(A) A simplified diagram of the neural network. Sensory input excites candidate motor actions and corresponding Go (blue circles) and NoGo (red circles) MSN units in the striatum. Distinct columns of Go and NoGo units provide the thalamus with positive and negative evidence for alternative motor responses, learned via DA reinforcement signals. Positive prediction errors result in increased DA during feedback and an increase of Go excitability relative to NoGo units. TANs are endogenously active units that modulate MSN excitability during reinforcement feedback, altering the efficacy of the reinforcement signal. (B) Stereotypical TAN response is temporally concomitant to reward-related phasic DA increase (Adapted from Figure 7 part C, Morris et al., 2004). (C) Schematic representation of TAN–MSN signaling (see below). TAN firing inhibits presynaptic glutamatergic signaling of D1 and D2 MSNs through M2 receptors, but also selectively excites D2 MSNs via M1 receptors. (D) Sensory input provides excitatory signaling to preSMA (representing two candidate motor actions) and corresponding Go and NoGo MSN units in the striatum. Each of (here, two) motor responses is coded by distinct columns of Go and NoGo units, representing positive and negative evidence for alternative that responses, learned via reinforcement conveyed by DA signals in the SNc. The basal ganglia contribute to action selection by disinhibiting thalamocortical activity for representing thatthe response having the largest Go–NoGo activation differential. Go units inhibit globus pallidus internal segment (GPi) units, which otherwise tonically inhibit the thalamus. NoGo units have the opposite effect by inhibiting the external segment (GPe), which in turn inhibits GPi. TANs are represented as a separate layer of endogenously active units that modulate MSN excitability during the dynamic burst–pause pattern windowing the dopaminergic reward prediction error signals. This pause duration can be fixed, or sensitive to the population uncertainty of MSNs (see below). Dotted black lines correspond to proposed feedback mechanism from MSNs to TANs. The STN modulates the threshold by which actions are disinhibited by the BG, included here for consistency with prior work. DA, dopamine; MSN, medium spiny neuron; preSMA, pre-suplementary motor cortex; SNc, substantia nigra pars compacta; STN, subthalamic nucleus.