July 2019
Volume 60, Issue 9
Open Access
ARVO Annual Meeting Abstract  |   July 2019
Refraction and Double Refraction of Light in the Human Macula Generate a Unique Functional Area in which Cones Can Specify Ocular Focus for Accommodation: the FOCAL Annulus
Author Affiliations & Notes
  • Marion Sangster Eckmiller
    C. and O. Vogt Institute for Brain Research, Heinrich Heine University Hospital Duesseldorf, Duesseldorf, NRW, Germany
  • Footnotes
    Commercial Relationships   Marion Eckmiller, None
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Investigative Ophthalmology & Visual Science July 2019, Vol.60, 1403. doi:https://doi.org/
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      Marion Sangster Eckmiller; Refraction and Double Refraction of Light in the Human Macula Generate a Unique Functional Area in which Cones Can Specify Ocular Focus for Accommodation: the FOCAL Annulus. Invest. Ophthalmol. Vis. Sci. 2019;60(9):1403. doi: https://doi.org/.

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      © ARVO (1962-2015); The Authors (2016-present)

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Purpose : Responses of foveal cones to light provide sensory signals about ocular focus for accommodation, but the specific signals used are not known. It was hypothesized that the special structure of the macula could provide it with optical properties that influence cone alignment, enabling foveal cones to specify ocular focus. To test this hypothesis, morphological and optical properties of the macula were analyzed by tracing the theoretical path of light from internal limiting membrane (ILM) to external limiting membrane (ELM).

Methods : Light micrographs of histological sections and published OCT data from human maculas were examined. Thickness of layers, slope of borders and Henle fiber layer (HFL) axons were used to construct a 2D macular template for light tracing. Refraction at the ILM was calculated by Snell's law. Double refraction in the birefringent HFL was calculated using Huygens’s principle.

Results : Light crossing the ILM along the foveal slope was bent peripherally by refraction. Light entering the HFL was split by double refraction into separate tangentially polarized ordinary (O) rays and radially polarized extraordinary (E) rays that bent more. E-rays exiting the HFL bent back and traveled parallel to O-rays to the ELM. At cone apertures O-rays were shifted most (by 0.32°) at 1° eccen., E-rays were shifted most (by 0.69°) at 1.25° eccen.. Cone apertures in an annulus from ∼0.75−1.75° eccen. received light rays shifted by 0.22°−0.69° that originated from eccentric ipsilateral locations in the pupil.

Conclusions : These findings reveal how macular structure can enable a subset of foveal cones to specify ocular focus, supporting the proposed hypothesis. Cones in this annulus are expected to be maximally sensitive to the divergent light they receive and to align parallel to it. This annulus of "misaligned" cones contradicts the Stiles-Crawford effect but exemplifies the Scheiner principle, used to determine focus in optometry. It is proposed that signals from the ensemble of cones in this annulus, termed the “Foveal Oculomonitor Cone Alignment Locus” or FOCAL Annulus, specify ocular focus for accommodation and likely for emmetropization. Assuming the FOCAL Annulus is required for sharp central vision, its dysfunction can disturb central vision and cause refractive disorders (e.g., myopia) in mature and growing eyes.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.


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