Abstract
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 neocortex1–4, but the prefrontal cortex seems to have a prominent role4,5 in this context. Specifically, in primates that are extensively trained on a categorization task, neurons in the prefrontal cortex rapidly and flexibly represent learned categories6,7. 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 execution8,9. 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.
Highlights
To determine whether mice had learned categorization, we tested a characteristic feature of category learning, rapid generalization to novel stimuli[10,11,12,13]
Encouraged by this finding, we tested whether the mouse medial prefrontal cortex (PFC) contained neurons that reflected the ability of the mouse to categorize visual stimuli as described above
We chronically monitored neuronal activity in cortical layer 2/3 using two-photon calcium imaging through a microprism implant inserted between the two hemispheres, which enabled optical access to mPFC29 (Fig. 2a–c, Extended Data Fig. 4)
Summary
To determine whether mice had learned categorization, we tested a characteristic feature of category learning, rapid generalization to novel stimuli[10,11,12,13]. All mice learned discriminating categories on the basis of two different rules, and they generalized these rules to novel stimuli, probably by selectively attending[16] to the relevant stimulus feature. Having established this training paradigm, we began tracking neuronal correlates of rule-based categories throughout learning. In naive mice (time point T1), mPFC neurons did not respond to visual stimuli (Fig. 2b, d, Extended Data Fig. 5), but some of these initially non-selective cells clearly showed category selectivity after learning (T5, rule 1) (Fig. 2c, e, Extended Data Fig. 5; neural correlates of other task-related aspects are described below). We defined neurons with a CTI value above 0.1 as category-selective a 16×
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