High-Resolution Retinal Imaging

To investigate the retina under natural conditions, we make strong use of adaptive optics instruments. Custom-engineered adaptive optics scanning light ophthalmoscopes (AOSLOs) can nowadays routinely resolve single rod and cone photoreceptors (smallest diameter ~1 µm). Combining different modalities, such as reflectance, scattering and fluorescence imaging, allows for a more detailed view of retinal structure and function.

Visual Cycle Function

One major challenge we are tackling is the objective assessment of visual cycle activity in rod and cone photoreceptors and the quantification of their mutual interaction. As functional and metabolic processes take place in the photoreceptors, highly efficient fluorophores are generated. By exciting these fluorophores via nonlinear absorption and capturing their fluorescence, we aim to interrogate the biochemistry of photoreceptors during visual cycle activity in their natural environment. In the first instance, we will benchmark visual cycle kinetics in the normal range in healthy retina. Measurements of photoreceptor autofluorescence should then enable the detection of attenuations or slowdowns of the visual cycle in the event of a pathological change in the retina.

Two-Photon Vision

Humans can visually perceive pulsed laser light in the infrared range that is focused onto the retina because it can stimulate the photoreceptors via single- and two-photon absorption by the visual pigment. Both types of absorption occur simultaneously and the probability for either process strongly depends on the illumination parameters. The likelihood for single-photon absorption increases linearly with the average laser power, whereas the likelihood for two-photon absorption increases with the average laser power squared. These distinct dependencies lead to changes in hue perception as the intensity of pulsed laser light projected onto the retina is altered. Pulsed infrared light opens the possibility to generate metamers and visual stimuli largely free from chromatic aberrations, which we aim to characterize and utilize for vision research.

Retinal Light Safety

Safety thresholds of retinal exposures with ultrashort pulsed light and/or scanning beams are still not well-established. The laser safety standards acknowledge a lack of biological data in this regime. The establishment of such thresholds and investigations as to the type of phototoxicity for different exposure paradigms are crucial for the development and translational perspective of novel imaging methods in ophthalmology.