Daniel M. Keller, PhD
Patients with age-related macular degeneration (AMD) exhibit abnormal eye movement patterns and visual fixations when viewing an image of a face. In a study published in the January 2013 issue of Optometry and Vision Science, researchers suggest that these abnormal patterns may help to explain the deficits in facial recognition experienced by patients with AMD.
William Seiple, PhD, from the New York Eye and Ear Infirmary and Lighthouse International in New York City, and colleagues recruited 9 control patients with normal vision and 9 patients with AMD and eccentric fixation, excluding patients with any other major ophthalmologic problem or any cognitive impairment. The participants with AMD (7 with exudative AMD and 2 with dry AMD; 4 women and 5 men) had an average age of 75 years (range, 61 - 87 years). Their average visual acuity was 0.48 logMAR, based on the Early Treatment Diabetic Retinopathy Study acuity chart.
The investigators used a scanning laser ophthalmoscope/optical coherence tomography instrument (OTI-OPKO Health, Inc) to track eye movement and fixation. Positions of fixation were mapped in the X and Y axes relative to the position of the anatomic fovea, and contour ellipses quantified fixation stabilities. Control participants had fixations in the fovea, and patients with AMD had eccentric loci peripheral to the fovea, designated as the preferred retinal locus (PRL).
The participants were shown a color image of the face of the Mona Lisa painting by Leonardo da Vinci at the center of a screen. The face subtended 8 degrees, which is the approximate size of a face at 1 m (a typical conversation distance). The team tracked fixations during a period of 16 seconds by making scanning laser ophthalmoscope movies of fundus movements, which were later overlaid on an image of the Mona Lisa face, thereby showing the points and durations of fixation.
Several differences distinguished the AMD cohort from the group with normal vision. First, all the patients with AMD had eccentric PRLs ranging from 0.5 to 4.1 degrees from the anatomic fovea. Speaking with Medscape Medical News, coauthor Richard Rosen, MD, vice chairman and director of research and surgeon director and chief of the Retina Service at the New York Eye and Ear Infirmary, explained that to get the best vision, the patients with AMD had to avoid macular lesions. "They will find an area where they see the best.... They're having to move the eye around in sort of an awkward way because the eye is designed for everything to come in right to the center."
Another difference is that fixation stability was significantly poorer among the patients (3.09 log minutes of arc squared; range, 2.3 - 3.7) than among the control participants (2.13 log minutes of arc squared; range, 1.77 - 2.45; P = .007). Among the participants with AMD, fixation was more stable with better visual acuity (P = .025) but decreased with greater PRL eccentricity (P = .01). Furthermore, eccentricity increased as acuity decreased (P = .003).
Fixating on Peripheral Areas
The investigators defined elliptical areas around Mona Lisa's eyes, nose, and mouth, which they designated as "internal features." All points outside of these areas were "external features."
The patients with AMD had fewer fixations on internal areas and more on external areas compared with control participants (P < .001). Thirty-eight percent of patients' fixations were on external areas vs 13% for control participants. For both groups, the greatest proportion of fixations were on the painting's eyes but were lower for patients than for control participants.
"Because [the patients] are not able to use the center of the fovea, they tend to look at more peripheral landmarks as opposed to the eyes, and nose, and mouth, which is what we do if we have normal vision," Dr. Rosen told Medscape Medical News.
Patients with AMD also had more eye movements than the control patients. During the measurement periods, patients with AMD had more (P = .008) and longer (P < .05) saccades compared with control patients, although the control participants averaged 26% longer fixations (P = .06). The angles of the saccades did not differ between the groups.
To eliminate the effects of optic blur as a cause of the differences in fixation patterns, control participants were tested with images blurred to acuity levels equivalent to 20/60 and 20/100. However, even under these blurred conditions, their fixations remained centered on the internal features of the face.
The authors conclude that "when control subjects and patients were given the same instructions, had no practice trials, and were shown an image of a well-known face, individuals with AMD have a different scanning behavior from control subjects."
Elucidating the abnormal scanning behavior of patients with AMD may lead to future treatments. "Potentially by understanding that this is the case...you can think of perhaps ways to train some sort of eye movements," Dr. Rosen said. "One of the projects [Dr. Seiple] has worked on is trying to retrain patients who have these PRLs that they use for their best vision how to re-establish some sort of normal scanning pattern." At this point the training is to help patients read more easily, but Dr. Seiple also plans to work on strategies for better facial recognition.
Pravin Dugel, MD, managing partner of Retinal Consultants of Arizona Ltd, in Phoenix and an expert correspondent for the American Academy of Ophthalmology, told Medscape Medical News that the study confirms "in a very elegant way" what has been known for some time. "What this really shows is that when you do have a lesion that sits in the macula but doesn't kill the entire macula...there's still enough plasticity [of the brain] that we can move our eye around enough to look at objects from those areas of the macula that either are not damaged or not as badly damaged and still put together a picture as best we can," he said.
Dr. Dugel pointed out some limitations of the study, including the small sample size. "And patients that are able to do this kind of task are already self-selected. So it doesn't tell you how common this is in real life," he said. "It doesn't tell you who is able to adapt like this and who's not."
He said the potential therapeutic value of the findings may lie in being able to use the technology to "marry the anatomy with the physiology" to predict who will be able, for example, to drive or not, "and I think we'll be able to make that step fairly soon."
Because of the prevalence of AMD and the importance of the fovea for good, detailed vision, "This is very, very appropriate stuff," Dr. Dugel noted. "It's a thesis-building study. It's not a definitive study, but it's a very important study because it's a very timely study."
There was no commercial funding of the study. Dr. Rosen has consulted in the past for OPKO. Dr. Dugel has disclosed no relevant financial relationships.
Optom Vis Sci. 2013;90:45-56.