Noninvasive imaging of the photoreceptor mosaic response to light stimulation.

In their latest work, Hillman et al. (1) observe optical path length changes of individual photoreceptor cells in response to light stimulation, noninvasively, in a living human subject. This is a remarkable technical feat, showing the reliable extraction of nanometer-scale changes across the photoreceptor mosaic, although involuntary motion of the target tissue is up to five orders of magnitude larger. The approach combines concepts from holography, optical coherence tomography (OCT), and computational adaptive optics, creating a powerful tool for studying vision, retinal disease, and response to treatment. The vision process is initiated by phototransduction, which starts with the absorption of photons in the photoreceptor outer segments by photopigments. Impairments of phototransduction or photopigment regeneration lead to vision loss in numerous inherited retinal conditions, such as blue cone monochromacy and achromatopsia. Importantly, the phototransduction process itself modifies the optical properties of the photoreceptors, by changing the amount of light that they transmit, scatter, or absorb; this allows one to study phototransduction noninvasively using light as a probe. Bleaching, the increase in probe light transmitted through the outer segment when high incident light levels drive a fraction of the photopigment to a nonabsorptive state, is one of the most studied of these phenomena (2). Although bleaching is the most direct result of a light stimulus, scattering (3, 4) and refractive index (5) may also change. These other optical changes, which are more prominent at nearinfrared probe wavelengths, have never been fully explained, in part due to contradictory results in the literature derived from different techniques and species. These poorly understood, nonbleaching signals in the photoreceptors have been grouped under the somewhat ambiguous title of “intrinsic optical signals” (IOSs). How can we pinpoint the origins of the IOSs? One fruitful path has been to use an interferometer, a very sensitive “clock” that measures the time difference, or optical path difference, between two light waves traveling along different paths, by adding them together (Fig. 1). If one or both paths change, the relative phase change of the waves translates into a measurable intensity change. Thus, path length changes can be measured with exquisite precision down to fractions of a wavelength. Jonnal et al. (6) elegantly used the photoreceptors themselves as a biological interferometer, building a flood illumination, adaptive optics ophthalmoscope where the two interfering paths were reflections from the proximal and distal ends of the photoreceptor outer segments (Fig. 1). The resulting oscillations in intensity after a light stimulus suggested that IOSs correlate with a change in the optical path length of the photoreceptors during phototransduction (6). OCT is also an interferometer-based approach in which one of the two interfering light waves is outside Depth-integra on