Dopamine precursors depletion impairs impulse control in healthy volunteers

The aim of the present study was to decipher the role of the dopamine system in impulse control. Impulsive actions entail (i) activation of the motor system by an impulse, which is an urge to act and (ii) a failure to suppress that impulse, when inappropriate, in order to prevent an error. These two aspects of action impulsivity can be experimentally disentangled in conflict reaction time tasks such as the Simon task, which measures susceptibility to acting on spontaneous impulses (as well as the proficiency of suppressing these impulses). In 12 healthy volunteers performing a Simon task, dopamine availability was reduced with an amino acid drink deficient in the dopamine precursors, phenylalanine and tyrosine. Classic behavioral measures were augmented with an analysis of the electromyographic activity of the response effectors. Electromyography allows one to detect covert activations undetectable with strictly behavioral measures and further reveals the participants' ability to quickly suppress covert activations before they result in an overt movement. Following dopamine depletion, compared with a placebo condition, participants displayed comparable impulse activation but were less proficient at suppressing the interference from this activation. These results provide evidence that the dopamine system is directly involved in the suppression of maladaptive response impulses.