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Susana T.L. Chung, Girish Kumar; Do Fixation Strategies Change with Target Size?. Journal of Vision 2016;16(4):38-39. doi: 10.1167/16.4.38.
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The resolving power of the retina is the highest at the center of the fovea and decreases with eccentricity from the fovea. In this study, we asked the question of whether normally sighted subjects use different retinal locations for fixation when targets have different resolution requirements, rather than always fixating using the retinal locus with the highest resolution capability. To address the question, we evaluated the fixation behavior of nine normally sighted young adults using a Tracking Scanning Laser Ophthalmoscope (TSLO) that allows for precise registration of the stimulus location on the retina. Stimuli were Sloan letters, presented singly or in groups of three (trigrams) or five (pentagrams). Letter sizes (full-height) ranged between 5 and 40 arcmin, corresponding to letter size of 20/20 to 20/160 on an acuity chart. Subjects were instructed to fixate at the center of the stimulus (single letter, trigram, pentagram), presented at a random location within the TSLO imaging raster. Three trials (10 sec each) were tested for each condition (letter size x stimulus type). Subjects' retina during each trial was recorded as a video, from which eye movements were extracted using a cross-correlation technique at a sampling rate of 540 Hz. The retinal locus for fixation; fixation stability (quantified using the Bivariate Contour Ellipse Area, BCEA); and the slope of the amplitude spectrum of the eye movements were compared across subjects, letter sizes and stimulus types. Across the different conditions and subjects, BCEA ranged from 0.04 to 0.76 deg2. There are no systematic changes in the retinal locus for fixation, fixation stability and the slope of the amplitude spectrum of fixational eye movements when letter size increases, or for the different stimulus types. Instead, fixation strategies of individual subjects appear to be idiosyncratic, even though the perceptual performance of the subjects was highly similar.
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