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M. S. Eckmiller; Integrating the Influences of Lens Pigment, Macular Pigment, and Accommodation in a Model for Cone Alignment. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3077.
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© ARVO (1962-2015); The Authors (2016-present)
How cones align is not known, but accurate alignment of human macular cones is essential for acuity, and alignment may be disturbed in age-related macular degeneration (AMD) (Eckmiller, Prog Ret Eye Res 23: 495, 2004). This study examined the physical factors (of light stimulus, dioptric apparatus, and cones) expected to influence cone alignment, considered how to integrate them in one mechanism, and derived a model for the mechanism of cone alignment.
The model assumed that individual cones compensate for ocular longitudinal chromatic aberration (LCA) by adjusting their alignment angle (Z), using a feedback mechanism (Teetz et al, ARVO 2007) to maximize their cumulative light catch. Accordingly, the optimal Z for a cone depended on stimulus wavelength () spectrum, ocular geometry and chromatic aberrations, cone spectral sensitivity, and optical filtering by lens and macular pigments. Spectral sensitivity functions of M and L cones and spectral filter functions of lens and macular pigments (from the literature) were used to calculate the effective cone spectral sensitivity functions after variable filtering. Ocular geometry and an equation from the literature for LCA (defocus) versus were used to calculate how the incidence angle of light rays of different on cones varied with retinal eccentricity (RE). The functions were combined to determine cone sensitivity (± optical filtering) as a function of defocus. These sensitivity curves were combined with geometrical optics and known ocular dimensions in a quantitative model that defined the photoresponse amplitude of cones at a given RE, as a function of radial Z, optical filtering, and accommodation state. With this model a cone showed its maximal integrated photoresponse at the optimal Z.
At all RE, cones were optimally aligned (approximately) towards the posterior nodal point. At a given RE, M cones aligned towards a more anterior point than L cones (at 3,5° RE their Z differed by 0,66 min). Accommodation changed optimal Z slightly. Optical filtering produced significant facilitative changes (Δ) in optimal Z, e.g., at RE of 3,5°, lens pigment led to ΔZ= 0,36 min for M cones and ΔZ= 0,29 min for L cones, whereas macular pigment led to ΔZ= 0,36 min for M cones and ΔZ= 0,30 min for L cones. The relative importance of the factors influencing cone alignment (and hence acuity) was lens pigment macular pigment > accommodation.
The proposed model for human cone alignment can explain how and why high foveal acuity depends on the presence of adequate ocular lens and macular pigments. These findings may clarify the involvement of aphakia, macular pigment, and refractive errors in AMD.
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