Contrast Sensitivity Function of Two-Photon Vision

Abstract

Purpose : Humans can perceive pulsed near-infrared light as a visible about halved wavelength by two-photon absorption in photoreceptors (PNAS 111(50) (2014) pp. E5445-E5454). The contrast sensitivity function (CSF) of two-photon vision is not yet known. This study aimed to provide the first data to compare CSF for one- and two-photon vision.

Methods : A custom-built optical system employing 1040 nm and 520 nm pulsed laser beams (200 fs, 76 MHz) was used to measure two-photon and one-photon CSF, respectively, of 3 healthy volunteers (2 women, 1 man, age 24-43). The stimuli were tumbling E of 5 spatial frequencies (3, 6, 8, 15, 18 cyc/deg) formed by scanning laser beam. The background was obtained by Maxwellian view illumination of the retina with white LED. A contrast threshold was determined in a four-alternative forced-choice procedure: by finding the minimum stimulus brightness for which the subject was able to state the letter orientation in 4 of 5 trials correctly. The study was approved by the Ethics Committee of the Collegium Medicum, NCU.

Results : The CSFs were calculated as the reciprocal of Weber contrast. Since the background was one-photon in both cases and there is a lack of luminosity function for two-photon vision, the two-photon CSF was normalized to equalize sensitivities for the maximum value of one-photon CSF. Both curves show a decline from 3 cyc/deg to 18 cyc/deg. However, the two-photon CSF has a slightly broader range than the one-photon CSF – the differences between these ranges were 21, 7, 5 for the individual subjects.

Conclusions : CSF for two-photon vision was shown the first time. The obtained result may suggest that the two-photon CSF has a broader range than CSF of normal vision.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.