Investigating the causes of stimulus-evoked changes in cone reflectance using a combined adaptive optics SLO-OCT system

In vivo functional imaging of human photoreceptors is an emerging field, with compelling potential applications in basic science, translational research, and clinical management of ophthalmic disease. Measurements of light-evoked changes in the photoreceptors has been successfully demonstrated using adaptive optics (AO) coherent flood illumination (CFI), AO scanning light ophthalmoscopy (SLO), AO optical coherence tomography (OCT), and full-field OCT with digital AO (dAO). While the optical principles and data processing of these systems differ greatly, and while these differences manifest in the resulting measurements, we believe that the underlying physiological processes involved in each of those techniques are likely the same. AO-CFI and AO-SLO systems are more widely used than OCT systems. However, those systems produce only two-dimensional images and so, less can be said about the anatomical and physiological origins of the observed signal. OCT signal, on the other hand, provides 3D imaging but at a cost of high volume of data, making it impractical to clinical purposes. In light of this, we employed a combined AO-OCT-SLO system–with point-for-point correspondence between the OCT and SLO images–to measure functional responses simultaneously with both and investigate SLO retinal functional biomarkers based on OCT response. The resulting SLO images reveal reflectance changes in the cones which are consistent with those previously reported using AO-CFI and AO-SLO. The resulting OCT volumes show phase changes in the cone outer segment (OS) consistent with those previously reported by us and others. We recapitulate a model of the cone OS previously proposed to explain AO-CFI reflectance changes, and show how this model can be used to predict the signal in AO-SLO. The limitations of the model is also discussed in this manuscript.