Complex oscillations in a human motor system

Experiments were performed to investigate the oscillations arising in a human motor system with delayed visual feedback. Eight subjects were instructed to maintain a constant finger position relative to a stationary baseline. The finger displacement was measured using a microdisplacement transducer connected to the index finger, and was displayed on an oscilloscope. Time delays between 40 and 1,500 ms were inserted in the visual feedback loop for 100 s. Results show that, as the time delays increase, irregular rhythms appear with short intermittent periods of regular oscillations. These regular low-frequency oscillations have an amplitude that increases with the time delays and a period that is consistently about 2 to 4 times the time delay. Fast Fourier Transforms (FFT) show a peak between 8 and 12 Hz, corresponding to physiological tremor in half the subjects. No systematic variations in the FFT for the 2 to 15 Hz range were observed as time delay increased. In the 0 to 2 Hz range, the FFT show a consistent increase in power with the time delay. These results indicate that, under the conditions of this experiment, tremor is not affected by time delays in the visuomotor system, and time delays in the visuomotor feedback loop give rise to complex oscillatory behaviors.