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Hand anthropometry and the limits of aperture separation determine the utility of Weber’s law in grasping and manual estimation

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Abstract

Recent work proposed that biomechanical constraints in aperture separation limit the utility of Weber’s law in determining whether dissociable visual codes support grasping and manual estimation. We tested this assertion by having participants precision grasp, manually estimate and complete a method of adjustment task to targets scaled within and beyond the range of their maximal aperture separation (i.e., from 20 to 140% of participant-specific maximal aperture separation: MAS). For grasping and manual estimation tasks, just-noticeable-difference (JND) scores were computed via the within-participant standard deviations in peak grip aperture, whereas method of adjustment JNDs were computed via the within-participant standard deviations in response output. Method of adjustment JNDs increased linearly across the range of targets; that is, responses adhered to Weber’s law. Manual estimation JNDs linearly increased for targets 20–100% of MAS and then decreased for targets 120–140% of MAS. In turn, grasping JNDs for targets 20% through 80% of MAS did not differ and were larger than targets 100–140% of MAS. That manual estimation and grasping showed a decrease in JNDs for the largest targets indicates that participants were at their biomechanical limits in aperture shaping, and the fact that the target showing the JND decrease differed between tasks (i.e., manual estimation = 100% of MAS; grasping = 80% of MAS) is attributed to the fact that grasping—but not manual estimation—requires a safety-margin task-set. Accordingly, manual estimations and grasping across a range of functionally ‘graspable’ targets, respectively, adhered to and violated Weber’s law—a result interpreted to reflect the use of dissociable visual codes.

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Notes

  1. The sample size used here was based on a previous study by our group (Holmes et al. 2011) showing that grasping JNDs did not reliably vary with target size, F(4,52) = 0.54, p = 0.70, ηp2 = 0.10—a finding indicating that the absence of reliable effect was not related to an inadequate replication sample size (Keppel 1991). In turn, Holmes et al. showed that manual estimations (for every participant) demonstrated linear JND/target size relations, F(4,52) = 9.61, p  < 0.001, ηp2 = 0.78.

  2. In an original iteration of this experiment, the duration of data capture for trials in grasping and manual estimation tasks was 1.5 s. Notably, however, the time required to complete the manual estimation task (i.e., lift hand from home position and separate thumb and forefinger to match target size) sometimes exceeded the 1.5 s interval. As a result, for some trials participants were in the process of completing their manual estimation before the end of the data collection window. Thus, we collected a second set of manual estimation trials with a data capture length of 4 s and those data represent that reported in the manuscript. The second set of manual estimation trials (i.e., the data presented here) involved the same participants and was completed 10–12 weeks following the original collection period.

  3. Bruno et al. (2016) do not provide measures of movement time or the timing of peak grip aperture.

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Acknowledgements

Supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (MH) and Faculty Scholar and Major Academic Development Fund Awards from the University of Western Ontario.

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Authors and Affiliations

  1. School of Kinesiology and Graduate Program in Neuroscience, University of Western Ontario, London, ON, N6A 3K7, Canada

    Naila Ayala & Matthew Heath

  2. Faculty of Health and Social Development, University of British Columbia, Kelowna, BC, Canada

    Gordon Binsted

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  1. Naila Ayala
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  2. Gordon Binsted
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  3. Matthew Heath
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Correspondence to Matthew Heath.

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Ayala, N., Binsted, G. & Heath, M. Hand anthropometry and the limits of aperture separation determine the utility of Weber’s law in grasping and manual estimation. Exp Brain Res 236, 2439–2446 (2018). https://doi.org/10.1007/s00221-018-5311-6

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