Midbrain dopamine firing activity codes reward expectation and motivation in a parametric working memory task

Little is known about how dopamine (DA) neuron firing rates behave in cognitively demanding decision-making tasks. We investigated midbrain DA activity in monkeys performing a vibrotactile frequency discrimination task that required comparing two frequencies presented sequentially in time. We found that DA activity was involved in reward prediction, motivation and working memory (WM). Further, DA phasic responses to the stimuli were greatly affected by a contraction bias. They were also related to motivated behavior on a trial-by-trial basis, exhibiting a larger engagement in more difficult trials. Otherwise, dopamine WM activity was neither tuned to the initial stored frequency nor affected by the bias, although it did code the motivation level and exhibited a ramp-like increase. This result is consistent with a DA role in stabilizing sustained activity, by increasing the gain of prefrontal neurons in a motivation-dependent way. All these DA responses could be potentially related to cognitive control.