Equal-magnitude size-weight illusions experienced within and between object categories.

Area of Science:

• Cognitive Psychology
• Human Perception
• Motor Control

Background:

• The size-weight illusion (SWI) is a phenomenon where smaller objects feel heavier than larger objects of equal mass.
• This illusion is often attributed to prior expectations linking object size with mass, common within object categories.

Purpose of the Study:

• To investigate if the SWI occurs across different object categories, challenging the reliance on category-specific size-mass expectations.
• To determine how visual cues, specifically color, influence both the perception of weight and the motor action of lifting.

Main Methods:

• Participants lifted same-colored or differently-colored cubes of identical mass but varying sizes.
• Force application rates during lifting were measured.
• Perceptual judgments of weight were recorded.

Main Results:

• Participants lifting same-colored cubes exhibited a robust SWI and applied greater initial force to the larger object.
• Participants lifting differently-colored cubes showed no difference in initial lifting force but still experienced a strong SWI.
• Color cues selectively modulated the rate of force application without altering the perceived heaviness.

Conclusions:

• The size-weight illusion can occur even when strong category-based size-mass expectations are absent.
• Visual cues like color can dissociate the control of motor actions (force application) from perceptual judgments (weight perception).
• This dissociation highlights distinct mechanisms underlying how prior information influences action versus perception.