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.

The interplay between strategic and adaptive control mechanisms in plastic recalibration of locomotor function.

We have previously shown that viewing simulated rotary self-motion during treadmill locomotion causes immediate strategic modifications (Richards et al. in Presence Teleoper Vir Real 13:371-384, 2004) as well as an after effect reflecting adaptive modification of the control of position and trajectory during over-ground locomotion (Mulavara et al. in Exp Brain Res 166:210-219, 2005). The process of sensorimotor adaptation is comprised of both strategic and adaptive control mechanisms. Strategic control involves cognitive, on-line corrections to motor outputs once one is aware of a sensory discordance. Over an extended period of exposure to the sensory discordance, new strategic sensorimotor coordination patterns are reinforced until they become more automatic, and therefore adaptive in nature. The objective of this study was to investigate how strategic changes in trunk control during exposure to simulated rotary self-motion during treadmill walking influences adaptive modification of locomotor heading direction during over-ground stepping. Subjects (n = 10) walked on a motorized linear treadmill while viewing a wide field-of-view virtual scene for 24 min. The scene was static for the first 4 min and then, for the last 20 min, depicted constant rate self-motion equivalent to walking in a counter-clockwise, circular path around the perimeter of a room. Subjects performed five stepping trials both before and after the exposure period to assess after effects. Results from our previous study showed a significant change in heading direction (HD) during post-exposure step tests that was opposite to the direction in which the scene rotated during the adaptation period. For the present study, we quantified strategic modifications in trunk movement control during scene exposure using normalized root mean square (R(P)) variation of the subject's 3D trunk positions and normalized sum of standard deviations (R (O)) variation of 3D trunk orientations during scene rotation relative to that during static scene presentation. Associated 95% confidence intervals, CI(P) and CI(O), were calculated to investigate the variation of strategic modifications during scene exposure. Repeated measures ANOVA and individual subject regression analyses showed that R(P) and R(O) (i.e. strategic modifications) for trunk fore/aft (X) positions and yaw rotations, respectively, decreased significantly over the exposure period. Furthermore, we found a significant correlation between the magnitude change in HD and the rate at which the variation of strategic modifications in trunk X decreased. We also found evidence of a correlation between HD and the rate at which strategic modifications in trunk yaw decreased. We infer that adaptive recalibration of locomotor trajectory using optic flow stimuli depends on the rate at which kinematic variability associated with strategic control is reduced.

Three-dimensional spatial skill training in a simulated space station: random vs. blocked designs.

BACKGROUND: Astronauts floating inside a spacecraft must be able to recall the direction to surrounding visual landmarks, regardless of their viewing perspective. If 3D orientation skills are taught preflight, should perspective sequences be blocked or randomized? Can standard spatial skill tests predict performance? METHODS: Undergraduates (40 men and 40 women; ages 19-24) learned 3D spatial relationships among landmark pictures in a cubic chamber simulating a space station node. Subjects learned to predict picture directions when told one picture's direction (the one behind them) and the subject's simulated roll orientation, which was changed between trials by rotating pictures. The dependent variable was the proportion of correct predictions. A between group (n=40 per group) independent variable was training type (random vs. blocked sequencing of perspectives). Experiment phase (familiarization, training, transfer, and 2 retention phases) was a within group variable. Subjects also took three standard spatial skill tests: Card Rotation, Cube Comparison, and Group Imbedded Figures. RESULTS: As hypothesized, during training, performance for the random group (0.56) was worse than the blocked group (0.83); during transfer, the random group (0.75) was better than the blocked group (0.56); during retention-i, the random group (0.70) was better than the blocked group (0.55); and during retention-2, the random group (0.76) was better than the blocked group (0.65). Spatial skill tests correlated differently across the two groups, indicating that random sequencing elicits different skills. CONCLUSION: Random presentation enhances 3D spatial skill transfer and retention. Standard spatial tests can predict performance and have the potential to customize training.

Three dimensional spatial memory and learning in real and virtual environments.

Human orientation and spatial cognition partly depends on our ability to remember sets of visual landmarks and imagine their relationship to us from a different viewpoint. We normally make large body rotations only about a single axis which is aligned with gravity. However, astronauts who try to recognize environments rotated in 3 dimensions report that their terrestrial ability to imagine the relative orientation of remembered landmarks does not easily generalize. The ability of human subjects to learn to mentally rotate a simple array of six objects around them was studied in 1-G laboratory experiments. Subjects were tested in a cubic chamber (n = 73) and a equivalent virtual environment (n = 24), analogous to the interior of a space station node module. A picture of an object was presented at the center of each wall. Subjects had to memorize the spatial relationships among the six objects and learn to predict the direction to a specific object if their body were in a specified 3D orientation. Percent correct learning curves and response times were measured. Most subjects achieved high accuracy from a given viewpoint within 20 trials, regardless of roll orientation, and learned a second view direction with equal or greater ease. Performance of the subject group that used a head mounted display/head tracker was qualitatively similar to that of the second group tested in a physical node simulator. Body position with respect to gravity had a significant but minor effect on performance of each group, suggesting that results may also apply to weightless situations. A correlation was found between task performance measures and conventional paper-and-pencil tests of field independence and 2&3 dimensional figure rotation ability.

Training, transfer, and retention of three-dimensional spatial memory in virtual environments.

Human orientation requires one to remember and visualize spatial arrangements of landmarks from different perspectives. Astronauts have reported difficulties remembering relationships between environmental landmarks when imagined in arbitrary 3D orientations. The present study investigated the effects of strategy training on humans' 1) ability to infer their orientation from landmarks presented ahead and below, 2) performance when subsequently learning a different array, and 3) retention of configurational knowledge over time. On the first experiment day, 24 subjects were tested in a virtual cubic chamber in which a picture of an animal was drawn on each wall. Through trial-by-trial exposures, they had to memorize the spatial relationships among the six pictures around them and learn to predict the direction to a specific picture when facing any view direction, and in any roll orientation. Half of the subjects ("strategy group") were taught methods for remembering picture groupings, while the remainder received no such training ("control group"). After learning one picture array, the procedure was repeated in a second. Accuracy (% correct) and response time learning curves were measured. Performance for the second array and configurational memory of both arrays were also retested 1, 7, and 30 days later. Results showed that subjects "learned how to learn" this generic 3D spatial memory task regardless of their relative orientation to the environment, that ability and configurational knowledge was retained for at least a month, that figure rotation ability and field independence correlate with performance, and that teaching subjects specific strategies in advance significantly improves performance. Training astronauts to perform a similar generic 3D spatial memory task, and suggesting strategies in advance, may help them orient in three dimensions.