene motion (both). Segmental responses significantly increased (*) at 0.1 Hz when platform and scene motion were combined.

Figure 8

Average head, whole body, and shank COM power for each of the three BOS conditions when the augmented visual motion was imposed on a stereo virtual scene. Subjects viewed the motion with a narrow (black line) and wide (dashed) FOV.

Figure 9

Average head, whole body, and shank COM power for the 100% (dashed line) and 45% (black line) BOS conditions in subjects that were able to maintain balance on the reduced BOS (typical subjects) and those that needed to take a step (steppers) while viewing the scene in stereo.

Figure 10

RMS of head velocity across a 1 sec period following a 30 deg/sec dorsiflexion tilt of the base of support while the scene was dark, matched to the head motion (0 deg/sec), matched to the velocity of the base of support (30 deg/sec), or moving at velocities greater than the base of support (45 and 60 deg/sec) in a healthy young adult (white bar), a subject with a history of vestibular dysfunction (grey bar), and a subject with visual sensitivity but no history of vestibular dysfunction (black bar).

Figure 11

Center of pressure responses of a BPPV subject before (top traces) and following (bottom traces) balance training. The subject stood on an unstable support surface while in the dark, viewing a scene matched to her head motion (still), viewing a scene moving counterclockwise (roll), and while pointing to a target in the rolling scene (pointing). N.B. the subject was unable to accomplish the pointing task prior to the balance training.