Value-based decision making consists of two broad steps of valuation and selection. Previous studies have shown value-related neuronal activity in a number of different brain structures such as striatum1–3
, parietal cortex4–6
, anterior cingulate cortex (ACC)7–8
, orbitofrontal cortex (OFC)8–10
and other parts of the prefrontal cortex (PFC)8,11
in rats and monkeys. Also, after the outcome of the animal’s choice is revealed, neuronal activity related to reward prediction error (RPE), namely the difference between the actual and predicted rewards, is found in multiple brain areas including midbrain dopamine regions12
. These results indicate that valuation of action outcomes engages multiple brain structures.
The neural system responsible for making a choice (selecting a single action out of multiple alternatives) based on values of expected outcomes has been more elusive. Although previous studies have found future choice-related neural signals in several brain areas such as the striatum1
, dorsolateral PFC13
, supplementary eye field (SEF)14
and parietal cortex6,15–16
in monkeys during a free-choice task, choice-related signals in multiple brain structures do not necessarily indicate that they are all involved in the final action selection process. Whereas valuation can be processed in parallel, choice must involve a process that selects a single action out of multiple alternatives that is to be executed by the motor system. Thus, although multiple brain regions might display upcoming choice signals in a given behavioral setting, it is likely that action selection takes place in a specific neural system and then the resulting choice signals propagate to other systems for the purpose of executing or evaluating the chosen action. Therefore, in order to identify the neural system responsible for final action selection, it would be important to compare relative time courses of choice signals across different brain regions and to examine effects of local lesion or inactivation on choice behavior of animals under the same behavioral setting.
Another important question is how different components of value-based decision making, namely value representation, action selection and action evaluation, overlap across different brain structures. Given that many brain areas conveying value signals tend to encode upcoming actions chosen by the animal (see above), it is likely that these functions are served by partially overlapping brain structures. However, the exact distribution of these functions remains unclear, in part because only a few studies have examined all of these signals in multiple brain areas under the same experimental setting (e.g., refs. 8,11,17
We investigated these issues in the rat brain. In our previous studies with rats2,8,18
, value-related neural signals have been found in the lateral OFC, medial PFC (dorsal ACC, prelimbic cortex and infralimbic cortex), dorsal striatum, and ventral striatum. By contrast, clear signals related to upcoming choice were found in none of these brain structures. We therefore hypothesized that upcoming choice signals would be found in a motor structure. The first aim of this study was to test this prediction, which was supported by the data. The second aim was to test whether upcoming choice signals co-exist with value signals at the final stage of decision making. Our results showed that the secondary motor cortex of the rat processes both action selection and value signals.