For the most part, neurons in an adult brain have a location, a cell type, a largely pre-determined set of long-range connections, and short range connections determined by location. Locations in the brain tend to have specific functions, or at least a list of possible functions. In that sense, they are not blank slates.
At the same time, how specifically they respond to input, generate an output, coordinate with other nearby cells, and broadly serve any function at all is so dependent on individual variability that it is hard to predict whether any one neuron is serving any specific role at all.
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