Changing difficulty level in a one-dimensional tracking task and corresponding heart rate changes.

Subjects were required to track a target consisting of two vertical bars moving with either a constant or a sinusoidally modulated speed across an oscilloscope screen with a joystick-controlled light-spot. Either target size (distance between bars-size guided tasks) or velocity of the target movement (velocity guided tasks), varied throughout different tasks determined by subject's performance. The target's initial movement period was either 2 or 3 seconds. The following parameters were studied: time on-target intervals, time off-target intervals, number of tracking errors, heart rate and tracking error incidence over the cardiac cycle. Time on-target intervals were longer for the velocity guided tasks than for the size guided ones. The same was true for time off-target intervals. Values for both types of intervals decreased gradually over the fixed sequence of tasks for velocity but not for size guided tasks. Heart rate was higher in the size guided task. Tracking error incidence did not depend on the phase of cardiac cycle.