Abstract
Purpose :
The 2-photon (2P) effect in vision occurs when light incident on the retina causes a direct photoisomerization of photoreceptor pigments involving two photons at a given wavelength, creating a percept of light corresponding to a 1-photon (1P) process at half that wavelength. Thus, a 2P effect elicited by 940-nm light may appear similar to 470-nm light. Typically, such an effect is achieved using pulsed lasers which compress photons in time. However, with adaptive optics (AO), we can also compress photons in space, further increasing the likelihood of 2P photoisomerization. We investigate the impact of AO correction on the 2P effect.
Methods :
We scanned a focused spot of 940-nm light in a 0.9° x 0.9° raster pattern using an AO scanning laser ophthalmoscope. A supercontinuum light source was used with a ~20-ps pulse and an average power of 198.2 μW. We varied the focusing depth of the raster in and out of the plane of best focus and had subjects make a color and luminance match by tuning the color of a projector-generated square, thereby revealing the 2P effect’s benefit from an AO correction.
Results :
Subjects’ matching data showed a clear increase in luminance and blue hue when the AO-corrected raster was focused on their photoreceptors. Their matches can be interpreted as having a relatively constant red component corresponding to 1P isomerizations at 940 nm superimposed with blue and green components that increase greatly as the raster comes into focus. For 3 subjects, a retinal irradiance of 0.29 W/cm2 of 940-nm light elicited a percept equivalent to an irradiance of 94 nW/cm2 of 470-nm light at best focus. Analysis of matching data yielded an average 1P luminance of 0.14 cd/m2 with the luminance of the 2P effect peaking at 0.57 cd/m2.
Conclusions :
2P vision was greatly enhanced by AO. As AO was used to focus a faintly-visible 940-nm scanning raster onto the retina, the raster appearance changed from red to blue, as if it were a mixture of 940- and 470-nm light. This effect was readily visible, despite our laser using pulses that are quite long compared to the conventional short-pulsed lasers used in 2P imaging (20 ps vs 200 fs). Our results are consistent with the hypothesis that this is a 2P isomerization process by the color appearance and by the strong focus dependency. These data set the technical specifications for future systems that aim to elicit even brighter color appearances via direct 2P activation of human photopigments.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.