A comment on ‘photoreceptor function in unilateral amblyopia’

Abstract

I would like to direct the readers attention to two misunderstandings that may have arisen from the paper published by DeLint, Weissenbruch, Berendschot and van Norren (1998). (1) Many years ago (1957–1959), Enoch introduced the term ‘receptor amblyopia’ (e.g. see Rynders, Grosvenor & Enoch, 1995, and items a, b for references). This phenomenon was later called by others ‘organic amblyopia’. Enoch had predicted that there were some individuals with amblyopia (really, modestly reduced visual acuity) or retinal disorders who had photoreceptors disturbed in alignment. And he suggested that this disturbance could alter resolution/contrast sensitivity (the term contrast sensitivity was not in use at the time). This effect, if it resulted in some degree of visual acuity loss or amblyopia, would of necessity involve affected groups of photoreceptors when making fine resolution judgements. DeLint et al. implied that Enoch and co-workers were in error, because DeLint et al. were not able to replicate the earlier findings by Enoch and his co-workers. Enoch never stated that such an anomaly might cause strabismic amblyopia, anisometropic amblyopia or visual deprivation amblyopia (the patients described in this report). There are many causes of amblyopia. Disturbed receptor alignment can accompany many conditions, and need not be the singular cause of amblyopia in a patient. Enoch never suggested that an anomaly of receptor alignment was a central issue in amblyopia. Rather, in an era (mid-to-late 1950s) when there was much confusion about the amblyopias as a class, the described retinal receptor anomaly represented one sub-category of visual acuity disorder that might be definable. Enoch and his co-workers have often found anomalous photoreceptor orientation and altered central VA/contrast sensitivity in the presence of ocular diseases or disorders such as wet age-related maculopathy, retinal detachment, central serous chorio-retinopathy, as well as secondary to retinal trauma, retinal degenerative processes, and a variety of tractional insults. Enoch’s arguments were (are) based strictly on the physical properties of fiber optics elements and waveguides. Each photoreceptor waveguide or fiber optics element has a limited (numerical) aperture, and hence exhibits directionally-sensitive/selective acceptance of energy. The efficiency of coupling of radiant energy into the receptor fiber optics element(s) acting as a waveguide(s), and ultimately the response generated in any single receptor element(s) is dependent upon the match and alignment between the aperture of the receptor waveguide(s) and the exit pupil aperture of the eye. In addition, the independence of transmission of radiant energy in a given receptor is dependent upon maintained separation of that fiber from its neighbors. If receptors are effectively separated by less than about one wavelength of light, one encounters ‘frustrated total reflection’ between fibers (a concept similar to ‘cross-talk’ in other settings). Photoreceptor crowding and/or ‘disarray’ of receptors will alter transmission ‘purity’ and efficiency, and affect contrast detection, and distinction between stimuli in affected receptor elements. These features are readily appreciated when viewing an imaged grating or * Tel.: +1-510-642-9694; fax: +1-510-643-5109. E-mail address: jmenoch@socrates.berkeley.edu (J.M. Enoch) 1 To limit the length of this communication, the writer references a book and a chapter which he helped edit some years ago (the following two references a and b). These references contain many added literature citations to points raised above. (a) Enoch, J.M., & Tobey, F.L. Jr. (1981). Vertebrate photoreceptor optics (483 pp.), vol. 23. Heidelberg: Springer-Verlag, Springer Series in Optical Sciences. (b) Enoch, J.M., & Lakshminarayanan, V. (1991). Retinal fibre optics. Chapter 13 in Vision and Visual Dysfunction, vol. 1. in Charman, W.N. (Ed.), Visual optics and instrumentation (pp. 280– 309). London: Macmillan Books.

DOI: 10.1016/S0042-6989(99)00003-6

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Cite this paper

@article{Enoch1999ACO, title={A comment on ‘photoreceptor function in unilateral amblyopia’}, author={Jay M. Enoch}, journal={Vision Research}, year={1999}, volume={39}, pages={2311-2312} }