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Alice Coakley, Uta Wolfe; A dissociation between haptic and visual distortion of perceived length. Journal of Vision 2008;8(6):1048. doi: https://doi.org/10.1167/8.6.1048.
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Analogues of many common visual illusions are found in the haptic modality possibly because haptic space is calibrated by vision. For the visual modality, we previously reported that deviations from a right angle in the L-configuration of the horizontal-vertical (HV) illusion led to an increase in the illusion if the angle was obtuse but to a decrease if the angle was acute. These findings are inconsistent with theories that postulate a visual anisotropy as the main basis for the HV illusion and they are furthermore not predicted by existing perspective theories. Here we test if this pattern reflects a distortion in length perception that also holds for the modality of haptic exploration. Method: Seven naïve participants using their index finger haptically scanned two joined edges (one horizontal, the other diagonal or vertical) arranged in six different configurations: an acute (45°), right (90°) or obtuse (135°) angle in an L or mirror-L configuration, all lying within the horizontal plane. Participants scanned the first edge (either the horizontal or diagonal/vertical) and then indicated its perceived length by scanning along the second edge for what they perceived to be an equal distance. Results: In contrast to the pattern observed for the visual distortion, haptic scanning errors indicated an HV illusion that was significantly larger for the right angle configurations (average illusion extent: 8.6%) than for the obtuse and acute angles (both 3%). Conclusions: Contrary to what would be predicted if haptic space was calibrated by vision or if a common distortion of space acted on both modalities, we report a dissociation between vision and haptics in the perception of relative length. The pattern of haptic errors followed known motor biases, specifically, the overestimation of radial relative to tangential movements, rather than the distortion observed in the visual sense.
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