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Toni Saarela, Michael Herzog; Crowding in multi-element arrays: Regularity of spacing. Journal of Vision 2009;9(8):1017. doi: 10.1167/9.8.1017.
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When a peripheral target is flanked by distracting elements, identification and discrimination of the target's attributes are impaired. This phenomenon is known as crowding. According to Bouma's law, the effectiveness of the distractors depends on their distance from the target, the “critical spacing” being roughly half the target eccentricity.
We have previously reported that the strength of crowding in multi-element arrays (e.g., a row of Gabor patches) depends critically on the spatial layout of the stimuli. In displays where the target forms a coherent texture with the distractors, crowding is strong. When the target stands out from the array because of a difference in for example length, crowding is weak.
Here, we asked whether element spacing in multi-element arrays has a similar effect on crowding. We measured orientation discrimination thresholds for a peripheral target (a line segment or a Gabor patch). The target was flanked on both sides by several distractors that were always vertical but otherwise identical to the target. Discrimination was measured with different element spacings. First, we used regularly spaced arrays, with either relatively tight or relatively wide inter-element spacing. Then, the regularity of the spacing was perturbed by jittering the positions of some or all of the distractors, or by introducing additional distractors into the regularly spaced arrays.
Tight, regular spacing produced stronger crowding (i.e., smaller threshold elevations) than wide regular spacing, as expected. However, with irregular spacing crowding was often weaker than with regular spacing. The average distance of the distractors from the target, or the number of distractors within the critical spacing, did not completely predict crowding strength. Thus, in addition to the effects of element distance, also the regularity of element spacing plays an important role in crowding.
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