Individual inner retinal neurons imaged in the living eye of monkey and human

Abstract

Purpose : Non-confocal adaptive optics (AO) imaging methods have permitted translucent cellular features to be imaged in the living eye of humans (Chui et al. 2012; Scoles et al. 2014), but inner retinal neurons have remained elusive. A safe in vivo method of imaging inner retinal neurons, particularly retinal ganglion cells (RGCs), is needed to better understand and monitor retinal diseases and evaluate vision restoration methods. We used AO-assisted multi-offset detection (MOD) to image the inner retina of humans and combined it with simultaneous two-photon autofluorescence (TPAF) in a macaque monkey to test the hypothesis that the structures seen in MOD are inner retinal neurons.

Methods : AO scanning light ophthalmoscopy (AOSLO) was used to image several humans and one monkey near the temporal raphe at ~15–20°. MOD used circular apertures of ~3–7 Airy disk diameters offset in radial or hexagonal patterns and combined images from two or more aperture positions to enhance image contrast. In humans, near infrared light (795 nm; ~290 µW) was used for MOD and a confocal image was acquired simultaneously with visible light (680 nm) for registration. In the monkey, the TPAF excitation light (730 nm; ~7mW) was acquired with MOD; a second channel collected TPAF (400–555 nm), while a third (789 nm) was used for registration. Power spectra and manual measurements evaluated the spatial frequency content of images and size of cellular features.

Results : Many visually discernable cell somas were observed in both the TPAF and MOD images in the monkey, permitting direct comparison of individual cells. Putative cells were also observed in humans, though less visible, likely due to the increased noise with the ~24 times lower light levels used for illumination and the increased blur from eye motion induced registration artifacts. The size and spacing of the cellular features seen in both monkey and human were consistent with the known dimensions of RGCs and displaced amacrine cells.

Conclusions : Inner retinal neurons can be imaged in monkey and human with MOD in AOSLO. This capability provides a new means for localizing inner retinal neurons for functional imaging of primates and with refinements could one day provide a safe method for examining the structure of inner retinal neurons in diseases such as glaucoma.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.