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Perceived numerosity, the sense of how many items are in a set, influences the automatic pupillary light reflex. This suggests numerosity is a fundamental visual feature processed even without conscious attention.

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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.