title: Smaller = denser, and the brain knows it: natural statistics of object density shape weight expectations.
abstract: If one nondescript object's volume is twice that of another, is it necessarily twice as heavy? As larger objects are typically heavier than smaller ones, one might assume humans use such heuristics in preparing to lift novel objects if other informative cues (e.g., material, previous lifts) are unavailable. However, it is also known that humans are sensitive to statistical properties of our environments, and that such sensitivity can bias perception. Here we asked whether statistical regularities in properties of liftable, everyday objects would bias human observers' predictions about objects' weight relationships. We developed state-of-the-art computer vision techniques to precisely measure the volume of everyday objects, and also measured their weight. We discovered that for liftable man-made objects, "twice as large" doesn't mean "twice as heavy": Smaller objects are typically denser, following a power function of volume. Interestingly, this "smaller is denser" relationship does not hold for natural or unliftable objects, suggesting some ideal density range for objects designed to be lifted. We then asked human observers to predict weight relationships between novel objects without lifting them; crucially, these weight predictions quantitatively match typical weight relationships shown by similarly-sized objects in everyday environments. These results indicate that the human brain represents the statistics of everyday objects and that this representation can be quantitatively abstracted and applied to novel objects. Finally, that the brain possesses and can use precise knowledge of the nonlinear association between size and weight carries important implications for implementation of forward models of motor control in artificial systems.
authors.affiliation: Department of Psychology, University of California Los Angeles, 1285 Franz Hall, Box 951563, Los Angeles, California, 90095-1563, United States of America.
authors.forename: Megan A K
authors.initials: MA
authors.lastname: Peters
authors.affiliation: Department of Computer Science, University of California Los Angeles, 4732 Boelter Hall, Los Angeles, California, 90095, United States of America.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Balzer
authors.affiliation: Department of Psychology, University of California Los Angeles, 1285 Franz Hall, Box 951563, Los Angeles, California, 90095-1563, United States of America; Department of Bioengineering, University of California Los Angeles, 420 Westwood Plaza, 5121 Engineering V, Los Angeles, California, 90095-1600, United States of America.
authors.forename: Ladan
authors.initials: L
authors.lastname: Shams
mesh_terms: D000293:Adolescent; D000328:Adult; D001921:Brain / Q000502:physiology*; D005260:Female; D006801:Humans; D017770:Lifting; D008297:Male; D011597:Psychomotor Performance / Q000502:physiology; D012858:Size Perception / Q000502:physiology*; D014892:Weight Perception / Q000502:physiology*; D055815:Young Adult