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College of Optometry, 320 W 10th Avenue, Columbus, OH, USA; traasch@optometry.osu.edu
The link between central scotomas, Troxler fading, and preferred retinal locus
Keywords: central scotoma; eccentric fixation; fixation instability; Troxler’s phenomenon
Every clinician who has worked with patients with central scotomas has observed the difficulty those patients experience. Not only have they lost the ability to resolve fine detail, they also see nothing "straight ahead". The resulting behaviour, most commonly called "eccentric viewing", typically involves the development of a "pseudofovea", or a preferred retinal locus (PRL). The use of a PRL is primarily an eye movement control issue: the individual must be able to place an object of interest on a particular fundus location. In addition to eye position control, however, successful use of an eccentric location must involve much more than just that. As Deruaz and colleagues have pointed out in this issue of BJO (p 461), higher level sensory processes are likely to be involved.
These authors have made the observation that some patients alternate between two retinal locations when deciphering a word. On further questioning, these patients reported that the displayed word was more distinct immediately after an eye movement. That phenomenon is attributed to the Troxler effect—that is, the fading of an object under stabilised conditions. This observation prompted the authors to conduct a laboratory experiment with normally sighted observers. This experiment is easy enough to replicate as a demonstration. Such a demonstration does seem to support the suggestion that Troxler fading might be experienced by those with central scotomas. We do not know, however, whether Troxler fading contributes to the development of multiple PRLs. As Deruaz et al suggest, saccades and other eye movements that occur naturally, even with a single PRL, probably prevent Troxler fading.
The efficient use of a PRL requires careful eye movement control. It must also require a shift of one’s attention to that location in the visual field. Consider a patient with two PRLs reading a full page of text. Words will fall on both PRLs simultaneously much of the time. In order to make sense of what is being viewed, that patient must be able to focus attention selectively on one or the other PRL. Without that ability, both regions of text would be equally salient.
An area that remains relatively unexplored is binocularity and PRLs. Most of what is known about PRL use applies to monocular viewing conditions. In an individual with bilateral central scotomas, how is the input from the two eyes integrated? Do PRLs tend to develop at corresponding retinal locations? Does a PRL tend to develop at a location that corresponds to a scotomatous area in the other eye? If PRLs develop in both eyes at non-corresponding locations, what happens under binocular conditions? Can both PRLs at non-corresponding points be used in a reading task? Can that person shift attention between those PRLs? Can this person experience confusion as the result of input from both PRLs? Is the input from one eye simply suppressed? We do not know the answers to those questions.
Eccentric viewing training is part of many comprehensive visual rehabilitation programmes. One wonders what should be the goal of eccentric viewing training. Does one reinforce to the trainee that the eccentric point is eccentric, preserving the normal oculocentric direction? Or does one attempt to replace the non-functioning fovea with that eccentric point? That is, should the goal be to shift the oculocentric direction to that eccentric point? If that is possible, it seems that would produce the most effective and effortless eccentric viewing behaviour possible. If the oculocentric direction shifts, however, is it a binocular shift, or does the shift occur independently (and perhaps unequally) for both eyes?
How one learns to view objects in the presence of a central scotoma is a complex issue. We have learned a great deal about this problem, but there is much more that we do not know. Perhaps we will learn whether training helps patients develop skills they otherwise would not develop, what type of training might be most effective, and in which patients training will be most effective. The growing number of visually impaired individuals will be better served when we have better answers to these questions.