Visual sensitivity modulates postural sway in a virtual environment in healthy elderly and individuals with stroke

We employed a virtual environment to examine the impact of visual sensitivity on postural behaviors in adults with chronic symptoms of stroke. Six adults at least 1 year post-stroke (52–70 yrs) and 6 healthy adults (50–70 yrs) were tested in a Rod and Frame test. They then stood quietly on a platform within a 3-wall virtual environment. The platform was tilted 3° into dorsiflexion while in the dark, with visual motion matched to head motion, or with pitch up and down visual field rotations at 30 and 45 °/sec. While the visual field rotated, the platform was held tilted for 30 sec and then slowly returned to a neutral position over 30 sec. Center of pressure (COP) was recorded and approximate entropy (ApEn) values were calculated and compared with visual error from the Rod and Frame test. No significant differences in visual errors were detected in the Rod and Frame task between the populations. However, in subjects with large visual errors (>8 deg) strong inverse correlations with ApEn values (r>−0.7) emerged with either a scene referenced to head motion or matched to the velocity of the platform tilt. ApEn values were typically below 1 indicating that COP responses were mostly predictable and reflecting a single input. This low ApEn with increased visual error suggests that the visual field serves as a meaningful reference for postural stabilization in visually dependent adults. Our results support the use of virtual environments to generate adaptive postural behaviors.