Crowding causes difficulties in judging attributes of an object surrounded by other objects. We investigated crowding for stimuli that isolated either S-cone or luminance mechanisms or combined them. By targeting different retinogeniculate mechanisms with contrast-matched stimuli, we aim to determine the earliest site at which crowding emerges. Discrimination was measured in an orientation judgment task where Gabor targets were presented parafoveally among flankers. In the first experiment, we assessed flanked and unflanked orientation discrimination thresholds for pure S-cone and achromatic stimuli and their combinations. In the second experiment, to capture individual differences, we measured unflanked detection and orientation sensitivity, along with performance under flanker interference for stimuli containing luminance only or combined with S-cone contrast. We confirmed that orientation sensitivity was lower for unflanked S-cone stimuli. When flanked, the pattern of results for S-cone stimuli was the same as for achromatic stimuli with comparable (i.e. low) contrast levels. We also found that flanker interference exhibited a genuine signature of crowding only when orientation discrimination threshold was reliably surpassed. Crowding, therefore, emerges at a stage that operates on signals representing task-relevant featural (here, orientation) information. Because luminance and S-cone mechanisms have very different spatial tuning properties, it is most parsimonious to conclude that crowding takes place at a neural processing stage after they have been combined.

中文翻译：

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拥挤的出现：对比和方向显着性的作用。

拥挤会导致判断被其他物体包围的物体的属性变得困难。我们研究了隔离 S 锥体或亮度机制或将它们组合起来的刺激拥挤情况。通过使用对比匹配的刺激针对不同的视网膜生成机制，我们的目的是确定拥挤出现的最早部位。歧视是在方向判断任务中测量的，其中 Gabor 目标在侧翼之间呈现在中央凹处。在第一个实验中，我们评估了纯 S 锥体和消色差刺激及其组合的侧翼和非侧翼方向辨别阈值。在第二个实验中，为了捕捉个体差异，我们测量了无侧翼检测和方向灵敏度，以及仅包含亮度或与 S 锥对比度相结合的刺激在侧翼干扰下的性能。我们证实，无侧翼 S 锥体刺激的方向敏感性较低。当侧翼时，S 锥体刺激的结果模式与具有可比较（即低）对比度水平的消色差刺激的结果模式相同。我们还发现，只有当方向辨别阈值被可靠地超过时，侧翼干扰才表现出真正的拥挤特征。因此，拥挤出现在对代表任务相关特征（此处为方向）信息的信号进行操作的阶段。由于亮度和 S 锥机制具有非常不同的空间调谐特性，因此最简单的结论是，拥挤发生在它们组合后的神经处理阶段。