Dynamic visual acuity during walking after long-duration spaceflight.

INTRODUCTION: Astronauts experience alterations in gaze control as a result of adaptive changes in eye-head coordination produced by microgravity exposure. This may lead to potential changes in postflight visual acuity during head and body motion.
METHODS: We gathered dynamic visual acuity (DVA) data from 14 astronauts and cosmonauts after long-duration (approximately 6 mo) stays in space. Walking was used to induce self-motion and visual acuity was determined by sequentially presenting Landolt ring optotypes on a computer display placed 4 m in front of subjects. Acuity assessments were made while seated (static condition) and walking (dynamic condition) at 6.4 km x h(-1) on a motorized treadmill. In each condition, a psychophysical threshold detection algorithm minimized the required number of optotype presentations by maximizing the amount displayed around the subject's acuity threshold. The difference between static and dynamic acuity measures provided a metric of change in the subjects' ability to maintain gaze fixation on the visual target while walking.
RESULTS: A decrement in postflight visual acuity during walking was found. A mean dynamic acuity decrement of approximately 0.75 eye-chart lines was observed 1 d after returning from space. The population mean showed a consistent improvement in DVA performance during the first postflight week. DISCUSSION: The recovery curves for individual subjects did not necessarily follow a pattern of continuous improvement with each passing day. When adjusted for previous long-duration flight experience, the population mean showed an unexpected DVA reduction in the re-adaptation curve that is similar to recovery patterns observed in prism adaptation studies.