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Stephanie Marie Marten-Ellis, Harold E Bedell; Do different mechanisms mediate contour interaction and crowding in the fovea and visual periphery?. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2213.
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© ARVO (1962-2015); The Authors (2016-present)
To compare the influence of nearby flanking lines (contour interaction) and flanking letters (crowding) on the identification of foveal and peripheral letters at different luminances.
Single Sloan letters were presented at the fovea or at 5 deg in the inferior visual field, with and without 4 flanking bars or flanking letters placed symmetrically to the right, left, above and below the target letter. Background luminance was either 200 or 0.5 cd/sq.m. For each combination of eccentricity and background luminance, letter size was adjusted to produce 80 - 90% correct identification when the flanking targets were absent. Percent correct letter identification was then determined for 5 to 8 edge-to-edge separations between the flankers and Sloan letters in 2 normal observers. The bar flankers subtended 1 x 5 letter-stroke widths and the letter flankers had the same dimensions as the letter targets.
At the fovea, the extent of interaction (the maximum target-to-flanker separation that influenced performance) was approximately 3 - 4 min arc for both bar and letter flankers. The magnitude of interaction (the maximum reduction of percent correct letter identification) also was similar for both types of flankers, but decreased dramatically at low compared to high background luminance. At 5 deg in the inferior field, the extent of contour interaction for bar flankers was approximately 4 times smaller than the extent of crowding for letter flankers. Reducing background luminance had no effect on either the extent or the magnitude of peripheral contour interaction or crowding.
At the fovea, bar and letter flanking targets interfere with letter recognition based on the same luminance-dependent mechanism. In the periphery, contour interaction and crowding are independent of background luminance but flanking bars impair identification much less strongly than flanking letters. The results suggest that flanking targets influence foveal and peripheral letter identification by different mechanisms.
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