The ability to categorize sensory stimuli is crucial for an animal’s survival in a complex environment. Memorizing categories instead of individual exemplars enables greater behavioural flexibility and is computationally advantageous. Neurons that show category selectivity have been found in several areas of the mammalian neocortex^1,2,3,4, but the prefrontal cortex seems to have a prominent role^4,5 in this context. Specifically, in primates that are extensively trained on a categorization task, neurons in the prefrontal cortex rapidly and flexibly represent learned categories^6,7. However, how these representations first emerge in naive animals remains unexplored, leaving it unclear whether flexible representations are gradually built up as part of semantic memory or assigned more or less instantly during task execution^8,9. Here we investigate the formation of a neuronal category representation throughout the entire learning process by repeatedly imaging individual cells in the mouse medial prefrontal cortex. We show that mice readily learn rule-based categorization and generalize to novel stimuli. Over the course of learning, neurons in the prefrontal cortex display distinct dynamics in acquiring category selectivity and are differentially engaged during a later switch in rules. A subset of neurons selectively and uniquely respond to categories and reflect generalization behaviour. Thus, a category representation in the mouse prefrontal cortex is gradually acquired during learning rather than recruited ad hoc. This gradual process suggests that neurons in the medial prefrontal cortex are part of a specific semantic memory for visual categories.

对感觉刺激进行分类的能力对于动物在复杂环境中的生存至关重要。记住类别而不是单个示例可以实现更大的行为灵活性并且在计算上是有利的。^{在哺乳动物新皮层1,2,3,4}的几个区域中发现了显示类别选择性的神经元，但在这种情况下，前额叶皮层似乎具有突出的作用^4,5。具体来说，在接受过广泛分类任务训练的灵长类动物中，前额叶皮层中的神经元可以快速灵活地表示学习到的类别^6,7. 然而，这些表征如何首先出现在幼稚的动物中仍未得到探索，因此不清楚灵活的表征是作为语义记忆的一部分逐渐建立起来的，还是在任务执行过程中或多或少地立即分配^8,9. 在这里，我们通过重复成像小鼠内侧前额叶皮层中的单个细胞来研究整个学习过程中神经元类别表示的形成。我们表明，小鼠很容易学习基于规则的分类并泛化到新的刺激。在学习过程中，前额叶皮层中的神经元在获得类别选择性方面表现出不同的动态，并且在后来的规则转换过程中表现出不同的参与。神经元子集选择性地和唯一地响应类别并反映泛化行为。因此，小鼠前额叶皮层中的类别表示是在学习过程中逐渐获得的，而不是临时招募的。这个渐进过程表明内侧前额叶皮层中的神经元是视觉类别的特定语义记忆的一部分。