Intermittency in Human Manual Tracking Tasks

• Published in: Journal of motor behavior 1993-03, Vol.25 (1), p.53-63
• Main Authors: Miall, R. Christopher, Weir, D. J., Stein, J. F.
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; feedback control; Fundamental and applied biological sciences. Psychology; Multimodal perception; Perception; Psychology. Psychoanalysis. Psychiatry; Psychology. Psychophysiology; sampling; visuomotor tracking
• ISSN: 0022-2895; 1940-1027
• DOI: 10.1080/00222895.1993.9941639

We confirm Craik's (1947) observation that the human manually tracking a visual target behaves like an intermittent servo-controller. Such tracking responses are indicative of "sampled" negative-feedback control but could be the result of other, continuous, mechanisms. Tracking performance therefore was recorded in a task in which visual feedback of the position of the hand-held joystick could be eliminated. Depriving the subjects of visual feedback led to smoother tracking and greatly reduced the signal power of their responses between 0.5-1.8 Hz. Their responses remained intermittent when they used feedback of their own position but not of the target to track a remembered (virtual) target. Hence, intermittency in tracking behavior is not exclusively a signature of visual feedback control but also may be a sign of feedback to memorized waveforms. Craik's (1947) suggestion that the intermittency is due to a refractory period following each movement was also tested. The errors measured at the start of each intermittent response, during tracking of slow waveforms, showed evidence of a small error deadzone (measuring 0.7 cm on the VDU screen or 0.8° at the eye). At higher target speeds, however, the mean size of starting errors increased, and the upper boundary of the distribution of starting errors was close to that expected of a refractory delay of approximately 170 ms between responses. We consider a model of the control system that can fit these results by incorporating an error deadzone within a feedback control loop. We therefore propose that the initiation of intermittent tracking responses may be limited by a positional error deadzone and that evidence for a refractory period between successive corrective movements can be satisfied without evoking an explicit timing or sampling mechanism.