Movement control in older adults: does old age mean middle of the road?

Old age is associated with poorer movement skill, as indexed by reduced speed and accuracy. Nevertheless, reductions in speed and accuracy can also reflect compensation as well as deficit. We used a manual tracing and a driving task to identify generalized spatial and temporal compensations and deficits associated with old age. In Experiment 1, participants used a hand-held stylus to trace a path. In Experiment 2, participants steered along paths in a virtual reality driving simulator. In both experiments, participants were required to stay within the boundaries while we manipulated task difficulty by changing path width or movement speed. The older group showed worse performance in the highly constrained conditions. Corner cutting effectively reduces the curvature of bends but yields a greater risk of error (i.e., clipping the path or road edge). Corner cutting is thus less risky on wider paths, and we found that corner cutting increased for both age groups in both tasks when paths were wider. Crucially, we observed a greater degree of corner cutting in the young group compared with the old, suggesting the old group compensated for decreased motor skill with "middle-of-the-road" behavior. Enforcing increased speed caused all participants to increase corner cutting. Thus, older participants showed spatial compensation for decreased skill by biasing their position toward the middle of the path in both a manual and steering task. External constraints (narrow paths and fast speeds) prevented this strategy and revealed age-related declines in skills central to manual control and driving.     

The role of gaze and road edge information during high-speed locomotion

Robust control of skilled actions requires the flexible combination of multiple sources of information. Here we examined the role of gaze during high-speed locomotor steering and in particular the role of feedback from the visible road edges. Participants were required to maintain one of three lateral positions on the road when one or both edges were degraded (either by fading or removing them). Steering became increasingly impaired as road edge information was degraded, with gaze being predominantly directed toward the required road position. When either of the road edges were removed, we observed systematic shifts in steering and gaze direction dependent upon both the required road position and the visible edge. A second experiment required fixation on the road center or beyond the road edges. The results showed that the direction of gaze led to predictable steering biases, which increased as road edge information became degraded. A new steering model demonstrates that the direction of gaze and both road edges influence steering in a manner consistent with the flexible weighted combination of near road feedback information and prospective gaze information.