Abstract

The material-weight illusion (MWI) is one example in a class of weight perception illusions that seem to defy principled explanation. In this illusion, when an observer lifts two objects of the same size and mass, but that appear to be made of different materials, the denser-looking (e.g., metal-look) object is perceived as lighter than the less-dense-looking (e.g., polystyrene-look) object. Like the size-weight illusion (SWI), this perceptual illusion occurs in the opposite direction of predictions from an optimal Bayesian inference process, which predicts that the denser-looking object should be perceived as heavier, not lighter. The presence of this class of illusions challenges the often-tacit assumption that Bayesian inference holds universal explanatory power to describe human perception across (nearly) all domains: If an entire class of perceptual illusions cannot be captured by the Bayesian framework, how could it be argued that human perception truly follows optimal inference? However, we recently showed that the SWI can be explained by an optimal hierarchical Bayesian causal inference process (Peters, Ma & Shams, 2016) in which the observer uses haptic information to arbitrate among competing hypotheses about objects’ possible density relationship. Here we extend the model to demonstrate that it can readily explain the MWI as well. That hierarchical Bayesian inference can explain both illusions strongly suggests that even puzzling percepts arise from optimal inference processes.

Highlights

  • Much of human perception—including illusions—is well described by optimal computations

  • That our competing density priors model can produce material-weight illusion (MWI) magnitudes qualitatively in line with MWI magnitudes observed in the literature supports the theory that the MWI, like the size-weight illusion (SWI), may occur as a result of optimal inference over competing density priors

  • We showed that the MWI can be explained by the exact same computational framework that neatly accounts for the SWI

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Summary

Introduction

Much of human perception—including illusions—is well described by optimal computations. There remained a class of weight-related visuo-haptic illusions that appeared to defy optimality (Buckingham, 2014), calling into question whether human perception is mathematically optimal Two such illusions are the size-weight illusion (SWI), in which the smaller of two equal-mass objects is perceived as feeling heavier (Murray et al, 1999), and the material-weight illusion (MWI), in which an object appearing to be made of a denser material is perceived as feeling lighter than an object that looks like it is made of a less dense material (Harshfield & DeHardt, 1970). A blanket conclusion that human perception is optimal is marred by these outliers, potentially suggesting an alternative computation underlying perceptual processing that may in some cases resemble Bayesian inference but in other cases does not

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