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Tzvi Ganel, Erez Freud, Eran Chazut, Daneil Algom; Visuomotor resolution for size is independent of conscious perception. Journal of Vision 2011;11(11):961. doi: https://doi.org/10.1167/11.11.961.
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© ARVO (1962-2015); The Authors (2016-present)
Human resolution power in the various senses is experimentally determined by a range of known psychophysical methods. A trivial yet critically important feature of classical psychophysics is that its methods are based on conscious perception. In contrast, grasping of the same stimuli might be less conscious. Recent findings from our laboratory showed that Weber's law, a pillar of classical psychophysics, does not necessarily hold when people grasp rather than perceptually evaluate the size of objects presented for view. Moreover, the trajectories of the fingers during grasping can be dissociated from people's conscious perception of size. In the current study, we integrated these two lines of evidence in order to compare grasping and perceptual sensitivity to differences in size that were set below the Just Noticeable Difference (JND). In Experiment 1, participants were asked to make size comparisons for pairs of objects and then to grasp the objects. In a second experiment, a similar paradigm was used but now subjects made perceptual estimations of size instead of grasping. The results showed that perceptual estimations were not sensitive to differences in size smaller than the JND. This pattern of results was found for both correct and erroneous size-comparisons trials. In contrast, grasping trajectories showed sensitivity to object size differences regardless of the JND. This pattern was observed even in trials in which perceptual size comparison were erroneous. Participants were not aware, therefore, whether their size discrimination via grasp was or was not veridical. These findings show that human resolution power is not fully captured by the classic perceptual JND. Grasping can exhibit greater resolution power than that measured via conscious visual perception.
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