Noninvasive, objective measurement of rod function is as significant as that of cone function, and for retinal diseases such as retinitis pigmentosa and age-related macular degeneration, rod function may be a more sensitive biomarker of disease progression and efficacy of treatment than cone function. Functional imaging of single human rod photoreceptors, however, has proven difficult because their small size and rapid functional response pose challenges for the resolution and speed of the imaging system. Here, we describe light-evoked, functional responses of human rods and cones, measured noninvasively using a synchronized adaptive optics optical coherence tomography (OCT) and scanning light ophthalmoscopy (SLO) system. The higher lateral resolution of the SLO images made it possible to confirm the identity of rods in the corresponding OCT volumes.

The integrated Stiles-Crawford function is commonly used as apodization model for vision through the natural eye pupil. However, this method does not account for possible effects related to the retinal thickness, the large length-to-diameter aspect ratio of the photoreceptors, or the use of nonMaxwellian illumination. Here, we introduce a geometrical optics model to calculate the fraction of overlap between light at the retina and the photoreceptor outer segments where absorption triggers vision. The model, which does not account for photoreceptor waveguiding, is discussed for both Maxwellian and nonMaxwellian illumination. The integrated Stiles-Crawford effect is analyzed experimentally with a uniaxial pupil-size flicker methodology and we find that the psychophysical measurements match better to the geometrical optics predictions than direct integration of a Stiles-Crawford function.

Optoretinography (ORG) is an emerging tool for testing neural function in the retina. Unlike existing methods, it is noninvasive and objective, and provides information about retinal structure and function at once. As such, it has great potential to transform ophthalmic care and clinical trials of novel therapeutics designed to restore or preserve visual function. Recent efforts have demonstrated the feasibility of ORG using state-of-the-art optical coherence tomography systems. These methods measure the stimulus-evoked movement of subcellular features in the retina, using the phase of reflected light to monitor their positions. Here we present an alternative approach that monitors the velocity of these features instead. This conceptual shift has significant implications for the nascent field of ORG. By avoiding the need to track specific cells over time, it obviates costly and laborious aspects of position-based approaches, such as adaptive optics, digital aberration correction, real-time tracking, and three-dimensional segmentation and registration. We used this velocity-based approach to measure the photoreceptor ORG responses in three healthy subjects. The resulting responses were reproducible and exhibited dependence on dose and retinal eccentricity. The possibility of reconstructing the position signal through numerical integration of velocity was explored.

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