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Exploring Visual Processes with Two-Photon Ophthalmoscopy

Periodic Reporting for period 2 - TrackCycle.2P (Exploring Visual Processes with Two-Photon Ophthalmoscopy)

Reporting period: 2021-08-01 to 2023-01-31

The project TrackCycle.2P aims to develop a method to investigate the first steps of light perception in the retina. In the long term, the method is intended to serve for early diagnosis of retinal diseases.
Many retinal diseases that lead to blindness, such as age-related macular degeneration or the genetic disease retinitis pigmentosa, can only be detected at a late stage or with genetic testing. So far, there are hardly any imaging methods for early objective diagnosis. Pathological changes in the retina affect the visual cycle that is initiated at the visual pigment in the photoreceptors of the retina: the visual pigment captures photons of light and subsequently changes to an excited state. This triggers the conversion of the light energy into an electrical impulse, which is transmitted to the brain via the optic nerve and produces the actual visual impression. The visual pigment is then regenerated and can absorb photons again.
It is this process that will be studied with two-photon ophthalmoscopy, a highly accurate method for examining the fundus of the eye. We will establish the measurement method and advance it for application to the human eye. Ultra-short laser pulses are used to cast light onto the retina and measure the visual cycle with a special ophthalmoscope. This is done by taking advantage of the fact that during the course of the visual cycle a molecule is produced, which can be excited to fluoresce.
A major goal of this project is to measure the strength of the fluorescent light in both types of photoreceptors in the retina, the rods and cones, in subjects with healthy eyes in order to learn more about the kinetics of the visual cycles in the normal range. The measurements should enable the detection of attenuations or slowdowns of the visual cycle in the event of a pathological change in the retina. Moreover, the visual experience in people exposed to two-photon ophthalmoscopy will be investigated. This will allow a better insight into the first steps of vision.
During the first 30 months of the project, we have been designing and building the Two-Photon Adaptive Optics Scanning Laser Ophthalmoscope (TP-AOSLO) for the project (see schematic). An optical design has been developed in Zemax and a mechanical design in Fusion 360. Electronical and optical components for the initial setup were carefully selected and characterized. All major parts of the optical system have now been assembled, and first images of a model eye have been acquired. A channel for visual stimulation has been added to the imaging system.
In addition to the work accomplished in the lab, progress has been made by advancing the development of software for computation of maximum permissible exposures, presentation of psychophysical tests and visual stimuli, data processing and estimation of the stimulation efficiency of pulsed infrared light for the distinct photoreceptor types.
An adaptive optics scanning laser ophthalmoscope optimized for safe two-photon imaging in the human eye will be developed. With this instrument, we will quantify the visual cycle in rods versus cones in response to stimulation in healthy human subjects. Particularly the cone visual cycle is not yet fully understood and requires further study. Moreover, the visual experience of subjects exposed to two-photon ophthalmoscopy will be investigated. The technique uses a pulsed laser as imaging source aimed to evoke nonlinear processes in the retina that can potentially be perceived by the subjects. A detailed understanding of these pathways may provide deeper insight into the first steps of vision and will help to design suitable stimulus paradigms to test visual cycle function with two-photon ophthalmoscopy.
Successful implementation of two-photon ophthalmoscopy and application to the human eye promises to broaden our knowledge of normal and abnormal visual cycle function and further our understanding of retinal biochemistry in health and disease.