Probing the causal contribution of premotor and parietal cortex in habit formation and expression.

Previous research suggests that habit formation is associated with a decreasing control by the goal-directed system and increasing control of the habit-related brain systems. However, the causal contribution of these systems in human habit formation and expression remains unclear. In the current study, we applied 1-Hz inhibitory repetitive transcranial magnetic stimulation (rTMS) targeting the inferior parietal cortex (IPL) and the dorsal premotor cortex (PMC) to interfere with goal- and habit-related brain systems, respectively. Two groups of participants (IPL/PMC) received real or sham-rTMS on separate days prior to habit training. We found that TMS did not induce significant learning-related changes during habit learning. Importantly, we found a significantly increased habit expression during goal-habit competition following IPL stimulation. In contrast, while numerically, habit expression decreased following PMC stimulation, this effect did not reach statistical significance. Thereby, the current study provides direct causal evidence suggesting that habit expression is significantly influenced by the release of control from the goal-directed system, while evidence regarding the influence of the habit-related system remains less conclusive.