INTERMITTENCY IN HUMAN MANUAL TRACKING TASKS

We confirm Craik's (1947) observation that the human manually tracking a visual target behaves like an intermittent servo-controller. Such t racking responses are indicative of ''sampled'' negative-feedback cont rol 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. Depriv ing the subjects of visual feedback led to smoother tracking and great ly reduced the signal power of their responses between 0.5-1.8 Hz. The ir responses remained intermittent when they used feedback of their ow n position but not of the target to track a remembered (virtual) targe t. Hence, intermittency in tracking behavior is not exclusively a sign ature 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, duri ng tracking of slow waveforms, showed evidence of a small error deadzo ne (measuring 0.7 cm on the VDU screen or 0.8-degrees at the eye). At higher target speeds, however, the mean size of starting errors increa sed, 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 intermitten t tracking responses may be limited by a positional error deadzone and that evidence for a refractory period between successive corrective m ovements can be satisfied without evoking an explicit timing or sampli ng mechanism.