Sinusoidal forearm tracking with delayed visual feedback. II. Dependence of the relative phase on the relative delay

The influence of delayed visual feedback on the phase relationships between target and response signal during a sinusoidal tracking task was analysed in five normal subjects. Target frequency was varied systematically between 0.3 and 1.5 Hz, and the delay between 0 and 120% of the target cycle duration. For each subject, 63 trials were recorded. Phase parameters (relative phase, absolute relative phase and percentage of positive relative phases) revealed a clear dependence on relative delay but not on target frequency. With relative delays close to 0 and 100% of the target cycle duration, subjects successfully tracked the target signal with a small phase lag. With delays in the 30-90% range, larger phase differences were observed. Furthermore, the (delayed) response signal usually preceded the target signal in this delay range. These findings provide further evidence for a dependence of tracking error on external delays in the visual feedback loop, and indicate that delays of about 50% of the movement cycle are more difficult to handle than are smaller or larger delays. The results are discussed with regard to the influence of different control strategies (feedback, feedforward) and different types of feedback (such as vision and proprioception) on motor control.