Directionality of individual cone photoreceptors in the parafoveal region

Abstract

The pointing direction of cone photoreceptors can be inferred from the Stiles-Crawford Effect of the First Kind (SCE-I) measurement. Healthy retinas have tightly packed cones with a SCE-I function peak either centered in the pupil or with a slight nasal bias. Various retinal pathologies can change the profile of the SCE-I function implying that the arrangement or the light capturing properties of the cone photoreceptors are affected. Measuring the SCE-I may reveal early signs of photoreceptor change before actual cell apoptosis occurs. In vivo retinal imaging with adaptive optics (AO) was used to measure the pointing direction of individual cones at eight retinal locations in four control human subjects. Retinal images were acquired by translating an aperture in the light delivery arm through 19 different locations across a subject’s entrance pupil. Angular tuning properties of individual cones were calculated by fitting a Gaussian to the reflected intensity profile of each cone projected onto the pupil. Results were compared to those from an accepted psychophysical SCE-I measurement technique. The maximal difference in cone directionality of an ensemble of cones, ρ̄, between the major and minor axes of the Gaussian fit was 0.05 versus 0.29 mm−2 in one subject. All four subjects were found to have a mean nasal bias of 0.81 mm with a standard deviation of ±0.30 mm in the peak position at all retinal locations with mean ρ̄ value decreasing by 23% with increasing retinal eccentricity. Results show that cones in the parafoveal region converge towards the center of the pupillary aperture, confirming the anterior pointing alignment hypothesis.

Vision science and adaptive optics, the state of the field

“…Stacey Choi & Nathan Doble (SC & ND): Recent work has centered on the objective measurement of the Stiles-Crawford Effect of the first kind by imaging individual cone photoreceptors and their orientation (Morris et al 2015). Another project has been extending the range of retinal eccentricities over which we can apply AO imaging, and we have imaged rods and cones out to 30° in the temporal and nasal retina (Wells-Gray et al 2015), which is important in the study of diseases such as retinitis pigmentosa (RP).…”

Volumetric integration model of the Stiles-Crawford effect of the first kind and its experimental verification

“…Complementary to psychophysical measurements, objective methods can determine a related optical SCE that has been examined via pupil imaging (Burns, Wu, Delori, & Elsner, 1995;Gorrand & Delori, 1995;van Blokland, 1986), fundus photography (Morris et al, 2015;Roorda & Williams, 2002), scanning laser ophthalmoscopy (Delint, Berendschot, & van Norren, 1997; and optical coherence tomography (Gao, Cense, Zhang, Jonnal, & Miller, 2008). Scattering and waveguiding (Marcos & Burns, 1999;Marcos, Burns, & He, 1998) or waveguide diffraction has been used to model this case (Vohnsen, Iglesias, & Artal, 2005).…”

“…The major reflective element within the photore-ceptors is the high-index mitochondria packed densely in the ellipsoid. This scattered light does not contribute significantly to vision triggered by absorption in the outer segments although a characteristic optical SCE directionality parameter can still be determined (Marcos, Burns, & He, 1998;Morris et al, 2015).…”

Multi-directional optical coherence tomography for retinal imaging

“…Concerning OCT, the IS/OS and the COST were previously found to be highly directionally reflective structures and thus very sensitive to the beam entry position in the pupil plane [ 42 ]. It seems the directionality in terms of reflectivity of the IS/OS-COST complex is more pronounced at peripheral regions in comparison to foveal regions [ 43 – 46 ]. However, without doubt further investigations to better understand the directional reflectivity of the IS/OS-COST complex are required.…”

“…However, without doubt further investigations to better understand the directional reflectivity of the IS/OS-COST complex are required. Nevertheless, already very small irregularities in tissue composition and arrangement will display as hyper- or hyporeflective areas, which might aid in early diagnosis of ophthalmic diseases [ 46 , 47 ].…”