Enumeration of very small quantities is a common task that we perform everyday. Much research has highlighted that in these conditions humans display fast, near errorless performance, a phenomenon dubbed subitizing. It has been suggested that this regime has a pivotal role in numerosity perception. Here we asked if this system can process multiple sets of items in parallel. At odds with what happens for moderate numerosities, we found a strong impairment caused already by the introduction of a second group of items marked by a different color. Adding shape as a cue provided no benefit. The only case in which subitizing was possible was when the target and distractor group were held constant through the experimental block. These results show the surprising fact that whilst being rapid and errorless, subitizing does not have the capability to disentangle multiple groups of items and deals only with coarse stimulus statistics.

枚举非常小的数量是我们每天执行的一项常见任务。许多研究强调，在这些条件下，人类表现出快速、近乎无差错的表现，这种现象被称为 subitizing。有人提出，该机制在数量感知中具有关键作用。在这里，我们询问该系统是否可以并行处理多组物品。与中等数量发生的情况不同，我们发现引入了以不同颜色标记的第二组项目已经造成了严重的损害。添加形状作为提示没有任何好处。只有当目标和干扰物组在整个实验块中保持不变时，才有可能进行 subitizing。这些结果显示了一个令人惊讶的事实，即在快速且无错误的同时，