Numerosity adaptation resists filtering: Insights from an illusory contour paradigm

Area of Science:

• Cognitive psychology
• Visual perception
• Neuroscience

Background:

• The mechanisms of numerosity perception are debated, with theories suggesting either a dedicated system or reliance on low-level visual features.
• Numerosity adaptation is often cited as evidence for a numerosity-specific mechanism, but alternative explanations involve filtering of previously processed information.

Purpose of the Study:

• To investigate whether numerosity adaptation is driven by a specific mechanism for processing discrete items or by low-level visual features.
• To dissociate numerosity adaptation from filtering effects using a novel adaptation paradigm.

Main Methods:

• Employed a novel adaptation paradigm using Ehrenstein-based illusory dots as adaptors and real dots as test stimuli to disentangle numerosity from low-level features.
• Utilized a second experiment where participants estimated numerosity of real or illusory dots in connected/unconnected configurations to confirm perception of discrete units.

Main Results:

• Found a significant increase in the point of subjective equality (PSE) after adaptation, indicating a genuine numerosity-driven effect.
• Observed a shift in the point of maximal reaction times (RTs) uncertainty in the same direction as PSE, suggesting sensory rather than decisional effects.
• Demonstrated that connected items, including illusory dots, were consistently underestimated, confirming their perception as discrete units.

Conclusions:

• Numerosity adaptation is not driven by low-level visual features or filtering mechanisms.
• Results strongly support the existence of a numerosity-selective system that processes the perceived number of discrete items.