The information capacity of the human motor system in controlling the amplitude of movement.

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Abstract
Author relates the traditional Weber function (variability of a response as a function of its amplitude) to parallel phenomena of variability as function of response duration, using certain concepts of information theory. An index of the difficulty of a movement is proposed on assumption that average amplitude, average duration, and amplitude variability of successive movements are related in manner suggested by information theory. Basic rationale is that minimum amt. of information required to produce movement having a particular avg. amplitude plus or minus a specified tolerance (variable error) is proportional to logarithm of ratio of tolerance to possible amplitude range. The specification of possible amplitude range is arbitrary and has been set at twice avg. amplitude. Avg. rate of information generated by series of movements is average information/movement divided by time/movement. Three expts. are reported which were designed to test hypothesis If amplitude and tolerance limits of a task are fixed and the S is instructed to work at max. rate, then avg. duration of responses will be directly proportional to min. avg. amt. of information/ response (i.e., degree of behavior organization) demanded by task conditions. Conditions studied covered range from 1 to 10 bits/response. Results indicate that rate of performance in a given type of task is approximately constant over a considerable range of movement amplitudes and tolerance limits, but falls off outside this opt. range. The level of opt. performance varied slightly among 3 tasks in range between ca. 10 to 12 bits/sec. Consistency of these results supports basic thesis that performance capacity of human motor system plus its associated visual and proprioceptive feedback mechanisms, when measured in information units, is relatively constant over considerable range of task conditions. This thesis offers plausible way of accounting for otherwise conflicting data on durations of different types of movements. Fixed information-handling capacity of motor system probably reflects fixed capacity of central mechanisms for monitoring results of ongoing motor activity while at same time maintaining necessary degree of organization with respect to magnitude and timing of successive movements.
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