Reward and punishment: investigating cortico-bulbar excitability to disclose the value of goods

Indignation for moral violations suppresses the tongue motor cortex: preliminary TMS evidence

We commonly label moral violations in terms of ‘disgust’, yet it remains unclear whether metaphorical expressions linking disgust and morality are genuinely shared at the cognitive/neural level. Using transcranial magnetic stimulation (TMS), we provide new insights into this debate by measuring motor-evoked potentials (MEPs) from the tongue generated by TMS over the tongue primary motor area (tM1) in a small group of healthy participants presented with vignettes of moral transgressions and non-moral vignettes. We tested whether moral indignation, felt while evaluating moral vignettes, affected tM1 excitability. Vignettes exerted a variable influence on MEPs with no net effect of the moral category. However, in accordance with our recent study documenting reduced tM1 excitability during exposure to pictures of disgusting foods or facial expressions of distaste, we found that the vignettes of highly disapproved moral violations reduced tM1 excitability. Moreover, tM1 excitability and moral indignation were linearly correlated: the higher the moral indignation, the lower the tM1 excitability. Respective changes in MEPs were not observed in a non-oral control muscle, suggesting a selective decrease of tM1 excitability. These preliminary findings provide neurophysiological evidence supporting the hypothesis that morality might have originated from the more primitive experience of oral distaste.

Pictures of disgusting foods and disgusted facial expressions suppress the tongue motor cortex

The tongue holds a unique role in gustatory disgust. However, it is unclear whether the tongue representation in the motor cortex (tM1) is affected by the sight of distaste-related stimuli. Using transcranial magnetic stimulation (TMS) in healthy humans, we recorded tongue motor-evoked potentials (MEPs) as an index of tM1 cortico-hypoglossal excitability. MEPs were recorded while participants viewed pictures associated with gustatory disgust and revulsion (i.e. rotten foods and faces expressing distaste), non-oral-related disgusting stimuli (i.e. invertebrates like worms) and control stimuli. We found that oral-related disgust pictures suppressed tM1 cortico-hypoglossal output. This tM1 suppression was predicted by interindividual differences in disgust sensitivity. No similar suppression was found for disgusting invertebrates or when MEPs were recorded from a control muscle. These findings suggest that revulsion-eliciting food pictures trigger anticipatory inhibition mechanisms, possibly preventing toxin swallowing and contamination. A similar suppression is elicited when viewing distaste expressions, suggesting vicarious motor inhibition during social perception of disgust. Our study suggests an avoidant-defensive mechanism in human cortico-hypoglossal circuits and its ‘resonant’ activation in the vicarious experience of others’ distaste. These findings support a role for the motor system in emotion-driven motor anticipation and social cognition.

Enhanced corticobulbar excitability in chronic smokers during visual exposure to cigarette smoking cues

BACKGROUND

Neuroimaging studies of chronic smokers report altered activity of several neural regions involved in the processing of rewarding outcomes. Neuroanatomical evidence suggests that these regions are directly connected to the tongue muscle through the corticobulbar pathways. Accordingly, we examined whether corticobulbar excitability might be considered a somatic marker for nicotine craving.

METHODS

We compared motor-evoked potential (MEP) amplitudes recorded from the tongue and the extensor carpi radialis (control muscle) of chronic smokers under drug withdrawal and intake conditions as well as a nonsmoker group. All participants were tested during passive exposure to pictures showing a smoking cue or a meaningless stimulus. In the intake condition, chronic smokers were asked to smoke a real cigarette (CSn: group 1) or a placebo (CSp: group 2).

RESULTS

Results show that MEP amplitudes recorded from the tongues of participants in the CSn and CSp groups under the withdrawal condition were selectively enhanced during exposure to a smoking cue. However, this effect on tongue MEP amplitudes disappeared in the intake condition for both the CSn and CSp groups.

LIMITATIONS

Limitations include the fact that the study was conducted in 2 different laboratories, the small sample size, the absence of data on chronic smoker craving strength and the different tastes of the real and placebo cigarettes.

CONCLUSION

These results suggest that, in chronic smokers, tongue muscle MEP amplitudes are sensitive to neural processes active under the physiological status of nicotine craving. This finding implicates a possible functional link between neural excitability of the corticobulbar pathway and the reward system in chronic smokers.