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S. T. L. Chung, Y. Lin; The Two-Dimensional Shape of Spatial Interaction Zones at the PRLs of Observers With Central Vision Loss. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1509.
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Patients with long-standing central vision loss often adopt a peripheral retinal location (the preferred retinal locus, PRL) as the locus for oculomotor and other visual tasks. Our previous work hinted that the PRL might exhibit properties resembling those of the normal fovea instead of the normal periphery as a result of adaptation. Here, we tested whether or not the properties of the spatial interaction zone measured at the PRL resemble those of the normal fovea or the periphery.
Spatial interaction zones were determined for five observers (age: 48 - 83, logMAR acuity: 0.46 - 0.98) with long-standing central vision loss by measuring the extent of spatial interaction along four meridians (0, 45, 90 and 135° from horizontal) with respect to each observer’s PRL. Stimuli were random sequences of three upright-oriented lowercase letters projected on the retina of the eye with better vision using a Rodenstock scanning laser ophthalmoscope (SLO). For each meridian, we used a staircase procedure to track the center-to-center separation between the middle target letter (at a fixed retinal location) and its flankers that yielded 71% correct for identifying the target letter. The retinal location of each observer’s PRL was determined from the SLO images for the correct trials (with the stimuli superimposed on the fundus), assuming that the target letter was centered on the PRL. For comparison, we also measured the spatial interaction zone at the fovea and 10° lower field in a group of 11 age-matched observers with normal vision.
Consistent with previous studies, the size of the spatial interaction zone is larger at 10° lower field than at the fovea for observers with normal vision, and that the shape of the interaction zone is elongated vertically at 10° lower field such that the ratio between the major and minor axes of the zone was 1.7. For observers with central vision loss, the PRLs clustered around the vertical meridian and located between 2.5 and 7.7° from the anatomical fovea. The sizes of the spatial interaction zones centered on the PRLs were similar to the predicted values at the same retinal eccentricities in the normal periphery; however, the shape of the interaction zone became circular instead of elliptical (averaged ratio between the major and minor axes = 1.1).
The circular instead of elliptical shape of the spatial interaction zones obtained at the PRLs of observers with central vision loss suggests that the shape of the interaction zone resembles closer that of the normal fovea instead of the normal periphery, indicating that the PRL might have undergone adaptive changes with time to become more fovea-like.
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