Two-photon microperimetry: sensitivity of human photoreceptors to infrared light

Abstract

Microperimetry is a subjective ophthalmologic test used to assess retinal function at various specific and focal locations of the visual field. Historically, visible light has been described as ranging from 400 to 720 nm. However, we previously demonstrated that infra-red light can initiate visual transduction in rod photoreceptors by a mechanism of two-photon absorption by visual pigments. Here we introduce a newly designed and constructed two-photon microperimeter. We provide for the first time evidence of the presence of a nonlinear process occurring in the human retina based on psychophysical tests using newly developed instrumentation. Since infra-red light penetrates the aged front of the eye better than visible light, it has the potential for improved functional diagnostics in patients with age-related visual disorders.

Clinical Application of Infrared-Light Microperimetry in the Assessment of Scotopic-Eye Sensitivity

“…Although this study was not specifically designed to ascertain the spectral sensitivity of the photoreceptor mechanisms mediating IR vision, the perception of a 1045-nm pulsed laser beam as green, suggests M-and L-cone activation. 7,15,20,21 Furthermore, recent publications confirm that both cones and rods play a role in this process. Spectrally sensitive measurements in wild type and rod transducing-α-subunit knockout, Gnat1-/-, mouse photoreceptors, and in primate L-and M-cones demonstrated that mammalian cones and rods can be activated by IR light by a twophoton excitation.…”

“…In Figure 3, we also observed that sensitivity increases beyond the fovea, which is consistent with the results by Rumiński et al who reported a steep increase in macular sensitivity as the stimulus moved from the center to 4°. 15 In that study, however, measurements beyond 5°eccentricity were not performed. Although publications on one-photon rod-mediated vision revealed a gradual increase of sensitivity with eccentricity, [22][23][24] we found that the differences between parafoveal responses were not statistically significant.…”

“…7 Moreover, a subject with documented autosomal cone-mediated recessive achromatopsia, perceived the pulsed IR beam. 7 In another paper, Rumiński et al 15 showed that, despite green color perception, scotopic eye sensitivity differs between IR and visible light. Furthermore, they demonstrated retinal topographic data obtained with both 522.5-and 1045-nm wavelengths showing decreased IR sensitivity as compared to visible light by 0.4.10 9 in the fovea center and 3.3.10 9 in the perifovea.…”

“…522.5 nm), fovea sensitivity to IR light is ninefold lower than to green light, as discussed earlier. 15 This must be realized in making comparisons with studies in the literature where solely visible light was used. Hammond et al measured the visibility threshold in patients aged 20 to 65 years old by projecting a 550-nm stimulus at only one retinal locus (6º temporally), 27 as an earlier study had shown that the sensitivity loss with age does not differ with eccentricity.…”

“…Indeed, a recent study has shown that the use of the method of adjustment yields excellent repeatability of IR-light sensitivity measurements. 15 In the current procedure, the light power was adjusted by rotation of a neutral-density filter (see Fig. 1), with the transmission gradually decreasing with the rotation angle.…”