Behavioral inhibition system sensitivity enhances motor cortex suppression when watching fearful body expressions

Influential theories suggest that a defensive behavioral inhibition system (BIS) supports the inhibition of action tendencies when facing potential threats. However, little is known about threat-related inhibitory mechanisms in humans and their relations to inter-individual differences in BIS sensitivity. To address this issue, we used paired-pulse TMS to investigate early human motor cortex (M1) responses to social signals of potential threats, like another's fearful body posture. In two experiments, participants observed pictures of fearful and happy postures, and neutral postures that were either dynamic (in Exp1) or static (in Exp2). To test suppression of M1 excitatory activity, we assessed intracortical facilitation (ICF) in an early phase of threat monitoring by administering TMS pulses at 100-125 ms from picture onset. We investigated the motor representation of hand and arm muscles that are differentially involved in flexion, extension, and abduction. As a control, we also assessed corticospinal excitability and short intracortical inhibition. In both experiments, and independently of the muscle, watching fearful bodies suppressed ICF relative to watching happy and non-emotional (dynamic or static) body expressions. Remarkably, greater fear-related ICF suppression was found in participants who scored higher on a self-report questionnaire assessing BIS sensitivity. These findings suggest that observing fearful body language activates a defensive suppression of M1 excitatory activity that is influenced by the personality disposition to experience fear and anxiety when facing potential threats. This BIS-related motor suppression may have the functional role of transiently suppressing action tendencies to promote threat monitoring and, ultimately, survival.