Abstract

The inclination to touch objects that we can see is a surprising behaviour, given that vision often supplies relevant and sufficiently accurate sensory evidence. Here we suggest that this ‘fact-checking’ phenomenon could be explained if touch provides a higher level of perceptual certainty than vision. Testing this hypothesis, observers explored inverted T-shaped stimuli eliciting the Vertical-horizontal illusion in vision and touch, which included clear-cut and ambiguous cases. In separate blocks, observers judged whether the vertical bar was shorter or longer than the horizontal bar and rated the confidence in their judgments. Decisions reached by vision were objectively more accurate than those reached by touch with higher overall confidence ratings. However, while confidence was higher for vision rather than for touch in clear-cut cases, observers were more confident in touch when the stimuli were ambiguous. This relative bias as a function of ambiguity qualifies the view that confidence tracks objective accuracy and uses a comparable mapping across sensory modalities. Employing a perceptual illusion, our method disentangles objective and subjective accuracy showing how the latter is tracked by confidence and point towards possible origins for ‘fact checking’ by touch.

Highlights

  • From museum visitors feeling compelled to touch statues that they can see, to the biblical account of the incredulous Thomas who would not accept that Jesus was alive unless he could touch him, tactile ‘fact-checking’ is frequent

  • We investigated how size estimation in both vision and touch was affected by the robust Vertical-Horizontal illusion[4,26,27,28] where a vertical bar appears to be longer than an adjoining horizontal bar of same length

  • Compared to touch, vision was better able to discriminate between different stimulus lengths, and was less subject to the Vertical-Horizontal illusion

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Summary

Introduction

From museum visitors feeling compelled to touch statues that they can see, to the biblical account of the incredulous Thomas who would not accept that Jesus was alive unless he could touch him, tactile ‘fact-checking’ is frequent. Among other factors underlying these complex behaviours, we suggest that the privilege of touch might come from it carrying more evidential weight than seeing when there is ambiguity[3] To test this hypothesis, we compared the confidence that observers put in their perceptual decisions after either seeing or touching stimuli that gave rise to a geometric illusion known as the Vertical-Horizontal (VH) illusion (Figs 1, 2a,b). An ideal observer should be able to decide which of two independent decisions to trust more, based on these comparable confidence ratings This is this common mapping assumption that we tested across touch and vision

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