Predicting two-photon retinal sensitivity from visible-light measurements

Two-photon (2P) vision enables human perception of pulsed infrared light through nonlinear absorption in retinal photopigments. To compare how retinal sensitivity varies across eccentricity under 2P and conventional one photon (1P) visible stimulation, we measured detection thresholds from −5° to +5° along the horizontal meridian in 14 participants using a dual wavelength perimeter. Sensitivity increased from the fovea toward the periphery in both modalities, but with markedly different gradients and dynamic ranges. To understand these differences, we extended existing nonlinear excitation models to predict 2P sensitivity from 1P measurements and found excellent agreement between theory and experiment (R 2 = 0.97). The broader and more compressed 2P sensitivity profile measured in this particular retinal area, spanning from the cone-rich fovea to the rod-dominated limits of the tested eccentricity range, appears to result directly from the quadratic dependence of 2P absorption.