Abstract
The observation of threatening expression in others is a strong cue for triggering an action response. One method of capturing such action responses is by measuring the amplitude of motor evoked potentials (MEPs) elicited with single pulse TMS over the primary motor cortex. Indeed, it has been shown that viewing whole body expressions of threat modulate the size of MEP amplitude. Furthermore, emotional cues have been shown to act on certain brain areas even outside of conscious awareness. In the current study, we explored if the influence of viewing whole body expressions of threat extends to stimuli that are presented outside of conscious awareness in healthy participants. To accomplish this, we combined the measurement of MEPs with a continuous flash suppression task. In experiment 1, participants were presented with images of neutral bodies, fearful bodies, or objects that were either perceived consciously or unconsciously, while single pulses of TMS were applied at different times after stimulus onset (200, 500, or 700 ms). In experiment 2 stimuli consisted of neutral bodies, angry bodies or objects, and pulses were applied at either 200 or 400 ms post stimulus onset. In experiment 1, there was a general effect of the time of stimulation, but no condition specific effects were evident. In experiment 2 there were no significant main effects, nor any significant interactions. Future studies need to look into earlier effects of MEP modulation by emotion body stimuli, specifically when presented outside of conscious awareness, as well as an exploration of other outcome measures such as intracortical facilitation.
Highlights
Perceiving threat cues from others will likely trigger a fight, flight or freeze response in the observer, supporting the notion that emotion and action readiness are tightly linked (Frijda, 2010; Eder and Rothermund, 2013)
motor evoked potentials (MEPs) are elicited with single pulses of transcranial magnetic stimulation (TMS) to M1, and they can be measured from the targeted muscle using electromyography (EMG)
A 2 × 3 × 2 RM ANOVA showed no main effects for either visibility [F(1,28) = 2.607, p = 0.118, η2p = 0.085], stimulus condition [F(2,56) = 1.002, p = 0.374, η2p = 0.035], or pulse time [F(1,28) = 1.133, p = 0.296, η2p = 0.039]
Summary
Perceiving threat cues from others will likely trigger a fight, flight or freeze response in the observer, supporting the notion that emotion and action readiness are tightly linked (Frijda, 2010; Eder and Rothermund, 2013) This action readiness is reflected in physiological responses such as changes in heart rate, pupil dilation, skin conductance, and muscle activations (Bradley et al, 2008; Roelofs et al, 2010; Huis In ’t Veld et al, 2014), and is evident in the state of the cortical motor system (Un-)conscious Threat Influences on MEPs (e.g., de Gelder et al, 2004; Goldberg et al, 2014). An increase in MEP amplitude in response to fearful faces compared to both other conditions was observed (Schutter et al, 2008)
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