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Abstract
The lawful continuous linear relation between movement time and task difficulty (i.e., index of difficulty; ID) in a goal-directed rapid aiming task (Fitts’ law) has been recently challenged in reciprocal performance. Specifically, a discontinuity was observed at critical ID and was attributed to a transition between two distinct dynamic regimes that occurs with increasing difficulty. In the present paper, we show that such a discontinuity is also present in discrete aiming when ID is manipulated via target width (experiment 1) but not via target distance (experiment 2). Fitts’ law’s discontinuity appears, therefore, to be a suitable indicator of the underlying functional adaptations of the neuro-muscular-skeletal system to task properties/requirements, independently of reciprocal or discrete nature of the task. These findings open new perspectives to the study of dynamic regimes involved in discrete aiming and sensori-motor mechanisms underlying the speed-accuracy trade-off.
Highlights
How movement kinematics and control mechanisms relate to the speed/accuracy trade-off in target-directed aiming tasks has been a central issue in human movement research for over a century [1]
We observed a discontinuity in the index of difficulty (ID)/acceleration phase (AT) relation for high difficulty levels, which did not result in an observable discontinuity in the ID/movement time (MT) relation, but was concomitant to an increase in the variability of movement trajectory during target approach
The fact that we failed to identify a discontinuity in MT probably reflects that it is harder to detect; after all, deceleration times (DT), which scales linearly with ID, accounts for most of the variance in ID-MT space
Summary
How movement kinematics and control mechanisms relate to the speed/accuracy trade-off in target-directed aiming tasks has been a central issue in human movement research for over a century [1]. Fitts’ law has been found to be pertinent to describe both reciprocal and discrete movements [2,4], and to apply to a large variety of motor tasks (see [5,6,7,8] for reviews) It has been widely used as a reference model to investigate sensori-motor processes underlying goal-oriented behavior (e.g., [9,10,11,12]). The breakpoint separated two dynamic regimes, one associated with a limit-cycle dynamics operating before the discontinuity, and another associated with a fixed-point dynamics afterwards These dynamic regimes are typically associated with rhythmic (performed at lower difficulty values) and discrete movements (performed at higher difficulty values), respectively [22,23]
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