Cortical shape adaptation transforms a circle into a hexagon: a novel afterimage illusion

Area of Science:

• Visual neuroscience
• Perception psychology

Background:

• Negative afterimages are typically thought to result from retinal bleaching.
• Previous models focused on retinal mechanisms for afterimage formation.

Purpose of the Study:

• To investigate the role of cortical processing in the shape of visual afterimages.
• To determine if cortical adaptation influences afterimage shape.

Main Methods:

• Participants viewed colored geometric shapes (circles, hexagons) on a gray background.
• Observed and reported the shapes of subsequent negative afterimages.
• Tested with both static and rotating stimuli to isolate retinal vs. cortical effects.
• Utilized interocular transfer to confirm binocular cortical involvement.

Main Results:

• Afterimage shapes systematically varied with the shape of the adaptation stimulus.
• Hexagonal afterimages were seen after viewing circles, and circular afterimages after hexagons.
• Rotating stimuli, despite identical retinal stimulation, produced shape-specific afterimages.
• Interocular transfer confirmed a binocular cortical contribution.

Conclusions:

• Afterimage shape is primarily determined by cortical adaptation, not retinal bleaching.
• Cortical mechanisms actively process and reconstruct shape information for afterimages.
• Specialized neural pathways likely detect shape features like corners and curves in the cortex.