Subretinal aspects of the optoretinographic response
Water movement in the living human retina and its regulation are important components of the tissue's structural integrity, optical properties, and homeostasis. In the outer retina there is a continuous flow of water from the subretinal space, through the retinal pigmented epithelium, and into the choroid. This flow is disrupted acutely in disorders such as retinal detachment and central serous retinopathy, and is also known to reduce dramatically with age and age-related macular degeneration. Optoretiongraphy is an emerging technique for measuring neural function in the retina by monitoring nanometer-scale deformations of the membranes of photoreceptors. These deformations have been hypothetically attributed, in part, to osmotic shifts that cause water to move into and out of the photoreceptor outer segment after light stimulation. In the present work, we describe a method for measuring changes in the lengths of the cone outer segment and subretinal space in parallel and results showing that light stimuli change the volume of the subretinal space. These results are consistent with earlier ex vivo measurements of its light-induced hydration. The magnitude of the latter changes depend on the rate of water clearance from the subretinal space, and thus may serve as an indicator of the health of the water transport system. In addition, they may help us understand the mechanisms underlying the photoreceptor optoretinogram. These findings add to a growing understanding of the ways in which light exposure leads to transient reconfigurations of the outer retinal layers lasting milliseconds to hours.