Ultra-high contrast retinal display system for single photoreceptor psychophysics

Abstract

Due to the enormous dynamic range of human photoreceptors in response to light, studying their visual function in the intact retina challenges the stimulation hardware, specifically with regard to the displayable luminance contrast. The adaptive optics scanning laser ophthalmoscope (AOSLO) is an optical platform that focuses light to extremely small retinal extents, approaching the size of single photoreceptor cells. However, the current light modulation techniques produce spurious visible backgrounds which fundamentally limit experimental options. To remove unwanted background light and to improve contrast for high dynamic range visual stimulation in an AOSLO, we cascaded two commercial fiber-coupled acousto-optic modulators (AOMs) and measured their combined optical contrast. By compensating for zero-point differences in the individual AOMs, we demonstrate a multiplicative extinction ratio in the cascade that was in accordance with the extinction ratios of both single AOMs. When latency differences in the AOM response functions were individually corrected, single switch events as short as 50 ns with radiant power contrasts up to 1:10 10 were achieved. This is the highest visual contrast reported for any display system so far. We show psychophysically that this contrast ratio is sufficient to stimulate single foveal photoreceptor cells with small and bright enough visible targets that do not contain a detectable background. Background-free stimulation will enable photoreceptor testing with custom adaptation lights. Furthermore, a larger dynamic range in displayable light levels can drive photoreceptor responses in cones as well as in rods.

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

“…The intensities of scattered and out-of-focus IR light are too small to induce 2P or 1P isomerization of visual pigments, resulting in smaller spread of IR visual sensitivity measurements in comparison to measurements with VIS light. Precise stimulation of individual foveal cone photoreceptors with VIS light at 543 nm can also be achieved with advanced optical instrumentation involving Adaptive Optics Scanning Laser Ophthalmoscopy (AO-SLO) and a cascade of acousto-optics modulators [33]. Currently, this method represents an excellent research tool, and also holds great potential for future routine testing of patients.…”

Adaptive Optics for Photoreceptor-Targeted Psychophysics

“…feeding the output of one AOM to the input of a second AOM, a multiplicative extinction ratio can be achieved. Single light switch events as short as 50 ns with radiant power contrasts up to 1:10 10 were demonstrated [77], which essentially spans the normal dynamic range of cone photoreceptors. Psychophysically, this contrast ratio was shown to be sufficient to stimulate single foveal photoreceptor cells with small and bright enough visible targets that do not contain a detectable background light.…”

Versatile Multi-Detector Scheme for Adaptive Optics Scanning Laser Ophthalmoscopy

“…The scanning beam in current AOSLO systems are always visible. The visibility of the scanning raster can confound certain applications of functional testing in AOSLO systems, including microperimetry [37] and visual psychophysics [38,39]. The ability to reduce light exposure and therefore visibility will be an important step towards invisible imaging.…”