Photoreceptor Orientation in Iris Coloboma

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Purpose. Investigate photoreceptor orientation and alignment in an observer with a simple coloboma of the iris and no retinal involvement. Method. Using a psychophysical procedure, extensive measurements of the Stiles-Crawford functions of the first kind (SCE I) were obtained from this observer. Results. In this observer, the usual center-of-the-exit-pupil-of-the-eye alignment tendency of photoreceptors (inferred from the peaks of the SCE I functions) is absent. The inferred orientation of photoreceptors at numerous retinal loci tested lies outside the pupil, opposite the direction of the downwardly displaced pupil center. Conclusions. These results add support to previous suggestions that nonphototropic forces such as retinal tractional forces should be considered as an integral part of the mechanism(s) subserving human photoreceptor alignment in both normal and anomalous observers.

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