Implications of infrared images of the living human cone mosaic for models of fundus reflectance

Abstract

Abstract: : Purpose:Models of fundus reflectance predict that the ratio of the light reflected from the choroid to that from the receptors increases with wavelength, for example by a factor of 6 from 550 nm to 750 nm (Van der Kraats et al., 1996). The light returning from layers behind the receptors is assumed to couple inefficiently back into the receptors. This assumption is supported by experiments that show that autofluorescence from lipofuscin in the RPE is not subject to photoreceptor waveguide effects visualized in the pupil (Prieto et al. OSA, 2000). These results predict that the contrast of the photoreceptor mosaic imaged in vivo with adaptive optics should dramatically decrease with increasing wavelength, by about a factor of 6 from 550 to 750 nm. We tested this prediction by imaging the cone mosaic with the Rochester Adaptive Optics Ophthalmoscope. Methods: We imaged bleached photoreceptors at 1⁰ in the temporal retina in two young human subjects at 550, 650, and 750 nm. To keep image noise constant, the amount of reflected light captured by the imaging camera was equalized for all wavelengths. The sharpest seven images for each wavelength were then registered and summed to increase signal to noise. The contrast of the cone mosaic was inferred from the power spectrum of these registered images. Results:In both subjects, there is a slight tendency for the contrast of images of the cone mosaic to decline with wavelength, but it is surprisingly small. The magnitude of the power spectrum at the cone sampling frequency decreased by less than a factor of 2 for one subject and 1.4 for the other from 550 nm to 750 nm, corresponding to a reduction in contrast of 1.4 and 1.2 respectively. Conclusions:The weak dependence of the contrast of our cone images on wavelength is inconsistent with models of fundus reflectance in which the light returning from layers behind the retina is not coupled efficiently into cones. The high contrast of cone images in infrared light implies that the light scattered back from deep retinal layers must predominantly return through the cone photoreceptors and not through the spaces between cones.

Keywords: photoreceptors • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • retina