Abstract
A gradual drop in visibility with obliquely incident light on retinal photoreceptors is namely described by the Stiles-Crawford effect of the first kind and characterized by a directionality parameter. Using a digital micromirror device in a uniaxial flicker system, here we report on variations of this effect with luminance levels, wavelengths within the visible and near-infrared spectrum and retinal regions ranging from the fovea to 7.5° parafoveal. Results show a consistent directionality in mesopic and photopic conditions. Higher directionality is measured for longer wavelengths, and a decrease with retinal eccentricity is observed. Results are discussed in relation to an absorption model for the visual pigments taking the outer-segment packing and thickness of the neural retina into account. Good correspondence is found without enforcing photoreceptor waveguiding.
Highlights
The efficiency of light to trigger vision when obliquely incident onto the retina is described by the psychophysical Stiles-Crawford effect of the first kind (SCE-I) [1]
Results for the SCE-I measured at the foveal and parafoveal retina at the three different wavelengths are fit to the volumetric overlap absorption model by adjusting the total cone density at each retinal location, the diameter of light beam to comply with percentage of each cone type, as well as the likelihood of having adjacent cones of the same kind
This differs from other parafoveal measurements done by others [3,4], which found that the directionality parameter at 10° nasal parafovea was up to 60% higher, on average, than that at the fovea
Summary
The efficiency of light to trigger vision when obliquely incident onto the retina is described by the psychophysical Stiles-Crawford effect of the first kind (SCE-I) [1]. The SCE-I is characterized monocularly using a subjective visibility comparison of retinal fields produced by two Maxwellian sources of light projected onto the eye’s pupil: one near its center (reference) and another (test) at various locations across the pupil plane [1,2]. The intensity of the test (Itest) and/or the reference (Iref) field is adjusted until the brightness perception of the two is matched and an effective visibility, η, is determined as the intensity ratio at each pupil point of entry, r. This visibility function is fitted to a Gaussian distribution in the pupil plane [2,12]:. The transition from Maxwellian (point) to Newtonian (normal) view is nontrivial and underestimates the role of the SCE-I in normal vision by up to an order of magnitude as we have recently shown using a mechanical flickering pupil with diameter in the range of 1.4 to
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