Laarni J, Näsänen R, Rovamo J, Saarinen J
Department of Psychology, University of Helsinki, Finland.
Invest Ophthalmol Vis Sci. 1996 Jul;37(8):1706-10.
Visual search for a target, orthogonal to distractors (nontargets) in orientation, has been shown to be independent of the number of distractors (set size). This finding has been thought to indicate that the search occurs spatially in parallel and without capacity limitations. The current study was designed to test whether an orientation difference of 90 degrees between the target and distractor gratings would produce a set-size effect when performance was measured at contrast threshold, that is, whether the threshold contrast at which the target was detected among distractors increased as a function of the number of distractors. The second question studied was whether signal-position uncertainty could explain the possible set-size effect.
The observer searched for a horizontal Gabor patch target in a two-interval, forced-choice task. In the search condition, the target patch was among seven vertical distractor Gabors, all positioned along an isoeccentric circle. The number of possible display locations monitored by the observer varied, and, before each block, he was informed which locations were relevant. In the single-element condition, the target appeared alone, but the number of possible target locations varied as above.
In both conditions, the contrast thresholds almost doubled when the number of possible target locations increased from 1 to 8.
Even though the orientation difference between target and distractors was maximal, a set-size effect was found. The effect could be explained by positional uncertainty.
视觉搜索与干扰项（非目标项）在方向上正交的目标时，已被证明与干扰项数量（集合大小）无关。这一发现被认为表明搜索在空间上并行进行且没有容量限制。本研究旨在测试当在对比度阈值下测量表现时，目标光栅与干扰光栅之间90度的方向差异是否会产生集合大小效应，即，在干扰项中检测到目标的阈值对比度是否会随着干扰项数量的增加而增加。研究的第二个问题是信号位置不确定性是否可以解释可能的集合大小效应。
观察者在两间隔的强制选择任务中搜索水平的加博尔斑块目标。在搜索条件下，目标斑块位于七个垂直的干扰加博尔斑块之中，所有斑块均沿等偏心圆定位。观察者监测的可能显示位置数量有所不同，并且在每个试次组之前，会告知他哪些位置是相关的。在单元素条件下，目标单独出现，但可能的目标位置数量如上述那样变化。
在两种条件下，当可能的目标位置数量从1增加到8时，对比度阈值几乎翻倍。
即使目标与干扰项之间的方向差异最大，仍发现了集合大小效应。这种效应可以用位置不确定性来解释。
