(A) In functional MRI, controls showed greater stop-related (Stop-Signal > Go, orange) and NoGo-related activations (NoGo > Go, green) in the right inferior frontal gyrus (P < 0.05 corrected). Activation overlap is in red. Coordinates are in MNI space. (B) The stop-related right inferior frontal gyrus activation was significantly weaker in Parkinson’s disease-placebo (PD-PLA) than in controls (disease effect, P < 0.05 corrected). Colour scale indicates t-values. (C) Citalopram’s enhancement of right inferior frontal gyrus activation in successful stops positively correlated with UPDRS-motor score (P < 0.05). (D) The drug-induced changes of SSRT (ΔSSRT) and NoGo-error (ΔNoGo-error) respectively related to right inferior frontal gyrus activation changes in successful Stop-Signal trials (ΔStop-Signal-activation) and those in NoGo trials (ΔNoGo-activation).