Adaptation to numerosity affects the pupillary light response.

Area of Science:

• Visual perception
• Neuroscience
• Psychophysics

Background:

• The pupillary light response (PLR) is an automatic reflex that regulates light entering the eye.
• Previous research indicated PLR gain is influenced by visual numerosity, with fewer items eliciting weaker responses.
• The role of perceptual adaptation in modulating this automatic response remained unclear.

Purpose of the Study:

• To investigate whether numerosity adaptation affects the pupillary light response.
• To determine if perceived numerosity, rather than actual item count, drives PLR modulation.
• To explore numerosity as a fundamental visual feature influencing subconscious behaviors.

Main Methods:

• Twenty-eight participants underwent numerosity adaptation to either low (10 dots) or high (160 dots) stimulus sets.
• Following adaptation, participants viewed dot arrays (10-40 dots) with constant luminance and either variable or homogeneous dot sizes.
• Pupil size was measured during passive viewing, and adaptation effects were verified psychophysically.

Main Results:

• Adaptation to high numerosity led to a systematic decrease in perceived numerosity compared to low numerosity adaptation.
• Pupillary light responses were significantly smaller after adaptation to high numerosity stimuli.
• This effect persisted regardless of dot size variability, indicating a robust influence of perceived numerosity.

Conclusions:

• Perceptual adaptation to numerosity influences the pupillary light response, demonstrating its automatic nature.
• Numerosity is a primary visual attribute spontaneously processed, affecting even task-irrelevant, unconscious behaviors.
• The findings support the view that the visual system actively estimates and encodes numerosity, impacting physiological responses.