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Susana T. L. Chung, Roger W. Li, Dennis M. Levi; Crowding between first- and second-order letter stimuli. Journal of Vision 2006;6(6):806. doi: 10.1167/6.6.806.
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Evidence abounds that the detection of first- and second-order visual stimuli is processed by separate pathways. This study asked whether first- and second-order stimuli remain independent at the stage of processing where crowding occurs. We measured thresholds for identifying a first- or second-order target letter in the presence of two second- or first-order flanking letters, for various letter separations. For comparison, we also measured thresholds when the target and flanking letters were all first- or second-order. Contrast of the flankers was 1.6x their respective contrast thresholds. First-order letters were defined by a luminance difference between the letter and its background. Second-order letters were defined by a differential noise contrast between the group of pixels that made up the letter and the group of pixels that made up the background. Measurements were obtained for each eye of four amblyopes, and at 10° lower field of four normally sighted observers. As expected, across all observers and for both eyes of the amblyopes, threshold elevation (magnitude of crowding) was maximal at the closest letter separation, and decreased as letter separation increased. Threshold elevation was greater for second- than first-order target letters, independent of the order type of flankers. Substantial threshold elevation occurred even when the target and flanking letters were of different order type. Our finding of substantial interaction between first- and second-order stimuli suggests that the processing of these stimuli is not independent at the stage of processing at which crowding occurs.
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