Stiles-Crawford effect in infrared two-photon vision.

Area of Science:

• Vision Science
• Photophysics
• Biophysics

Background:

• Pulsed infrared (IR) light can be perceived visually via non-linear two-photon (2P) absorption.
• The Stiles-Crawford effect of the first kind (SCE-I) describes how perceived brightness varies with the pupil entry point of light.
• SCE-I is well-established for visible light but less understood for 2P vision.

Purpose of the Study:

• To compare the intensity of the Stiles-Crawford effect (SCE-I) for visible light versus pulsed IR light perceived through 2P absorption.
• To investigate whether photoreceptor directionality differs between conventional one-photon and 2P excitation pathways.

Main Methods:

• Recruited 11 participants to assess the visual perception of visible and IR light stimuli.
• Measured the Stiles-Crawford effect by analyzing perceived brightness changes based on the location of light entry within the pupil.
• Quantified the characteristic directionality parameter for both visible and IR light conditions.

Main Results:

• The average directionality parameter for IR light was 0.080 ± 0.049 mm⁻².
• The average directionality parameter for visible light was 0.059 ± 0.016 mm⁻².
• The mean difference between IR and visible light was 0.021 ± 0.039 mm⁻² (p=0.11), indicating no statistically significant difference.

Conclusions:

• Photoreceptor directionality responses appear similar for both conventional one-photon (visible light) and two-photon (IR light) excitation.
• The Stiles-Crawford effect intensity is comparable across visible and IR light perceived via 2P absorption.
• These findings support the notion of unified visual processing mechanisms regardless of excitation pathway.