Figure 3.

(a) Standard BG model with additional simulated cortical noradrenaline (NA) effects. The locus coeruleus (LC) fires phasically upon sufficient activation of premotor units and reciprocally modulates the gain of these units via simulated NA. (b) Normalized distributions for model reaction times (number of processing cycles before the BG facilitates a response). The LC phasic mode is associated with a narrow distribution of reaction times, peaking at 50 cycles. In the tonic mode (LC units tonically 50% activated), noisy activation of both competing responses leads to a bimodal distribution and overall more RT variability, potentially explaining the variability seen in ADHD. In the ‘supra-tonic’ mode, LC activity was tonically set to maximal firing rates, leading to faster RTs. (c) Per cent accuracy in the same simple choice discrimination simulated to generate RT distributions in panel (b). High accuracy is seen in the phasic LC mode, as premotor responsiveness is boosted only in the presence of a task-relevant stimulus–response association. The tonic and supra-tonic modes lead to activation of alternative noisy responses, which can get inappropriately executed if not dynamically modulated by the LC.