The relationship between retinal receptor orientation and photoreceptor optics.

At this time, based on still-restricted studies, it would seem that a fundamental property of vertebrate receptor optics is anterior pointing by receptors. The anterior pointing locus is most probably a point approximating the center of the exit pupil of the eye. There is evidence for the recovery of orientation when that orientation is disturbed. One or more mechanisms mediate that orientation, and we must seek to define and understand their functional properties. We must better define how disturbance in orientation influences vision. Briefly stated, substantially disturbed receptors have reduced light-guiding capability (and hence reduced sensitivity), reduced contrast sensitivity (caused by increased cross talk and so forth), and reduced resolution capability. These combine to creat a detectable (but not necessarily common) form of amblyopia [30]. In a sense, these cases of poor acuity would seem to represent a failure of the photoreceptor alignment system. An interesting set of theories has evolved in relation to receptor alignment in the neonate. There is considerable mechanical hydraulic stress on the newborn ocular vascular system at the time of delivery. Retinal hemorrhages and transient edema of the optic papilla following intracranial pressure rise and rapid decompression later during delivery of the head commonly occur. These can readily causd disturbances in receptor alignment. In some instances the macula is involved. While hemorrhages are apparently rapidly absorbed, if foveal receptors remain misaligned during the critical period (not yet really defined) for the development of vision, central fine-resolution capability may fail to develop. That is, subsequent elaboration of the visual system is dependent upon the quality of the optically transmitted and neurally transformed retinal image during the critical period for visual development. If the capability for realignment exists and occurs after part or all of the critical period, then signs of a prior receptor disarray may no longer be present in later years, but reduced resolution capability can persist. There is interesting literature on the subject [63-68]. K. Simons is currently reviewing this literature as part of his dissertation. Simons points out that the rupture of vessels may in fact partially serve as a safety valve protective of the infant in the presence of these substantive forces, i.e., fine-vessel rupture may limit the potential for damage to the eye. At this stage of investigation it is dangerous to carry speclation too far. Thus not only must we consider the mechanism for orientation, the nature of a possible error signal, and the mechaniisms for maintaning orientation and relating the retina to its substrate, but we must also analyze the consequences of disturbances in these mechanisms in relation to the individual's visual capability. Of equal importance is the need to understand the mechanisms and processes leading to failure of recovery of proper photoreceptor orientation...