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Rachel M. Foster, Volker H. Franz; Systematic biases occur when variability is compared across early and late portions of grasp trajectories. Journal of Vision 2012;12(9):605. doi: 10.1167/12.9.605.
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© ARVO (1962-2015); The Authors (2016-present)
Finding the variability of the maximum grip aperture (MGA) to be independent of object size, Ganel and Goodale (2003) suggested that the motor system does not follow Weber’s law. Others asked whether this claim can be generalized to the unfolding of the grasping movement (Heath, Mulla, Holmes, & Smuskowitz, 2011; Holmes, Mulla, Binsted, & Heath, 2011; Holmes, Mulla, Smuskowitz, & Heath, 2011) and tested whether the variability of the grip aperture (aperture-variability) depends on object size at different time points of the movement. Because they found such a dependence for early but not late time points, they concluded that Weber's law holds only for early stages of a movement and grasping is controlled by two different processes. We suggest that this reasoning is based on an ill-specified null hypothesis. The dependence of aperture-variability on object size at early time points can be fully explained by the fact that, as movement time is roughly constant, the fingers must open faster for large objects than for small objects. This faster opening corresponds to a larger first derivative of aperture with respect to time (aperture-velocity). We show that aperture-variability is approximately proportional to aperture-velocity (more specifically: proportional and low-pass filtered). Therefore, even if we assume a perfectly unitary motor process that does not obey Weber's law at the time of the MGA, it will exhibit a pattern that resembles Weber’s law for earlier time points (i.e., aperture-variability will depend on object size because aperture-velocity depends on object size). This effect is not solved by time normalization, which alters the apparent aperture-velocity by compressing and stretching trajectories. While motivated by a single line of studies, our observations show that it is inherently difficult to compare variance calculated for different time points in the grasping movement, and thus impact the interpretation of many time-dependent analyses in our field.
Meeting abstract presented at VSS 2012
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