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Philip B Kruger, Lawrence R Stark; Optical vergence detection across the pupil for accommodation, emmetropization and visual perception. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3778. doi: https://doi.org/.
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The standard “camera” model of vision and the eye is that the intensity and wavelength distributions of the blurred retinal image alone provide all the optical information for vision. Several lines of research suggest that the visual system detects optical vergence directly across the exit pupil. The purpose of this modeling study was to investigate the nature of ray angles at the retina, their relationship to the exit pupil, and their potential to specify the sign of wavefront vergence.
We developed an optical model in which a reduced eye with various levels of spherical defocus views a monochromatic edge target. Given the paraxial, geometrical-optical point-spread function, it is simple to obtain the intensity profile of the retinal image by numerical convolution. The current method extends the convolution concept by cataloging—for each point in the retinal image—the frequency distribution of the incoming ray angles. These ray angles are expressed by the starting point of each incoming ray within the exit pupil.
The bright side of the blurry edge collects rays from all parts of the exit pupil. However, the dark side of the blurry edge has a frequency distribution that becomes progressively restrained to incoming rays from one side of the pupil. For a given target edge profile (e.g. light-dark or dark-light) and for a given frequency distribution on the darker side of the blurry edge, the sign of defocus (myopic or hyperopic) is completely specified. A hypothetical mechanism on the retina (e.g. groups of cone waveguides with different pointing directions in the exit pupil) able to detect ray angle in the exit pupil, also would be able to specify amplitude and sign of defocus based on the characteristic gradual changes of ray angle across the pupil.
The mechanism arising from this geometrical-optical analysis may be conceptualized as an “observer” on the retina who “watches” the exit pupil and sees “with” motion across the pupil in myopic defocus and “against” motion across the pupil in hyperopic defocus. Thus relative motions across retina and pupil provide the sign of defocus, and speed of motion across the pupil provides the amplitude of defocus.
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