The Truth-Telling Motor Cortex: Response Competition in M1 Discloses Deceptive Behaviour

Recent studies have suggested that circuits associated with response conflict and response inhibition are strongly implicated in deception. Using single-pulse transcranial magnetic stimulation (TMS), we examined whether conflict between competing responses in primary motor cortex (M1) can be used for discriminating between intentionally false and true facial recognition. Participants used little finger and thumb key-presses to lie or tell the truth regarding their familiarity with a series of famous and nonfamous faces. Single-pulse TMS was administered to M1 at three intervals prior to response execution in order to evoke motor evoked potentials (MEPs) in both Abductor Digiti Minimi (ADM) and first dorsal interosseous (FDI) of the right hand. As predicted, we found that the MEP of the nonresponding digit was greater than the MEP of the responding digit when participants prepared to engage in deception, while a mirror-reversed pattern was observed for truth telling. This effect did not interact with the stimulation interval suggesting consistent activation of the motor plan representing the truth throughout the response preparation process. We discuss these results with reference to models of response selection and procedures for the detection of deception.