Task-dependent changes in visual feedback control: A frequency analysis of human manual tracking

Prominent components in the frequency spectrum of human manual tracking responses are thought to reflect the visual feedback control loop and have been used in estimations of the visual feedback loop delay. The frequency structure of human tracking was therefore examined here in two tasks: visually guided tracking of slow and fast pseudorandom targets. Visually related frequency components were identified by testing, in each condition, the effect of adding additional feedback delays on the frequency spectrum. The major frequency components of the responses consisted of a fundamental component and its odd harmonics. These components were related to the visual feedback loop delay and shifted in concert toward lower frequencies as the feedback delay was increased. Furthermore, there were no differences in responses between 3 normal subjects and 1 subject with peripheral sensory loss. This implies that the frequency structure is dominated by the visual feedback control loop, without significant influence from proprioceptive control loops. However, the feedback-loop delay was shown to decrease from around 341 to 264 ms as the task speed doubled. Thus the estimates of visual-feedback delays are influenced by the target being followed, and this suggests that the subjects can ''tune'' their feedback system to suit the demands of the tracking task.