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Julia Trommershauser, Sergei Gepshtein, Laurence T. Maloney, Michael S. Landy, Martin S. Banks; Optimal compensation for changes in effective movement variability in planning movement under risk. Journal of Vision 2004;4(8):145. doi: 10.1167/4.8.145.
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In a previous study, we examined human movement planning in video-game-like situations in which there were explicit monetary rewards and penalties associated with touching objects at various locations in space. Performance did not differ from optimal, i.e., subjects acted so as to maximize expected gain (Trommershäuser et al., 2003, JOSA, 20, 1419). To use such an optimal movement strategy, subjects must not only take into account the consequences of an intended movement, but also the consequences of their own movement variability, which may carry the movement into “costly regions.” In two experiments, we tested whether humans compensate for changes in their effective movement variability. In both experiments, subjects had to touch a target region rapidly while trying not to hit a nearby penalty region. Each target hit yielded a monetary reward; each penalty hit yielded a monetary penalty. Late responses were penalized. In the first experiment (six subjects), we manipulated movement variability by perturbing visual feedback of the finger position during the movement. Rewards and penalties were scored based on the perturbed (and noisier) visually specified finger position. In the second experiment (six subjects), we compared performance with two stimulus configurations, one smaller in scale than the other. Reducing the scale of the configuration has the same effect, in relative terms, as increasing the subject's motor variability. The subject's actual motor variability did not change. In both experiments, subjects compensated for visually imposed (Exp. 1) and relative (Exp. 2) increases in variability. Overall, subjects' performance did not differ significantly from optimal. We conclude that humans take their effective movement variability into account in planning movement, that their performance remains optimal across changes in scale, and that they update their estimates of movement variability in response to externally imposed increases in effective variability.
Grant EY08266 from the National Institute of Health; Grant RG0109/1999-B from the Human Frontiers Science Program; Deutsche Forschungsgemeinschaft (Emmy-Noether-Programm); AFOSR Research Grant F49620
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