Effects of Delayed Visual Feedback on Jaw, Finger, and Toe Positioning in Man

The importance of visual cues in a positioning task was established by performing experiments in which the visual feedback display was varied. Thirty subjects had to maintain a constant jaw or finger position coinciding with the target level provided on an oscilloscope screen. A linear variable displacement transducer recorded, amplified, and digitized the position signals. Spectral analysis and calculation of the median frequency detected possible shifts in the power spectrum. Positioning accuracy was evaluated by means of the root-mean-square (RMS) error, which calculated the deviation from the target position. The median frequency of the finger tremor spectrum increased after withdrawal of visual feedback display, when eyes were either open or closed. Increased power in the low-frequency range of the finger signal was observed when delays were introduced in the feedback display. For jaw-closing muscles, the median frequency was not significantly altered when a delayed visual display was introduced or when the visual feedback display was abolished. Furthermore, RMS errors for finger positioning increased significantly in the 0.25-, 0.50-, and 1.00-s delayed display condition. Jaw-positioning errors significantly increased only after introduction of a 1.00-s delayed display. Results of a supplementary experimental set-up indicated a significant improvement of finger-positioning performance during 4 min of training, whereas no such effect was established for jaw positioning. These findings indicate that modification or withdrawal of visual feedback affects finger positioning more than jaw positioning. The spectral composition of a jaw position signal is not visually determined.