Mass and density estimates contribute to perceived heaviness with weights that depend on the densities' reliability

Abstract

People perceive a smaller and denser object to be heavier than a larger, less dense object of the same mass. We developed a new model of heaviness perception that can explain this size-weight illusion. Modeling followed recent insights on principles of information integration. Perceived heaviness is modeled as a weighted average of one heaviness estimate derived from object mass and another one derived from object density with weights that follow estimate reliabilities. In an experiment, participants judged the heaviness of 18 objects using magnitude estimation methods. Objects varied in mass and density. We also varied the reliability of density information by varying visual reliability: Participants were blindfolded or had strongly impaired, mildly impaired or full vision. Because participants lifted each object via a string they required visual information on object size to assess object density. The pattern of mass and density influences on judged heaviness confirmed model predictions. Also as predicted, density influences on judged heaviness increased with increasing reliability, whereas mass influences decreased. Individual and average data were well fit by the model (r²s > 0.96). Density information contributed for 14%, 21% and 29% to heaviness, when vision was strongly impaired, mildly impaired or not impaired, respectively. Overall, the results highly corroborate our model, which appears to be promising as unifying framework for a number of findings on the size-weight illusion.

Density Estimation is Influenced More by Mass When Objects are Denser

“…We found that density estimates depended both on density and mass. This is similar to previous studies that found that heaviness estimates depended on these two object properties as well [3,13] and the finding that participants had more difficulties reporting a difference in heaviness with objects of different compared to equal densities [9]. Hence, density and heaviness perception might be closely related.…”

“…In contrast, accuracy declines when the factors change in opposite directions [9]. More recently, it was proposed that heaviness estimation is formed from a weighted combination of mass and density information, depending on the properties' reliability [3,13].…”

The weight of expectation: Implicit, rather than explicit, prior expectations drive the size–weight illusion

“…Bottom-up explanations reject the idea of illusions as a failure of the perceptual system, reclassifying the illusory weight difference as the detection of some other action-relevant variable that varies between the stimuli. Candidate variables include the detection of difference in an object's density (Drewing & Tiest, 2013;Grandy & Westwood, 2006), inertia tensor (Amazeen & Turvey, 1996), or some reflection of our perceptual expertise in detecting an object's "throwability"-the relationship between volume and mass, which allows some items to be thrown further than others- (Zhu & Bingham, 2011). By contrast, the top-down explanation is conceptually similar to that put forward for the MWI and golf ball illusion, with cognitive (rather than sensorimotor) expectations interacting with sensory information from the hands and arms to drive the illusion that the objects have different weights from one another.…”