Fixation locus in patients with bilateral central scotomas for targets that perceptually fill in.

Patients who develop central scotomas as a consequence of macular degeneration frequently lose their ability to read, drive, and recognize faces. To compensate for the loss of central vision, these patients must learn to use the peripheral retina to perform tasks normally performed with the fovea or parafovea. Patients typically adopt one or more specific eccentric retinal loci to track and view objects and to read. These loci have been termed preferred retinal loci (PRLs) and their characteristics have been described.1,2 In most patients, the use of a PRL as a “pseudofovea” is incomplete, as their oculomotor behavior demonstrates a lack of complete re-referencing from the fovea. Some patients may continue to image objects of interest in the scotoma by making foveating saccades.3 One of the most frequently used techniques for teaching a patient to use his or her remaining functional vision is eccentric viewing training, in which the patient is taught to move his or her eyes or head so that objects are imaged on healthier peripheral retina.4 Despite its wide implementation as a regular part of low vision services, the benefit of current techniques to train eccentric viewing is debatable.5 Much of the current eccentric viewing training is done without the use of fundus imaging and focuses on helping the patient recognize where he or she has to look to perceive objects and perform visual tasks, such as reading. During such training the specific area on the retina the patient uses to perform tasks is uncertain and the examiner is left to estimate the magnitude of the patient's eye movements, and hence the retinal locus being used, by gross observation. Without dedicated instrumentation, it is not possible to tell if a patient has moved his or her eye sufficiently or too much to perform a specific visual task. In general it remains unclear where precisely the scotoma is in relation to the targets being viewed and whether the patient's scotoma overlaps the target of interest. Scotoma location relative to the PRL also is not clear when performing perimetry using a tangent screen, Humphrey field analyzer or other device that does not image the fundus because the retinal location of fixation is uncertain. A method to predict the retinal location of fixation without fundus imaging would be useful both in training eccentric viewing and in field analysis. The use of a scanning laser ophthalmoscope (SLO) or Nidek MP-1 microperimeter allows the examiner to evaluate eccentric viewing with greater accuracy.6–9 With these instruments the examiner is able to observe an image of the patient's fundus while the patient is performing a task such as fixating a target or reading a sentence. The patient is still unable to perceive the location of his/her scotoma, but the examiner can coach the patient to move his/her eye to a more “optimal” location. These instruments have yet to become implemented widely in routine low vision evaluations, possibly because of limited accessibility, high cost, increased in-office time, and the need for substantial technical skill. Perceptual filling-in, the perception that the color and texture of the surrounding visual field extend into the area of the scotoma, likely complicates training eccentric viewing and oculomotor control and may contribute to the lack of substantial improvement in some patients. Zur and Ullman demonstrated that gratings and regular dot patterns that span the scotomatous region of the visual field are nevertheless perceived by patients with AMD to be uniform and continuous.10 Investigators have also shown that the commonly used Amsler grid is perceived as complete by many patients with dense central scotomas.11,12 Because of perceptual filling-in, patients are generally unaware of their scotoma, which makes it difficult for them to understand how they must move and position their eyes to use their remaining peripheral vision.11 Research to evaluate the effect of perceptual filling-in on visual functioning and to develop techniques that allow patients to visualize their pathological scotomas is needed. On the other hand, perceptual filling-in may allow us to elicit fixation using the vestigial fovea in patients who have not shifted their oculocentric visual direction to the PRL. Large targets that extend beyond the central scotoma are expected to perceptually fill in and patients with absolute scotomas should see these targets as complete and continuous. For example it is expected that when fixating a large cross patients will perceive each of the legs of the cross as complete and continuous without a gap in the middle despite the scotoma being positioned over part of the target. If a patient perceives a target as filled in, we predict that the patient should position the retinal locus that corresponds to the primary oculocentric visual direction near the center of the target. This would be at the vestigial fovea for patients who have not experienced a shift in their oculocentric visual direction. In contrast, the use of small targets that cannot fill in, such as letters, is expected to elicit the use of the patients' PRL. A better understanding of the retinal locus patients use to fixate targets that are expected to perceptually fill in and those that are not may allow us to develop targets that give increased confidence about the retinal location of fixation. These targets may in turn be used when eccentric viewing training and perimetry are performed without imaging the fundus. In addition, it is desirable to determine whether fixation stability is better with any one target, as stability frequently limits what clinical tests can be performed. If targets expected to fill in are imaged at or near the vestigial fovea, these target types could be used to better approximate the projected location of the vestigial fovea and scotoma during eccentric viewing training and visual field testing outside of fundus imaging devices. If on the other hand patients use an alternate retinal locus to view the center of the fill-in targets, it would imply that fill-in targets are unsuited for eliciting fixation at the vestigial fovea and shouldn't be used in eccentric viewing training or perimetry, during which it often is assumed that patients fixate with their vestigial fovea.