Visuomotor adaptation in normal aging

Abstract

Visuomotor adaptation to a gradual or sudden screen cursor rotation was investigated in healthy young and elderly subjects. Both age groups were equally divided into two subgroups; one subgroup was exposed to 11.25 degrees step increments of visual feedback rotation, every 45 trials (up to a total of 90 degrees), whereas a second subgroup was subjected to 90 degrees rotation from the onset of exposure. Participants performed discrete, horizontal hand movements to virtual targets in four randomized directions. Targets appeared on a computer screen in front of them, and a board prevented vision of the hand at all times. Differential effects of aging on visuomotor adaptation were found, depending on the time course of the visual distortion. In both age groups, early exposure to the sudden visual feedback distortion resulted in typical spiral-like trajectories, which became straighter by late exposure. However, the final adaptation level was reduced in the aged group, although the aftereffects were similar. When subjects were exposed to the gradual distortion, no statistically significant differences in measures of adaptation with advancing age were found. In this case, both age groups appeared to adapt equally. However, after removal of the distortion, elderly subjects showed reduced aftereffects as compared with the young group. These findings suggest differential effects of aging on adaptation to gradual versus sudden visual feedback distortions, and may help to explain the conflicting results obtained in previous visuomotor adaptation studies.

Figure 1

(A) Depiction of the experimental set-up. (B) Time-course of the sudden and gradual visual feedback distortion as a function of trial.

Figure 2

Screen cursor movement paths (mean ± SD) and normalized shifts of initial direction of movement (mean ± SD; insets) for the young and aged groups as a function of distortion type and stage of visuomotor adaptation. (A) Pre-exposure, (B) early-, and (C) late-exposure to a 90° CCW screen cursor rotation, and (D) post-exposure following removal of the screen cursor rotation. In these figures, the last two trials of pre- and late-exposure, and the first two trials of early and post-exposure for each subject per target direction (10 traces total per target direction for each condition) have been overlaid to show typical movement paths at different stages of learning (e.g., sharp reversals, spirals, loops, etc). (Insets) The dark vectors represent mean normalized shifts of the initial movement direction, collapsed across targets, from pre-exposure to early- (B), late- (C), and post-exposure trials (D), respectively. The mean initial direction of movement at pre-exposure (represented as a broken dark line) is set to 0°.

Figure 3

Trial block means, standard errors, and fitted curves for the standardized initial directional error (IDE) and root mean square error (RMSE) scores during exposure and post-exposure conditions as a function of age group and distortion type. The group data for each measure has been subdivided into two sections and fit separating the exposure (double exponential fit), and post-exposure (single exponential fit) conditions as stated by the horizontal axis label (left).

Figure 4

Aftereffect (mean and SEM) for standardized IDE and RMSE scores for the first aftereffect trial and the first aftereffect trial-block mean.