Abstract
Densitometry is a powerful tool for the biophysical assessment of the retina. Until recently, this was restricted to bulk spatial scales in living humans. The application of adaptive optics (AO) to the conventional fundus camera and scanning laser ophthalmoscope (SLO) has begun to translate these studies to cellular scales. Here, we employ an AOSLO to perform dynamic photopigment densitometry in order to characterize the optical properties and spectral types of the human cone photoreceptor mosaic. Cone-resolved estimates of optical density and photosensitivity agree well with bulk estimates, although show smaller variability than previously reported. Photopigment kinetics of individual cones derived from their selective bleaching allowed efficient mapping of cone sub-types in human retina. Estimated uncertainty in identifying a cone as long vs middle wavelength was less than 5%, and the total time taken per subject ranged from 3–9 hours. Short wavelength cones were delineated in every subject with high fidelity. The lack of a third cone-type was confirmed in a protanopic subject. In one color normal subject, cone assignments showed 91% correspondence against a previously reported cone-typing method from more than a decade ago. Combined with cone-targeted stimulation, this brings us closer in studying the visual percept arising from a specific cone type and its implication for color vision circuitry.
Highlights
Light capture by photoreceptors is the gateway for vision
Retinal reflectance is not entirely attributed to photopigment kinetics. Other factors such as interference artifacts from cone outer segment tips, [30, 31, 33, 34]light-evoked intrinsic signals [32], light scattered from inner retina and the anterior ocular optics all contaminate the reflectance signal attributable to photopigment
The light dose used for bleaching and imaging was low, i.e. 3–6 μW at the cornea allowing the measure of high apparent cone optical density and dense sampling of pigment kinetics following dark adaptation
Summary
Light capture by photoreceptors is the gateway for vision. The optical properties, spatial sampling and spectral topography of the photoreceptor mosaic set the primary constraints on the fidelity of luminance and color vision. The distribution and arrangement of short wavelength cone (S-cone) photoreceptors has been studied extensively in the last three decades [1,2,3]. The similarity in the morphological and biochemical characteristics of long and middle-wavelength cones (L- and M-cones) has made their differentiation challenging. Global and indirect measures of cone activity from electroretinogram, microspectrophotometry and perception have been used to determine relative cone numerosity [4,5,6,7].
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