| Summary: | Decision-making not only involves deciding about which action to choose but when and whether to initiate an action in the first place. Macaque monkeys tracked number of dots on a screen and could choose when to make a response. The longer the animals waited before responding, the more dots appeared on the screen and the higher the probability of reward. Monkeys waited longer before making a response when a trial's value was less than the environment's average value. Recordings of brain activity with fMRI revealed that activity in dorsal raphe nucleus (DRN)-a key source of serotonin (5-HT)-tracked average value of the environment. By contrast, activity in the basal forebrain (BF)-an important source of acetylcholine (ACh)-was related to decision time to act as a function of immediate and recent past context. Interactions between DRN and BF and the anterior cingulate cortex (ACC), another region with action initiation-related activity, occurred as a function of the decision time to act. Next, we performed two psychopharmacological studies. Manipulating systemic 5-HT by citalopram prolonged the time macaques waited to respond for a given opportunity. This effect was more evident during blocks with long inter-trial intervals (ITIs) where good opportunities were sparse. Manipulating systemic acetylcholine (ACh) by rivastigmine reduced the time macaques waited to respond given the immediate and recent past context, a pattern opposite to the effect observed with 5-HT. These findings suggest complementary roles for serotonin/DRN and acetylcholine/BF in decisions about when to initiate an action.
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