Relation of stimulus and response amplitude to tracking performance.

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Abstract
The present study investigated the effects of visual stimulus and motor response amplitude and their interactions in a continuous tracking task. Determinations of scale effects were made for 8 secondary task conditions including both pursuit and compensatory tracking displays, 3 categories of intricacy of the target course, and 3 frequencies of target motion. With only 1 exception relative performance improved significantly as display amplification was increased from 1/4 to 4 in. and as the amplitude of required arm control motion was increased from 5[degree] to 80[degree]. Thus, both scale effects were progressive and a Weber-type function did not hold. However, the extent of this effect varied markedly with different tracking tasks. As performance improved with practice on the standard task (30-cpm target motion with a pursuit display) the use of larger amplitudes of movement and larger display magnifications became increasingly beneficial. Larger amplitudes of control motion also became increasingly beneficial as the visual display scale was increased. As the target motion was made more intricate by the addition of lower harmonics (and concomitantly was reduced in average power) the advantage of large amplitude control movements was reduced; however, the beneficial effects of display magnification increased slightly with increasing intricacy of target motion. The over-all performance level was severely reduced when a compensatory display was used and the target course was made more intricate, whereas when a pursuit display was used the more intricate but somewhat slower target motion was no more difficult to track. Pursuit tracking was markedly superior to compensatory tracking and this superiority was greatest for the most intricate pattern of target motion. As the frequency of the single-component target motion was increased from 15 to 30 to 60 cpm, the progressive nature of the visual scale effect decreased. Rates of target motion in the region of 60 cpm appear to exceed an optimum discriminatory range when large Sd values are used. Although visual and motor scale effects could be varied independently, the present experimental procedure did not permit the independent study of the effect of variations in proprio-ceptive feedback apart from the effect of a changed activity (energy) level in the motor system. The relations among the different sets of data, however, provide a basis for inferences regarding the relative value of position vs. rate information, and the relative value of visual vs. proprioceptive cues, in a tracking task. Changes in visual and motor scale effects are postulated to accompany a shift from primary dependence on visual cues to increased dependence on proprioceptive information in controlling sequences of movements.
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