Stiles-Crawford effect in infrared two-photon vision

Pulsed IR light can be perceived as visible through a non-linear two-photon (2P) absorption process. Similar to visible light, the Stiles-Crawford effect of the first kind (SCE-I) predicts that the perceived brightness in 2P vision depends on the location of the incoming beam in the pupil. This study compared the SC effect intensity for both visible and IR light. The average (±1 SD) characteristic directionality parameter obtained from 11 participants was 0.080 ± 0.049 mm^-2 for the IR light and 0.059 ± 0.016 mm^-2 for the visible light. The mean difference was 0.021 ± 0.039 mm^-2 (95% CI: [-0.005, 0.047]) with a p-value of 0.11, suggesting similar photoreceptor directionality responses to both conventional one-photon and 2P excitation.