Miller Lab alumnus David Freedman and colleagues present a model that shows how categorical neural activity can develop through learning.   As a result of top-down influences from decision neurons, categorical representations develop in neurons that show choice-correlated activity fluctuations.  They test the model via recordings from parietal cortex.

Choice-correlated activity fluctuations underlie learning of neuronal category representation
Tatiana A. Engel, Warasinee Chaisangmongkon, David J. Freedman & Xiao-Jing Wang