Effective adaptation to the demands of a changing
environment requires flexible cognitive control. The medial and lateral frontal
cortices are involved in such control processes, putatively in close interplay
with the basal ganglia. In particular, dopaminergic projections from the
midbrain (i.e., from the substantia nigra (SN) and the ventral tegmental area
(VTA)) have been proposed to play a pivotal role in modulating the activity in
these areas for cognitive control purposes. In that dopaminergic involvement
has been strongly implicated in reinforcement learning, these ideas suggest
functional links between reinforcement learning, where the outcome of actions
shapes behavior over time, and cognitive control in a more general context,
where no direct reward is involved. Here, we provide evidence from functional
MRI in humans that activity in the SN predicts systematic subsequent
trial-to-trial response time (RT) prolongations that are thought to reflect
cognitive control in a Stop-signal paradigm. In particular, variations in the
activity level of the SN in one trial predicted the degree of RT prolongation
on the subsequent trial, consistent with a modulating output signal from the SN
being involved in enhancing cognitive control. This link between SN activity
and subsequent behavioral adjustments lends support to theoretical accounts
that propose dopaminergic control signals that shape behavior both in the
presence and absence of direct reward. This SN-based modulatory mechanism is
presumably mediated via a wider network that determines response speed in this
task, including frontal and parietal control regions, along with the basal
ganglia and the associated subthalamic nucleus.