Department of Psychology, University of Giessen , Giessen , Germany.
Brain and Mind Institute, Western University , London, Ontario , Canada.
J Neurophysiol. 2019 Mar 1;121(3):996-1010. doi: 10.1152/jn.00199.2018. Epub 2019 Jan 23.
The material-weight illusion (MWI) occurs when an object that looks heavy (e.g., stone) and one that looks light (e.g., Styrofoam) have the same mass. When such stimuli are lifted, the heavier-looking object feels lighter than the lighter-looking object, presumably because well-learned priors about the density of different materials are violated. We examined whether a similar illusion occurs when a certain weight distribution is expected (such as the metal end of a hammer being heavier), but weight is uniformly distributed. In experiment 1, participants lifted bipartite objects that appeared to be made of two materials (combinations of stone, Styrofoam, and wood) but were manipulated to have a uniform weight distribution. Most participants experienced an inverted MWI (i.e., the heavier-looking side felt heavier), suggesting an integration of incoming sensory information with density priors. However, a replication of the classic MWI was found when the objects appeared to be uniformly made of just one of the materials ( experiment 2). Both illusions seemed to be independent of the forces used when the objects were lifted. When lifting bipartite objects but asked to judge the weight of the whole object, participants experienced no illusion ( experiment 3). In experiment 4, we investigated weight perception in objects with a nonuniform weight distribution and again found evidence for an integration of prior and sensory information. Taken together, our seemingly contradictory results challenge most theories about the MWI. However, Bayesian integration of competing density priors with the likelihood of incoming sensory information may explain the opposing illusions. NEW & NOTEWORTHY We report a novel weight illusion that contradicts all current explanations of the material-weight illusion: When lifting an object composed of two materials, the heavier-looking side feels heavier, even when the true weight distribution is uniform. The opposite (classic) illusion is found when the same materials are lifted in two separate objects. Identifying the common mechanism underlying both illusions will have implications for perception more generally. A potential candidate is Bayesian inference with competing priors.
当一个看起来很重（例如石头）的物体和一个看起来很轻（例如泡沫塑料）的物体具有相同的质量时，就会出现物质重量错觉（MWI）。当提起这些物体时，看起来较重的物体比看起来较轻的物体感觉更轻，这大概是因为人们对不同材料密度的先验知识被违反了。我们研究了当预期某种重量分布（例如锤子的金属端较重）但重量均匀分布时是否会出现类似的错觉。在实验 1 中，参与者提起看起来由两种材料（石头、泡沫塑料和木材的组合）制成但被操纵为具有均匀重量分布的二分物体。大多数参与者经历了颠倒的 MWI（即，看起来较重的一侧感觉较重），这表明传入的感觉信息与密度先验知识的整合。然而，当物体看起来均匀地由一种材料制成时（实验 2），发现了经典 MWI 的复制。这两种错觉似乎都与提起物体时使用的力无关。当提起二分物体但要求判断整个物体的重量时，参与者没有体验到错觉（实验 3）。在实验 4 中，我们研究了具有不均匀重量分布的物体的重量感知，再次发现了先前和感觉信息的竞争密度先验知识的整合证据。总的来说，我们看似矛盾的结果挑战了大多数关于 MWI 的理论。然而，用传入感觉信息的可能性对竞争密度先验知识进行贝叶斯整合，可能解释了相反的错觉。新的和值得注意的是：我们报告了一种新的重量错觉，它与物质重量错觉的所有现有解释都相矛盾：当提起由两种材料组成的物体时，看起来较重的一侧感觉较重，即使真实的重量分布是均匀的。当相同的材料在两个单独的物体中提起时，会发现相反的（经典）错觉。确定这两种错觉的共同机制将对更普遍的感知产生影响。一个潜在的候选者是具有竞争先验知识的贝叶斯推理。
