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Abstract:
Recent studies have investigated the neural basis of value-based decision making using two primary approaches: electrophysiological recordings in monkeys working for juice rewards and fMRI measurements in humans working for monetary rewards. In an effort to better connect these two branches of the literature, we obtained fMRI measurements from both species during performance of a simple foraging task.
Monkeys and humans foraged on red and green visual icons that represented two competing reward sources. At any time only one of these reward sources was presented to the subject, paired with an abstract "switch" icon whose selection changed the reward source presented on the subsequent trial (referred to as changeover-key procedure). Each reward source was independently baited on a variable interval reward schedule with baiting probabilities that changed unpredictably over time. This dynamic element forced the subject to maintain an internal representation of the value of the reward sources in order to harvest rewards efficiently. The only material difference between the tasks used with the two species was the form of the rewards: juice rewards for monkeys, monetary rewards for human. Consistent with our previous work, both species reported their choices with eye movements to the desired icon.
Monkey imaging data were acquired using a purpose built vertical bore 7T magnet, while the human data were gathered in a standard horizontal bore 3T magnet. Identical and common practice techniques were used to analyze both sets of data. Interestingly, the network of brain areas showing task-related BOLD activation was quite similar between the two species. Activation preceding option selection included the frontal eye fields (FEF), the supplemental eye fields (SEF), and regions of the parietal lobe. Activation associated with reward delivery included the striatum and the anterior insula for both monetary and juice rewards. A distributed network of brain areas was activated in correlation with slow variations in local reward rate, a task variable that accounts for behavior and neural activity in similar games (e.g. Sugrue et al, Science, 2004). This network includes many of the areas involved in option selection and reward delivery, but also recruits new areas - most strikingly the Anterior Cingulate Cortex (ACC) of both species.
Our results demonstrate an interspecies correspondence of brain areas activated in value-based decision making, establishing a stronger basis for relating existing monkey and human literature. Importantly, these fMRI results suggest the ACC as high priority target for electrophysiological study (see Sugrue et al, elsewhere at this meeting).
