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Vision for action and perception elicit dissociable adherence to Weber’s law across a range of ‘graspable’ target objects

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Abstract

A number of studies have reported that grasps and manual estimations of differently sized target objects (e.g., 20 through 70 mm) violate and adhere to Weber’s law, respectively (e.g., Ganel et al. 2008a, Curr Biol 18:R599–R601)—a result interpreted as evidence that separate visual codes support actions (i.e., absolute) and perceptions (i.e., relative). More recent work employing a broader range of target objects (i.e., 5 through 120 mm) has laid question to this claim and proposed that grasps for ‘larger’ target objects (i.e., >20 mm) elicit an inverse relationship to Weber’s law and that manual estimations for target objects greater than 40 mm violate the law (Bruno et al. 2016, Neuropsychologia 91:327–334). In accounting for this finding, it was proposed that biomechanical limits in aperture shaping preclude the application of Weber’s law for larger target objects. It is, however, important to note that the work supporting a biomechanical account may have employed target objects that approached —or were beyond—some participants’ maximal aperture separation. The present investigation examined whether grasps and manual estimations differentially adhere to Weber’s law across a continuous range of functionally ‘graspable’ target objects (i.e., 10,…,80% of participant-specific maximal aperture separation). In addition, we employed a method of adjustment task to examine whether manual estimation provides a valid proxy for a traditional measure of perceptual judgment. Manual estimation and method of adjustment tasks demonstrated adherence to Weber’s law across the continuous range of target objects used here, whereas grasps violated the law. Thus, results evince that grasps and manual estimations of graspable target objects are, respectively, mediated via absolute and relative visual information.

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Notes

  1. The term just-noticeable-difference (i.e., JND) was not employed by Bruno et al. (2016). Instead, the authors used the term “variable error”. For consistency, and given the findings of the present work, we use JND as a standard term to reflect trial-to-trial variability in peak grip aperture.

  2. Skewness and interquartile range (IQR) statistics were computed via the MATLAB Statistics Toolbox (7.9.0 The MathWorks, Natick, MA, USA). The skewness statistic was bias corrected and the equation documentation can be found at: https://www.mathworks.com/help/stats/skewness.html. The interquartile range represented the mid-spread or middle 50% of all values for a given participant and object size combination and was computed as the difference between the 75th (Q 3) and 25th (Q 1) percentiles (i.e., Q 3 minus Q 1).

  3. Skewness for each object size was contrasted to zero. Given the number of comparisons, Fig. 5 depicts 99% between-participant confidence intervals to prevent inflation of our experiment-wise error rate. Notably, however, even when a more liberal confidence interval (i.e., 95% confidence interval) was adopted the only reliable difference was that the grasping task produced a negative skew for the 80% target object size.

  4. Westwood et al. (2001) represents the only study we are aware of providing a tabular summary of mean peak grip aperture values as a function of object size for open-loop manual estimation and grasping. These data revealed that PGA for manual estimation and grasping were 1.10 and 1.56 times larger than the physical size of target objects.

  5. The present work as well as previous open-loop grasping studies by our group (Holmes and Heath 2013; Holmes et al. 2011) employed a movement time (MT) criterion of 600–800 ms. As a result, MTs did not reliably vary as a function of target object size. Bruno et al. (2016) do not report MT values and it is therefore unclear whether target object size influenced the overall timing of grasping.

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Acknowledgements

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

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Correspondence to Matthew Heath.

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Heath, M., Manzone, J., Khan, M. et al. Vision for action and perception elicit dissociable adherence to Weber’s law across a range of ‘graspable’ target objects. Exp Brain Res 235, 3003–3012 (2017). https://doi.org/10.1007/s00221-017-5025-1

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