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Force can overcome object geometry in the perception of shape through active touch

Abstract

Haptic (touch) perception normally entails an active exploration of object surfaces over time. This is called active touch1,2,3. When exploring the shape of an object, we experience both geometrical4 and force cues. For example, when sliding a finger across a surface with a rigid bump on it, the finger moves over the bump while being opposed by a force whose direction and magnitude are related to the slope of the bump5. The steeper the bump, the stronger the resistance. Geometrical and force cues are correlated, but it has been commonly assumed that shape perception relies on object geometry alone. Here we show that regardless of surface geometry, subjects identified and located shape features on the basis of force cues or their correlates. Using paradoxical stimuli, for example combining the force cues of a bump with the geometry of a hole, we found that subjects perceived a bump. Conversely, when combining the force cues of a hole with the geometry of a bump, subjects typically perceived a hole.

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Figure 1: Side (a) and front (b) views of the apparatus.
Figure 2: Experimental conditions 2–9.
Figure 3: Subjects used force to identify or locate shape features.
Figure 4: Force perceptually transformed geometrical shape.

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Acknowledgements

We thank M. Cynader, B. Frost and L. Requadt for helpful comments, and D. Pavlasek and J. Boka for help in designing/building the manipulandum and surfaces. The research was funded by Canada's Network of Centers of Excellence programme, Institute for Robotics and Intelligent Systems, and the Natural Sciences and Engineering Research Council of Canada.

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Correspondence to Gabriel Robles-De-La-Torre.

Supplementary information

Scatter plots of datasets used to compute tracking performance in Experiment 1, Conditions 6 and 7.

Figures 6 and 7 present all the data used to compute the correlations shown in Fig. 3c (Condition 6) and Fig. 3d (Condition 7). In Conditions 6 and 7, a force-masked physical shape and a control virtual shape were presented side by side. Subjects were asked to determine the location of the highest/deepest feature. Each panel in Figures 6 and 7 displays scatter plots of localization errors for one subject versus D = LFM − LV, the distance between the force-masked and virtual bumps (Fig. 5). One error (filled circles, Figs. 6 and 7) was EV = LV - LS, the difference between the position of the virtual bump, LV, and LS, the subject's perceived bump position (Fig. 5). The other error (empty circles, Figs. 6 and 7) was EFM = LFM − LS, the difference between the position of the force-masked physical bump, LFM, and the subject's perceived bump position, LS (Fig. 5). The panels in Figure 6 have been sorted in decreasing order by the value of the correlations between the location of control virtual bumps and subject's localization (Fig. 3c, Condition 6). The value of these correlations is indicated on top of each panel.

For most subjects (Fig. 6a-g), the localization error EV with respect to the virtual bump was consistently close to zero (filled circles), while the error EFM with respect to the force-masked physical bump (empty circles) grew as the distance between virtual and physical bumps increased. These subjects consistently tracked the virtual and not the force-masked bumps, as indicated also by the correlations (top of the panels) between subject's localization and the position of the virtual bumps (Fig. 3c, Condition 6, thick circles). The data of the subject exposed to stimuli with spurious correlations (main text, Fig. 3c, Condition 6, Force-masked physical bump) are shown in panel d. This subject also clearly tracked the virtual and not the force-masked bumps.

EV and EFM were frequently close to zero for some subjects (Fig. 6h-j). These subjects sometimes located the virtual bump and sometimes the force-masked physical bump. The data clustering below and above zero error in Fig. 6h-j suggests that these subjects were frequently not accurate when locating either virtual or force-masked stimuli.

Figure 7 shows equivalent error plots for all subjects exposed to virtual and force-masked holes. The error definitions and the order of the panels are the same as those used in Fig. 6. The top of each panel shows the correlations between the location of control virtual holes and subject's localization (Fig. 3d, Condition 7, thick circles). Most subjects (Fig. 7a-f and j) tracked the virtual holes. However, one subject consistently tracked virtual holes (Fig. 7j), but not virtual bumps (Fig. 6j), while a different subject consistently tracked virtual bumps (Fig. 6g) but not virtual holes (Fig. 7g). Also, one subject tracked the force-masked holes (Fig. 7h) and frequently tracked the force-masked bumps, too (Fig. 6h). Finally, one subject tracked both force-masked and virtual stimuli part of the time when presented either with bumps (Fig. 6i) or holes (Fig. 7i).

Figure 5

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The workspace positions of virtual (LV) and force-masked (LFM) shapes were compared in each trial to the subject's perceived position of the shape feature (LS) to compute localization errors.

Figure 6

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Subjects' localization errors with respect to the position of virtual (filled circles) and force-masked bumps (empty circles). Each panel presents data from one subject. Most subjects (a-g) tracked the virtual bumps. Some subjects tracked force-masked bumps to a varying degree (h-j). Each circle represents a trial's error. The horizontal axis indicates the distance between the location of virtual and force masked stimuli (D=LFM-LV, Fig. 5). Panels are sorted by the correlations between subjects' localization and the position of the control virtual bump (Fig. 3c, Condition 6). These correlations are shown on top of each panel.

Figure 7

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Subjects' localization errors with respect to the position of virtual (filled circles) and force-masked holes (empty circles). Each panel presents data from one subject. Most subjects (a-f, j) tracked the virtual holes. Some subjects tracked force-masked holes to a varying degree (g-i). Each circle represents a trial's error. The order of panels is the same as that of Fig. 6. The horizontal axis indicates the distance between the location of virtual and force-masked stimuli (D=LFM-LV, Fig. 5). The correlations between subjects' localization and the position of the control virtual hole (Fig. 3d, Condition 7) are shown on top of each panel.

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Robles-De-La-Torre, G., Hayward, V. Force can overcome object geometry in the perception of shape through active touch. Nature 412, 445–448 (2001). https://doi.org/10.1038/35086588

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