Is there a planning control following Fitts’ law? - Rehabilitation Institute of Chicago

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Wed, Jan 13

Speaker: Matteo Bertucco, PhD (Postdoctoral Candidate)

Title: Is there a planning control following Fitts’ law? The timing of anticipatory postural adjustments follows Fitts’ law for step initiation.

Abstract: Everyday human movements often involve a trade-off between speed and accuracy; frequently the actions are performed while people standing requiring a combined postural control. A well-known motor control model, called Fitts’ law (Fitts, 1954), establishes a linear relationship between velocity and accuracy in movement performance. The model defines that the movement time (MT) to reach a specific point in space is a linear function of the ratio between movement amplitude and accuracy defined by the target width such that:

(1) MT = a + b log2(2A/W) where A and W are respectively the amplitude and target width, a and b are constants from the regression and the ratio log2(2A/W) defines the Index of Difficulty (ID). Traditionally Fitts’ law has been tested at the behavioural level, while very little is known about its influence for action planning. Action planning maybe disclosed by considering the modulation of the Anticipatory Postural Adjustments (APAs) before the actual movement takes place. Here we test whether the two APA parameters, APA timing and APA magnitude, are independently controlled and modulate by following Fitts’ law. Twelve subjects were asked to perform a fast and accurate pointing movement to a target with their preferred foot under a classical Fitts’ law paradigm with different combinations of target distances and widths to obtain several index of difficulties ranged from 1 to 6.64 bits. Kinematics variables, as movement time, velocity and variability at the target were analyzed. Moreover, analysis of the APAs was performed to assess their modulation given the different IDs as indices of feed-forward central commands. Results revealed that movement time and peak of velocity scaled with the index of difficulty (ID) but by considering each movement distance separately. APAs amplitude and duration, scaled according to movement parameters but not in the same way. APA magnitude scaled with movement velocity while APA duration was sensitive to the ratio amplitude/accuracy by following the ID for movements performed below 250 ms of time where an on-line feedback control is supposed not to be available during the performance. In conclusion we showed that when movement time does not allows for an on-line feedback control, the anticipatory temporal muscle activation acts as an independent central command that triggers a fine-tuning for speed-accuracy trade-off following Fitts’ law.

Host: Dr. Zev Rymer