Defining filled and empty space: reassessing the filled space illusion for active touch and vision

Area of Science:

• * Psychology
• * Haptic Perception
• * Visual Perception

Background:

• * The filled space illusion typically demonstrates that filled extents are perceived as longer than empty ones.
• * Previous research often used solid surfaces to represent empty space in tactile studies.
• * The effect of comparing filled stimuli to truly empty space (air) in active touch remains underexplored.

Purpose of the Study:

• * To investigate if the filled space illusion in active touch is affected by the nature of the comparator (solid surface vs. empty space).
• * To determine if the illusion's direction changes when comparing gratings to air versus solid surfaces.
• * To compare the tactile findings with the effects observed in visual perception.

Main Methods:

• * Participants actively explored gratings and compared their perceived length to either smooth, solid surfaces or empty space.
• * Direct comparisons were made between perceived lengths of empty space and solid surfaces.
• * Visual perception experiments were conducted using solid and empty extents as comparators.

Main Results:

• * In Experiments 1 and 2, gratings were perceived as longer when compared to smooth, solid surfaces than when compared to empty space.
• * Experiment 3 revealed that empty space was perceived as longer than solid surfaces in direct comparison.
• * Experiment 4 showed gratings were perceived as longer than both solid and empty extents in vision, unaffected by the comparator.

Conclusions:

• * The standard filled space illusion in touch likely requires comparison to a solid surface.
• * Comparing gratings to empty space may reverse the typical filled space illusion.
• * Active touch exhibits a dual nature influencing spatial perception, distinct from visual perception.