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Ailin Deng, Evan Cesanek, Fulvio Domini; Sensory feedback reduces scalar variability effects in perception and action tasks. Journal of Vision 2019;19(10):110. doi: https://doi.org/10.1167/19.10.110.
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According to Weber’s law, the variability in a perceptual response scales with physical stimulus magnitude. Such scalar variability effects are widely observed in visual perception, but recent work shows that the kinematics of visually guided actions instead show equivalent variability across stimulus magnitudes. To account for this surprising violation of a supposedly lawful phenomenon, resistance to Weber’s law has been touted as a feature of specialized vision-for-action processing, separate from perceptual processing. In this study, we investigated the alternative hypothesis that limited scalar variability is achieved by movement recalibration based on sensory feedback. In Experiment 1, participants performed a pantomime grasping block without any feedback and a feedback grasping block with visual and haptic feedback. These were alternated with three manual size estimation (MSE) blocks, counterbalancing for task order. Scalar variability affected the maximum grip apertures of pantomime grasps, but not normal grasps. More interestingly, a reduction in Weber’s fraction was observed for MSEs following normal grasping, but not following pantomime grasping. When pantomime and normal grasping were intermixed (but with explicit cues to trial type), scalar variability was absent for both. Next, we investigated whether haptic feedback from grasping plays an irreplaceable role in reducing scalar variability, or if visual feedback in a perceptual task will also suffice. Experiment 2 consisted of three MSE blocks, with no feedback in the first and last blocks, but with visual feedback of the grip aperture relative to the target object in the middle block (two lines around the target displayed immediately upon entering the estimate). Visual feedback significantly enhanced accuracy and precision in the second block while also reducing scalar variability; these effects carried over into the third block. Clearly, sensory feedback plays a central role in the reduction of scalar variability, regardless of any functional distinction between perception and action tasks.
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