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A Simplified Method of Identifying the Trained Retinal Locus for Training in Eccentric Viewing

In the typical human visual system, the macula allows for high visual resolution. Damage to this area from diseases, such as age-related macular degeneration (AMD), causes the loss of central vision in the form of a central scotoma (Kanski, 2008). Since no treatment is available to reverse AMD, providing low vision rehabilitation to compensate for the loss of central vision is invaluable for individuals with this condition. Teaching persons with a central scotoma the technique of eccentric viewing to use their remaining peripheral retina to read and perform tasks of daily living has been shown to be effective (Nilsson, Frennesson, & Nilsson, 2003; Petre, Hazel, Fine, & Rubin, 2000; Vukicevic & Fitzmaurice, 2002). It has been reported that persons with AMD can unconsciously adopt an eccentric area of the uncompromised retina, allowing them to achieve better vision. This eccentric area acts as a pseudo-fovea and is termed the preferred retinal locus (PRL) (Schuchard, 1995; Timberlake et al., 1986). Although useful, this spontaneous choice of location may not provide optimal vision for individuals to perform various visual tasks, such as reading, recognizing faces, or performing activities of daily living. Optimizing the location of the retinal area used can be addressed with formal training in eccentric viewing to teach individuals to use a more suitable part of their retina to see by introducing a trained retinal locus (TRL) (Culham, Silver, & Bird, 1990; Fletcher, Schuchard, & Watson, 1999; Nilsson et al., 2003; Vukicevic & Fitzmaurice, 2005, 2009). Although there are many methods for training eccentric viewing, the fundamental principles are equivocal and involve two stages: 1. Identification of the residual areas of the healthy retina by mapping the individual's visual field and 2. Location of the most suitable area to be used as the TRL on the basis of the proximity to the fovea. A number of methods and apparatuses are available to chart the visual field, but the preferred method is by microperimetry with such devices as the Macular Integrity Assessment Microperimeter (CenterVue SpA, Padova, Italy) or the Nidek MP-1 Micro Perimeter (Nidek Technologies Srl, Padova, Italy). Alternately, the visual field can be mapped by tangent screen perimetry, such as the Bjerrum tangent screen. Microperimetry allows clinicians to precisely delineate the borders of the scotoma and the corresponding visible pathology on the retina. The technique is also capable of displaying direct, real-time observation of the retina, and stimuli can be placed on the retina for the purposes of training in eccentric viewing. For this reason, microperimetry is useful (Sunness, Bressler, & Maguire, 1995; Timberlake et al., 1986). Nevertheless, microperimetry is not widely used clinically because it is considered to be the least popular method of identifying the retinal locus for eccentric viewing training (Weisser-Pike, 2008). Low vision practitioners have reported that microperimeters are difficult to use and difficult for clients to understand how to perform the required task. The preference is for simpler methods, such as the Amsler grid or the Bjerrum tangent screen. Clinicians' preference is not the only issue that restricts the clinical use of microperimetry in a low vision setting: it is expensive equipment and hence is generally reserved for research purposes (Manivannan et al., 2001). Low vision practitioners commonly use the Bjerrum tangent screen as a method of mapping the central visual field or the central scotoma or both. The Bjerrum tangent screen is a black felt screen that evaluates the central 30 degrees of the retina at a one- or two-meter (about 3 feet or 6.5 feet) testing distance. The Bjerrum has statico-kinetic properties that allow the borders of the scotoma to be accurately charted (kinetically) and individual test points to be grossly quantified (statically). …