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.

Purpose: Two-photon vision relies on the perception of pulsed infrared light due to two-photon absorption in visual pigments. This study aimed to measure human pupil reaction caused by a two-photon 1040-nm stimulus and compare it with pupil responses elicited by 520-nm stimuli of similar color. Methods: Pupillary light reflex (PLR) was induced on 14 dark-adapted healthy subjects. Three types of fovea-centered stimuli of 3.5° diameter were tested: spirals formed by fast scanning 1040-nm (infrared [IR] laser) or 520-nm (visible [VIS] laser) laser beams and uniformly filled circle created by 520-nm LED (VIS light-emitting diode [LED]). The power of visible stimuli was determined with a dedicated procedure to obtain the same perceived brightness equivalent as for 800 µW used for two-photon stimulation. Results: The minimum pupil diameter for IR laser was 88% ± 10% of baseline, significantly larger than for both VIS stimuli: 74% ± 10% (laser) and 69% ± 9% (LED). Mean constriction velocity and time to maximum constriction had significantly smaller values for IR than for both VIS stimuli. Latency times were similar for IR and VIS lasers and slightly smaller for VIS LED. Conclusions: The two-photon stimulus caused a considerably weaker pupil reaction than one-photon stimuli of the same shape, brightness, and similar color. The smaller pupil response may be due to weaker two-photon stimulation of rods relative to cones as previously observed for two-photon vision. Contrary to normal vision, in a two-photon process the stray light is not perceived, which might reduce the number of stimulated photoreceptors and further weaken the PLR.

The recently introduced term “two-photon vision” relates to the visual perception resulting from a simultaneous absorption of two photons by photoreceptors. In this study, we determined two-photon retinal sensitivity in age-related macular degeneration (AMD) and compared it that in normal aging. Microperimetry was performed with visible (white) light and infrared (IR) light, which was perceived as green in the two-photon stimulation. In total, 45 subjects were included with one (better) eye studied. Furthermore, best-corrected visual acuity (VA) and ocular straylight were assessed. AMD resulted in decreased median (interquartile range) logMAR VA, i.e., 0.15 (0.05; 0.24), which in normal eyes was −0.02 (−0.06; 0.02). The two groups showed comparable straylight levels. Sensitivity to IR light was significantly lower in the AMD group (p < 0.001): 8.3 (7.4, 9.3) dB than in controls 10.7 (9.7, 11.2) dB. AMD also significantly affected visible light sensitivity (p < 0.001): 14.0 (11.0; 15.5) dB vs. 18.0 (16.3; 18.9) dB. Notably, the two-photon approach yielded a lower data spread. In conclusion, AMD considerably impairs retinal sensitivity measured in the single- and two-photon realm. However, two-photon-vision microperimetry may improve the testing accuracy and offer an additional diagnostic parameter (beyond VA measurements) for retinal function assessment.

Intraocular lens (IOL) simulators are gaining increasing interest among researchers and clinicians, as they allow users to experience vision through IOLs without the need for implantation. This study examined the correlation between a look-through simulator and the optical properties of modern IOLs, measured using an optical bench. Subjective defocus responses from five observers using a look-through simulator were compared with the through-focus optical quality of IOLs, assessed by measuring the optical transfer function (OTF) and using its components to predict postoperative visual acuity. The analysis of the agreement between OTF metrics and defocus curves confirmed a strong relationship.

Two-photon absorption occurring in photoreceptors of human eye is responsible for perception of pulsed infrared laser beams. Color of perceived light corresponds to about half of wavelength of stimulating beam. This recently discovered phenomenon is called two-photon vision. However it is yet not clear how such phenomena could impact an ophthalmic diagnosis and therefore further studies are required. In this paper we present the system that enables future psychophysical experiments to extend knowledge about two-photon vision and its diagnostic value by identifying differences between two-photon and single-photon vision. Full Text: PDF ReferencesD. A. Atchison, G. Smith, Optics of the human eye (Butterworth-Heinemann 2000) CrossRef G. Palczewska et al. "Human infrared vision is triggered by two-photon chromophore isomerization", Proceedings of the National Academy of Sciences 111, 1428 (2014). CrossRef P. Artal, S. Manzanera, K. Komar, A. Gambín-Regadera, M. Wojtkowski, "Visual acuity in two-photon infrared vision", Optica 4, 1488 (2017). CrossRef K. Komar et al. "Posterior Eye Shape Measurement With Retinal OCT Compared to MRI", Investigative Ophthalmology & Visual Science 57, 196 (2016). CrossRef K. Komar et al. "Two-photon visual sensitivity of human cones - a psychophysical study", Investigative Ophthalmology & Visual Science 59, 4049 (2018). DirectLink S. A. Burns,R.H. Webb, Handbook of optics 28 (New York, McGraw Hill 1994) DirectLink R. W. Nygaard, T. E. Frumkes, "Calibration of the retinal illuminance provided by maxwellian views", Vision Research (1982) CrossRef P. Melillo et al. "Pupillometric analysis for assessment of gene therapy in Leber Congenital Amaurosis patients", Biomedical engineering online 11, 40 (2012) CrossRef P. A. Stanley, A. K. Davies, "The effect of field of view size on steady-state pupil diameter", Ophthalmic and Physiological Optics, 15, 601 (1995) CrossRef S. Mathôt, "Pupil mimicry is the result of brightness perception of the iris and pupil" Journal of Cognition 1 (2018) CrossRef M. M. Thomas, T. D. Lamb, "Light adaptation and dark adaptation of human rod photoreceptors measured from the a‐wave of the electroretinogram", The Journal of physiology 518, 479 (1999) CrossRef

The aim of this paper is to present a novel method, called Adaptive Edge Detection (AED), of extraction of precise pupil edge coordinates from eye image characterized by reflections of external illuminators and laser beams. The method is used for monitoring of pupil size and position during psychophysical tests of two-photon vision performed by dedicated optical set-up. Two-photon vision is a new phenomenon of perception of short-pulsed near infrared laser beams focused on human retina as having color of half of their wavelength and it is caused by two-photon absorption occurring in human photoreceptors during safe illumination conditions. The AED method is constructed of four basic image processing operations of computational complexity of order approx. O(width*height), which makes possible to use it in real time applications. Furthermore, to achieve high resistance of AED method for presence of light reflections on images as well as other difficulties, we apply machine learning model - Haar feature based cascade (Hfbc) classifier. After implementation of trained Hfbc classifier, our method is able to find pupil edge on images, for which it was failed with this before. Finally, we obtain pupil radius and central pixel coordinates even for images containing eyelashes, changed position of illuminator or presence of reflections caused by stimulating laser beam, which significantly improves pupil edge detection efficiency.