Abstract
When people judge the weight of two objects of equal mass but different size, they perceive the smaller one as being heavier. Up to date, there is no consensus about the mechanisms which give rise to this size-weight illusion. We recently suggested a model that describes heaviness perception as a weighted average of two sensory heaviness estimates with correlated noise: one estimate derived from mass, the other one derived from density. The density estimate is first derived from mass and size, but at the final perceptual level, perceived heaviness is biased by an object’s density, not by its size. Here, we tested the models’ prediction that weight discrimination of equal-size objects is better in lifting conditions which are prone to the size-weight illusion as compared to conditions lacking (the essentially uninformative) size information. This is predicted because in these objects density covaries with mass, and according to the model density serves as an additional sensory cue. Participants performed a two-interval forced-choice weight discrimination task. We manipulated the quality of either haptic (Experiment 1) or visual (Experiment 2) size information and measured just-noticeable differences (JNDs). Both for the haptic and the visual illusion, JNDs were lower in lifting conditions in which size information was available. Thus, when heaviness perception can be influenced by an object’s density, it is more reliable. This discrimination benefit under conditions that provide the additional information that objects are of equal size is further support for the role of density and the integration of sensory estimates in the size-weight illusion.
Highlights
Human perception of our environment does not match one to one with its physical description, and discrepancies between the two reveal how the brain processes perceptual information [1]
Compared to the string condition just-noticeable differences (JNDs) values of both sets were reduced in the precision grip, small set: t(9) = 3.63, p = 0.005, d = 1.148, large set: t(9) = 3.30, p = 0.009, d = 1.043, and enclosure condition, small set: t(9) = 3.30, p = 0.009, d = 1.043, large set: t(9) = 2.814, p = 0.02, d = 0.89
We observed no difference in JND values between the precision grip and enclosure condition, small set: t(9) = 0.382, p = 0.712, d = 0.121, large set: t(9) = 0.384, p = 0.71, d = 0.121
Summary
Human perception of our environment does not match one to one with its physical description, and discrepancies between the two reveal how the brain processes perceptual information [1]. One very salient discrepancy is given in the size-weight illusion: When people lift two objects of identical mass but different size, they perceive the smaller one as being heavier [2,3,4]. This size-weight illusion can be observed when objects are lifted simultaneously or sequentially using the same hand [5]. The illusion persists when you know that two objects are equal in mass [6] This illusion was first described more than one century ago [7, 8], there is still no consensus on the processes that give rise to this illusion.
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