A unified model for self-paced movements
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While performing hand-control movements, humans tradeoff the movement speed and accuracy to meet either the high-speed criterion or the high-accuracy criterion. Although researchers have been developing many models for predicting different speed-accuracy tradeoff relationships, most of the models were studied independently and thus restricted to specific types of movements. To better understand the different speed-accuracy tradeoff relationships of hand-control movements as a natural result of our motor system, a general model was proposed by unifying existing findings, models and further investigations. Based on the concepts of the intermittent correction servo, the general model explains that while performing a movement, humans behave like a mechanical servo that intermittently utilizes visual feedback to correct movement misalignments until the movement is completed. Associated with the servo are four motor properties that determine the speed-accuracy tradeoff relationships: (1) corrective reaction time - the time required to utilize visual feedback to process a movement impulse, (2) ballistic movement time - the time required to perform a ballistic movement, (3) ballistic movement variability- the movement accuracy of a ballistic movement, and (4) moving behavior and strategy - the way a movement consists of ballistic movements. The first three properties are general properties that keep consistent for individuals. The last property differs for different types of movements. The general model with the four motor properties was expected to be generalized to different types of movements that have been previously studied independently. To validate the proposed general model, the three general properties were studied and the last motor property of moving behavior and strategy was specified for two types of well-studied movements: self-paced aiming movements and self-paced tracking movements. The two specified moving behaviors and strategies along with the three general properties were considered as two specified models and were programmed as two simulations. With three general properties individuals' measured as model inputs, the simulations were expected to predict the individuals' the speed-accuracy tradeoff relationships while performing both types of movements. Six experiments were conducted in which each participant performed all the experimental tasks on a drawing tablet with a stylus. The experiment of corrective reaction time and the experiment of ballistic movement time and variability were used to measure the individuals' three motor properties. The experiment of normal aiming movement, experiment of ballistic aiming movement, experiment of normal tracking movement, and experiment of ballistic tracking movement were used to measure the same individuals' speed-accuracy tradeoff relationships while performing both types of self-paced movements. For the experiments with "normal" settings, the movements were performed with continuous visual feedback in which movement time or movement speed was measured. For the experiments with "ballistic" settings, the movements were performed ballistically in which the number of ballistic movements or the average ballistic movement distance were measured. The results of the four experiments were treated as "answer keys" and compared with the simulated predictions to test the validity of the proposed models. The results showed that (1) the experiment of corrective reaction time did not measure the individuals' corrective reaction time well in a pilot study. Thus, the reasonable corrective reaction time range reported in the literature was utilized for the model testing, (2) based on Gan & Hoffmann's (1988) model, a ballistic movement time model was proposed that predicted very well the relationship between ballistic movement time and ballistic movement distance, (3) based on Howarth et al.'s (1971) model, a ballistic movement variability model was proposed that simultaneously predicted very well both the ballistic movement variability (measured perpendicular to and parallel to the moving direction) according to ballistic movement distance, (4) the two specified models predicted were valid; based on the reasonable range of corrective reaction time, the measured ballistic movement time and the measured ballistic movement variability, the two simulations predicted well the results measured in the four experiments with either "normal" or "ballistic" settings, and (5) this research demonstrated that the speed-accuracy tradeoff relationships of self-paced aiming movements and self-paced tracking movements can be explained and predicted by the proposed general model with the same source of three motor properties.
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