Tagging items in spatial working memory: a unit-recording study in the rat medial prefrontal cortex.

The rat medial prefrontal cortex has been suggested to be involved in executive functions and, more specifically, in working memory and response selection. Here, we looked for prefrontal neural correlates as rats performed a modified radial arm maze task that taxed such functions. Rats had to learn the position of four rewarded arms among eight, and visit each rewarded arm only once, thus avoiding repeated visits. In addition, rats were left on the maze after the four successful visits to baited arms until they had visited all the arms twice. Prefrontal neural activity was examined during choice periods, i.e. 2s before the rat entered the arms. We found that a substantial proportion of recorded medial prefrontal neurons were selectively activated before either the first or second visit to the arms irrespective of their reward status, thereby tagging already visited arms. These behavioral correlates show that, within the rodent medial prefrontal cortex, neuronal populations demonstrate behavioral correlates suggestive of its role in guiding prospective search behavior and thus executive functions.