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Ailin Deng, Evan Cesanek, Fulvio Domini; Sensory feedback reduces scalar variability in grasping. Journal of Vision 2018;18(10):63. doi: 10.1167/18.10.63.
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© ARVO (1962-2015); The Authors (2016-present)
Visual perception of spatial properties obeys Weber's law: variability in the perceptual response scales with the physical stimulus magnitude. Yet some recent studies have shown that motor responses (e.g., the grip aperture of grasping movements) do not show this scalar variability effect. It remains an open question whether the absence of scalar variability in motor responses is due to visual processing that does not obey Weber's law or if it is due to other unaccounted-for factors. Since visual and haptic feedback are known to calibrate the accuracy of reach-to-grasp movements over repeated trials, we hypothesized that sensory feedback from repeated movements may also reduce scalar variability. In virtual reality, participants repeatedly reached-to-grasp visual targets of varying lengths (21-42 mm) along their vertical axes, and we analyzed within-subject variability in maximum grip apertures (MGAs) and in grip-aperture trajectories. We manipulated feedback availability in four blocks: no feedback, visual feedback only, haptic feedback only, or visual and haptic feedback. When visual feedback was available, participants could see their fingertip positions as the hand approached the target. When haptic feedback was available, participants encountered a physical object that matched the visual target. Our results demonstrate that sensory feedback does reduce scalar variability. We observed strong scalar variability in the no-feedback condition, both in early stages of the movement and at the terminal grip aperture. In all other feedback conditions, scalar variability was reduced across the entire movement. Notably, the fact that visual feedback alone reduced scalar variability just as much as haptic feedback indicates that the presence of haptic feedback is not critical for calibration to occur. In follow-up trajectory analyses, we explored whether various models of grasp planning could explain our results. We conclude that scalar variability in the grip aperture arose from motor processes, not the visual size estimation process.
Meeting abstract presented at VSS 2018
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