No abstract

Quick inhibition of approach tendencies in response to signals of potential threats is thought to promote survival. However, little is known about the effect of viewing fearful expressions on the early dynamics of the human motor system. We used the high temporal resolution of single-pulse and paired-pulse transcranial magnetic stimulation (TMS) over the motor cortex to assess corticospinal excitability (CSE) and intracortical facilitation (ICF) during observation of happy, fearful and neutral body postures. To test motor circuits involved in approach tendencies, CSE and ICF were recorded from the first dorsal interosseous (FDI), a muscle involved in grasping, and the abductor pollicis brevis (APB), which served as a control. To test early motor dynamics, CSE and ICF were measured 70–90 ms after stimulus onset. We found a selective reduction in CSE in the FDI when participants observed fearful body expressions. No changes in ICF or in the excitability of APB were detected. Our study establishes an extremely rapid motor system reaction to observed fearful body expressions. This motor modulation involves corticospinal downstream projections but not cortical excitatory mechanisms, and appears to reflect an inhibition of hand grasping. Our results suggest a fast visuo-motor route that may rapidly inhibit inappropriate approaching actions.

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.

The ability to rapidly process others’ emotional signals is crucial for adaptive social interactions. However, to date it is still unclear how observing emotional facial expressions affects the reactivity of the human motor cortex. To provide insights on this issue, we employed single-pulse transcranial magnetic stimulation (TMS) to investigate corticospinal motor excitability. Healthy participants observed happy, fearful and neutral pictures of facial expressions while receiving TMS over the left or right motor cortex at 150 and 300 ms after picture onset. In the early phase (150 ms), we observed an enhancement of corticospinal excitability for the observation of happy and fearful emotional faces compared to neutral expressions specifically in the right hemisphere. Interindividual differences in the disposition to experience aversive feelings (personal distress) in interpersonal emotional contexts predicted the early increase in corticospinal excitability for emotional faces. No differences in corticospinal excitability were observed at the later time (300 ms) or in the left M1. These findings support the notion that emotion perception primes the body for action and highlights the role of the right hemisphere in implementing a rapid and transient facilitatory response to emotional arousing stimuli, such as emotional facial expressions.