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Ravi Sankar Jonnal, Mehdi Azimipour, Denise Valente, Kari V Vienola, Robert Zawadzki, John S Werner; Optoretinography: noninvasive in vivo measurement of light-evoked responses in human rods and cones. Invest. Ophthalmol. Vis. Sci. 2020;61(7):217.
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© ARVO (1962-2015); The Authors (2016-present)
Imaging photoreceptor function has potential applications in basic science, translational research, and clinical management of ophthalmic disease. It is believed that outer segments (OS) elongate in response to visible stimuli. Here we present a custom adaptive optics (AO) optical coherence tomography (OCT) system equipped with a scanning light ophthalmoscope (SLO), demonstrate its capability to measure these light-evoked responses of rods and cones, and provide evidence that this is a sign of neural function.
The OCT sub-system was a 1.6 MHz swept laser (Optores, λ=1060 nm; Δλ=75 nm), while the AO sub-system was a custom wavefront sensor and 97-actuator deformable mirror (Alpao). The SLO source was a superluminescent diode (Superlum, λ=840 nm; Δλ=30 nm), which also served as the AO beacon. Two subjects were imaged at retinal locations between the foveal center and 8.0°. After AO correction, acquisition of OCT volumes and SLO frames was initiated. After 2 s, a 10-ms stimulus flash was delivered, and imaging continued for 5-8 s. The power of the stimulus was varied to control photopigment bleaching. Images were motion-corrected to permit tracking of single cells over time, whose phase was extracted to measure light-evoked OS elongation.
Rod and cone OS elongated in response to stimuli, with an observed dependence of elongation velocity and maximum excursion on bleaching. Rod responses were found to be larger and slower than cones', and a small minority cones were found to be unresponsive to 555 nm light. Rod responses were observable when bleaching as little as 0.005% of photopigment, and increased with increasing bleaching up to 1%. The corresponding range for cones was ~1% to ~50%.
The optoretinographic responses of the rods and cones may indicate cellular function over a range of stimulus flash intensities. The differences between rod and cone responses agree with known differences in their sensitivity and speed. Given its three-dimensional cellular resolution and high sensitivity, the optoretinogram has the potential to transform the state of objective functional ophthalmic testing.
This is a 2020 ARVO Annual Meeting abstract.
En face projections of the cone-rod mosaic paired with plots of light-evoked OS elongation of individual cells (top and middle). Plots of average elongation of ~10 rods or cones in response to two bleaching flashes (bottom) illustrate the slower but more sensitive response of rods.
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