Emergence of categorical representations in parietal and ventromedial prefrontal cortex across extended training.

How do the neural representations underlying category learning change as skill develops? We examined perceptual category learning using a prototype learning task known to recruit a corticostriatal system including the posterior striatum, motor cortex, visual cortex, and the intraparietal sulcus (IPS). Male and female human participants practiced categorizing stimuli as category members or nonmembers (A versus not-A) across three days, with fMRI data collected at the beginning and end. Univariate analyses found that corticostriatal activity in regions associated with habitual instrumental learning were recruited across both sessions, but activity in regions associated with goal-directed instrumental learning decreased from day 1 to day 3. Multivoxel Pattern Analysis (MVPA) indicated that after training the trained category could be more easily decoded from the IPS when compared with a novel category. Representational Similarity Analysis (RSA) showed development of category representations in IPS and motor cortex. In addition, RSA revealed evidence for category-related representations including prototype representation in the ventromedial prefrontal cortex which may reflect parallel development of schematic memory for the category structure. Overall, the results converge to show how performance of category decisions and representations of the category structure emerge after extensive training across the corticostriatal system underlying perceptual category learning.Significance Statement We compared activity during initial category learning with that after an extended training session and used multivariate methods to characterize representational changes. We found that representations changed in the intraparietal sulcus (IPS) and ventromedial prefrontal cortex (VMPFC). The IPS became sensitive to category membership and distinguished between the trained category and a novel category. The VMPFC showed sensitivity to the prototype as well as other category-related features. In addition, motor cortex coded for category membership decisions and making associated motor responses. Overall our results go beyond previous research that established what brain regions are recruited during the initial phases of perceptual category learning to characterize how category representations emerge as participants become highly skilled.