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Rhythmic fluctuation of orientation to a continuous manual control task.

Fluctuation of orientation to a tracking task was made observable by varying the controlled system characteristics according to the error level of the moment. Male adult subjects performed either compensatory or pursuit tracking for 15 min, and large error increases were seen between intervals of half a minute to a few minutes. Orienting to the task was examined in two ways. In the compensatory tracking, the subject was to find, as quickly as possible, randomly appearing, flickering illumination of the target spot. The critical flicker frequency thus measured was significantly lower during 15-sec periods preceding error increases than during steady control phases, low values from the former periods deviating from a normal distribution. In the pursuit tracking, saccadic eye movements were found less frequently during 15-sec periods prior to large error increases, resulting in lowered instantaneous saccade rates computed from distribution of saccade intervals. These results suggest that a recurrent phase exists in which orienting to the tracking temporarily declines prior to an apparent performance decrement, presumably due to spontaneous lapse of attention.