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W.H. Seiple, J.P. Szlyk, M.F. Rabb; The Importance of Accounting for Eccentric Fixation When Measuring Local Electrophysiological and Psychophysical Function . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4760.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To demonstrate the possible errors that can be made when interpreting local electrophysiological and psychophysical results in patients with macular disease. Methods: Local electrophysiological (multifocal electroretinograms – mfERG) and local psychophysical (letter acuity perimetry and Humphrey visual fields) functions were assessed within the central retina of twenty control subjects and five patients with North Carolina Macular Dystrophy – MCDR1. The patients’ results were compared to those of the control subjects in two ways: First, for all tests, equivalent stimulus regions were compared. For example, the mfERG response of a patient’s central hexagon was compared to the averaged responses of the central hexagon for the control subjects. This was then done for all stimulus positions. Second, equivalent retinal regions were compared. For example, a patient’s mfERG response for the central hexagon was compared to the interpolated responses of control subjects’ at the location of the patient’s eccentric fixation. Control responses were then also calculated for all hexagons shifted relative to the patient’s point of fixation. Results: When equivalent stimulus areas were compared, the patients showed significantly decreased function in the central areas for all tests. However, when control subjects were asked to fixate eccentrically and equivalent stimulus areas compared, they also showed central areas of significantly reduced function on all tests. This was true even though the control subjects had completely normal visual function. This erroneous result was caused by comparing responses of different retinal regions. That is, the responses obtained from the peripheral retina will always be reduced relative to the response of the central retina. As expected, when equivalent retinal areas were compared in the eccentrically fixating control subject, there were no regions of abnormal response. A comparison of equivalent retinal areas between the MCDR1 patients’ and the controls subjects yielded areas of dysfunction that corresponded to the funduscopically visible lesions. Conclusions: When assessing local function in patients with macular disease it is important to quantify fixation location and to compare corresponding retinal locations.
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