Abstract
Purpose :
Two-photon vision originates from two-photon absorption of near infrared light in photoreceptors causing visual sensation. Offering highly localized excitation and no glare, two-photon stimuli are a promising tool in diagnostics of retinal health. Recently we demonstrated pupillary light reflex (PLR) for two-photon stimuli. In contrast to microperimetry which relies on conscious feedback from a patient, PLR can aid objective measurement of retinal sensitivity. Before its benefits can be accessed, careful characterization of two-photon PLR is needed. In this work, pupillary light reflexes upon two-photon excitation with infrared light are compared with those registered for visible light stimuli at half the wavelength.
Methods :
Custom system was built employing two femtosecond laser beams: 1040 nm for two-photon stimulation and its second harmonic as well as 520 nm LED for comparisons. Spiral shaped laser stimuli of outer size 3.5 deg were scanned over the fovea with no perceptible flickering. Projected LED stimulus was also fovea centered and of equal size. Intensities of visible stimuli were adjusted to obtain brightness close to IR ones. PLR of 8 dark-adapted subjects aged 24-41 (4f, 4m) were registered upon 2 s stimulation. Pupil diameter and center was determined with custom software.
The study was approved by the Ethics Committee of the Collegium Medicum, NCU.
Results :
Mean peak constriction between subjects for two-photon stimulus was 17±12% while for visible laser and LED stimuli it was 34±11% and 38±10%, respectively. Mean latencies (~0.4 s) and times to peak constriction (~2 s) were similar for one- and two-photon optotypes.
Conclusions :
Constrictions for 1040-nm two-photon stimuli were significantly smaller than for both 520-nm ones. The latter two optotypes yielded almost equal pupil reactions despite different methods of light delivery to the retina (scanned vs projected). In two-photon case only focal region of the laser beam contributes to the visual sensation, i.e. scattered light is not perceived by the visual system. On the contrary, for one-photon stimuli stray light is perceived and might contribute to the magnitude of PLR effect.
This is a 2020 ARVO Annual Meeting abstract.