The pupil responds spontaneously to perceived numerosity.

Area of Science:

• Visual perception
• Neuroscience
• Ophthalmology

Background:

• Pupil size is primarily determined by luminance.
• The pupillary light response (PLR) is also affected by stimulus characteristics and attention.
• The influence of perceived numerosity on the PLR is not well understood.

Purpose of the Study:

• To investigate whether perceived numerosity modulates the pupillary light response.
• To determine if physical or illusory numerosity affects the PLR.
• To explore numerosity as a spontaneously encoded visual feature.

Main Methods:

• Participants passively viewed dot arrays with matched luminance but varying physical or illusory numerosity.
• Stimuli included connected shapes (inducing illusory underestimation) and disconnected dots.
• Pupillary responses (constriction/dilation) were measured in relation to array properties.

Main Results:

• Pupillary constriction to white dot arrays and dilation to black dot arrays were stronger for higher perceived numerosity.
• The magnitude of the pupillary light response scaled with perceived numerosity.
• Both physical and illusory numerosity influenced the PLR.

Conclusions:

• Perceived numerosity significantly modulates the pupillary light response, an automatic reflex.
• This modulation occurs even without explicit tasks or conscious attention.
• Numerosity appears to be a spontaneously encoded feature within the visual system.