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O. Ehrt, R.F. Hess; Contour interaction and contrast masking in amblyopia . Invest. Ophthalmol. Vis. Sci. 2004;45(13):2583.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: A very important feature of the amblyopic vision is the strong negative effect of nearby stimuli on the discrimination of optotypes. The mechanisms of this contour interaction are still poorly understood. We were interested to determine the contribution of contrast masking (measured as change in detection thresholds) to contour interaction (measured as change in discrimination thresholds) in amblyopes. Methods: We measured contrast thresholds for detecting the presence of a Landolt–C and contrast thresholds for discriminating its orientation on five amblyopic subjects (2 strabismic, 2 anisometropic, 1 mixed; age 22 – 58 y). The dominant eye was tested with a C–size of 0.5° and 0.25°. The C–size for testing the amblyopic eye was scaled to its single optotype VA (0.2 – 0.8). The Landolt–C was presented for 500 ms without and with flanking high contrast bars at separations of 0 – 12 –fold the gap size. Results: Contrast threshold for detection was 0.04 – 0.06 for the dominant as well as for the amblyopic eyes and independent of bar separation. Discrimination threshold however did show significant differences between the dominant and amblyopic eyes. The dominant eyes showed a 60% mean increase (SD 10%) of the threshold for abutting bars compared to the "no–bar" condition for both stimulus sizes. This was identical to normals. The mean increase for amblyopic eyes was 130% (SD 30%) for large an 180% (SD50%) for the discrimination of the small Landolt–C. The size of the interaction zone was much larger for amblyopic as for dominant eyes. Conclusion: Simple contrast masking does not seem to contribute to contour interaction in amblyopia. Most likely higher level visual processes (e.g. feature integration, surround suppression) are involved in this effect that is of major clinical importance for amblyopia screening and monitoring of amblyopia treatment.
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