Stable and dynamic representations of value in the prefrontal cortex

The ability to associate positive and negative outcomes with predictive stimuli allows us to make optimal decisions. These stimulus-value associations are kept up to date by comparing an expected value with the experienced outcome. When a stimulus and its outcome are separated by a delay, the value associated with the stimulus must be held in mind for such comparisons to be possible, however little is known about the neural mechanisms that hold value representations online across delays. Temporarily remembering task-relevant information has been extensively studied in the context of item-specific working memory, and different hypotheses have suggested this ability requires either persistent or transient neuronal activity, with stable or dynamic representations respectively. To test these different hypotheses in the context of value representations, we recorded the spiking activity of neurons in the orbitofrontal and anterior cingulate cortex of two monkeys performing a task in which visual cues predicted a reward delivered after a short delay. We found that features of all hypotheses were simultaneously present in prefrontal activity and therefore no single hypothesis was exclusively supported. Instead, we report mixed dynamics that support robust, time invariant value representations while also encoding the information in a temporally specific manner. We suggest that this hybrid coding is important for optimal behavior and might be a critical mechanism supporting flexible cognitive abilities.