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

Introduction

Disgust is a disease-avoidance emotion affecting several aspects of life, as it may influence how we select what we eat, our friends, sexual partners, what social group we adhere to, the clothing we wear, the music we listen to and, probably, our concept of morality (Rozin and Fallon, 1987^; Rozin et al., 2000^; Oaten et al., 2009). Despite the 140 years that have elapsed since Charles Darwin published his influential work, The Expression of the Emotions in Man and Animals (Darwin, 1872), our understanding of the psychobiology of disgust has progressed rapidly only in the last decades.

Studies have revealed that disgust processing is mediated by a complex network of cortical (including insular, cingulate and orbitofrontal cortex) and subcortical regions (e.g. the striatum) implicated in the processing of reward and aversion. Regions within this network are recruited when experiencing unpleasant odors and liquids (O’Doherty et al., 2001^; Wicker et al., 2003^; Jabbi et al., 2007^, 2008a), and when feeling nausea (Krolak-Salmon et al., 2003^; Napadow et al., 2013) but also during imaginative experiences of disgust, e.g. when watching or imagining disgusting foods and scenarios (Schienle et al., 2006^; Calder et al., 2007^; Jabbi et al., 2008a). Moreover, regions like the cingulate cortex and the insula are also vicariously active when watching disgusted expressions (Krolak-Salmon et al., 2003^; Wicker et al., 2003^; van der Gaag et al., 2007^; Jabbi et al., 2008a^; Jabbi and Keysers, 2008b), suggesting that perception of disgust in others may tap into shared emotion representations between self and other (Calder et al., 2000^; Keysers and Gazzola, 2009^; Lamm et al., 2011^; Avenanti et al., 2013b). Such overlapping activations for one’s own experience of disgust and the disgusted expressions of others may also reflect the emotional and homoeostatic salience of first- and third-person experiences of disgust. The insula and the cingulate cortex are recruited in a wide variety of tasks involving the subjective awareness of both positive and negative feelings (Craig, 2009^; Ibañez et al., 2010^; Menon and Uddin, 2010^; Cauda et al., 2012^; Couto et al., 2013^; Lamm and Majdandžić, 2015^; Terasawa et al., 2015^; Zaki et al. 2016) and could thus play a domain-general role, that is, to identify the most salient among several internal and extrapersonal stimuli in order to guide behavior. Interestingly however, electrical stimulation of the insular cortex can induce genuine disgust-related sensations (i.e. nausea, unpleasant tastes and sensations in the mouth and stomach) and corresponding orofacial motor responses (Penfield and Faulk, 1955^; Ostrowsky et al., 2000^; Selimbeyoglu and Parvizi, 2010), pointing to disgust-specific mechanisms in the insula and its strong link with motor circuits controlling oral motor behavior (Caruana et al., 2011^; Jezzini et al., 2012).

Indeed, the insula and other regions that are recruited when sensing or imagining disgust possess reciprocal connections with a number of motor areas, including somatomotor orofacial regions (Mesulam and Mufson, 1982^; Alipour et al., 2002^; Jabbi and Keysers, 2008b^; Cauda et al., 2011^; Deen et al., 2011^; Cerliani et al. 2012) that control ingestion through several descending cortico-bulbar pathways. These include cortico-pharyngeal and cortico-vagal pathways that regulate swallowing (Mistry et al., 2007^; Steele and Miller, 2010) and cortico-hypoglossal projections controlling the tongue, which is the primary organ of taste in the gustatory system and plays a major role in the preparatory oral activity that precedes swallowing (Matsuo and Palmer, 2008). Remarkably, the tongue has a major role not only in favoring swallowing but also in preventing it, e.g. when tasting disgusting and potentially toxic food. In these circumstances, ingestion has to be inhibited in advance to prevent contamination from toxic and potentially lethal substances.

However, little is known about the neurophysiological mechanisms underlying motor reactions to the sight of potentially toxic foods. Moreover, no studies have tested whether the human motor system undergoes similar neurophysiological modulations when vicariously sensing distaste in others. To address this issue, in this study we used transcranial magnetic stimulation (TMS) to monitor changes in primary motor cortex (M1) excitability while participants watched disgust-related and control pictures. These included pictures representing gustatory disgust/revulsion (namely, spoiled foods and facial expressions of distaste), non-gustatory disgust (i.e. disgusting invertebrates inducing repulsion, such as worms), corresponding emotionally positive stimuli (i.e. fresh foods, facial expressions and invertebrates) and neutral control stimuli (fixation cross). This way, we investigated the corticomotor correlates of imaginative and social observational aspects of gustatory and non-gustatory disgust. We targeted the tongue representation in M1 (tM1) and recorded motor-evoked potentials (MEPs) from the apex of the tongue as a measure of tM1 cortico-hypoglossal excitability. As a non-oral somatotopic control, we also targeted the arm representation in M1 (aM1) and recorded MEPs from the extensor carpi radialis (ECR) as a measure of aM1 cortico-spinal excitability.

TMS-induced MEPs are an established method for investigating the excitability of the human motor system during emotion processing (Borgomaneri et al., 2012^, 2015b^; Coelho et al., 2010^; van Loon et al., 2010). Previous TMS studies have shown that processing salient and emotionally threatening stimuli is often associated with inhibition of corticomotor output (Makin et al., 2009^; Serino et al., 2009^; Avenanti et al., 2012). For example, viewing social cues of potential threats (i.e. a fearful body posture) was found to rapidly freeze grasping-related cortico-spinal circuits, suggesting an automatic suppression of approach tendencies (Borgomaneri et al., 2015c). Similarly, pain administered on one’s own body part reduces the excitability of motor circuits controlling that body part (Urban et al., 2004) and watching pain in others triggers similar body part-specific motor inhibition (Avenanti et al., 2006^, 2009a^; Minio-Paluello et al., 2009). However, to date, no study has specifically tested whether similar somatotopic inhibitory modulations occur at the level of tM1 cortico-hypoglossal projections during imaginative and social observational aspects of disgust.

Based on the above-mentioned evidence of somatotopically specific motor inhibition when processing noxious or threatening stimuli and the specific role of the tongue in preventing ingestion of contaminants, we hypothesized that being exposed to rotten foods eliciting distaste and revulsion might specifically reduce tM1 cortico-hypoglossal excitability (but not aM1 cortico-spinal excitability). Because of the strong link between personal experience, imagery and social perception of emotions (e.g. Wicker et al., 2003^; Schienle et al., 2006^; Calder et al., 2007^; Jabbi et al., 2008a), we also predicted a similar ‘vicarious’ tM1 suppression when watching facial expressions of gustatory disgust. No similar tM1 modulations were expected for disgusting invertebrates—which are less related to distaste and revulsion, and more to repulsion and physical withdrawal (Rozin and Fallon, 1987).

Moreover, based on the evidence that brain activations during perception of disgusting stimuli are influenced by inter-individual differences in disgust sensitivity (Schienle et al., 2005^; Calder et al., 2007^; Schäfer et al., 2009^; Borg et al., 2013), we predicted that greater disgust sensitivity would be associated with stronger suppression of tM1 cortico-hypoglossal output during perception of gustatory disgust.

Materials and methods

Visual stimuli

The experimental stimuli consisted of three categories of pictures (subtending a visual angle of 18.53 × 12.19° and with comparable luminance; Figure 1) presented in separate blocks. One block included pictures of disgusting foods (i.e. moldy/rotten foods that can elicit distaste and revulsion), while pictures of fresh food and a fixation cross were used as emotionally positive and neutral controls. Another block included pictures of facial expressions of gustatory disgust, i.e. facial reactions of distaste/revulsion for unpleasant tastes (showing a gaping mouth and protruded tongue), while a fixation cross and pleasant facial expressions were used as control stimuli. A third block included disgusting stimuli not associated with distaste/revulsion (i.e. worms and cockroaches, which can elicit repulsion and withdrawal tendencies but not gustatory disgust), while a fixation cross and positive-looking butterfly and ladybug pictures were used as control stimuli.

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Facial expressions were selected from the Nimstim set (Tottenham et al., 2009), whereas food and invertebrate stimuli were selected from a picture database on the internet. We carried out a pilot study to select disgusting pictures with comparable disgust and arousal ratings across the three categories (Supplementary Materials and Supplementary Table S1). However, in a second pilot, when we distinguished between two different aspects of disgust, namely distaste/revulsion and repulsion/withdrawal, we found the former aspect to be more associated with gustatory disgust stimuli (particularly rotten foods, but also expressions of distaste) and the latter with disgusting invertebrates (Supplementary Table S2). In the second pilot study, we also ensured that the disgust-related pictures and the corresponding positive pictures had comparable arousal ratings but opposite valence (Supplementary Table S2).

Within each block, we used fixation crosses as emotionally neutral stimuli. We chose fixation crosses because some ‘neutral’ stimuli, e.g. neutral facial expressions, are often perceived as ambiguous rather than emotionally neutral (Cooney et al., 2006; Toki et al., 2012). Even more critically, the use of the same neutral stimulus across the three blocks allowed us to monitor any possible changes in excitability over time.

Results

The Friedman ANOVA performed on tongue MEPs was significant (χ²₈=17.45, P = 0.026), whereas the analysis on ECR MEPs was not (χ²₈= 12.98, P = 0.11), suggesting that visual conditions consistently modulated tM1 cortico-hypoglossal excitability but not aM1 cortico-spinal excitability (Figure 2). Further Friedman ANOVAs and Wilcoxon matched-pairs tests were performed to isolate significant tM1 modulations within the three experimental blocks. Friedman ANOVAs restricted to food (χ²₂= 13.00, P = 0.0015) and face blocks (χ²₂= 10.71, P = 0.0047) were significant. Wilcoxon tests showed that MEP amplitudes from the tongue were lower for pictures of rotten foods (mean ± SD: 0.171 mV ± 0.156) than for pictures of pleasant foods (0.186 mV ± 0.162; Z= 2.17, P = 0.03) and the food block baseline (0.182 mV ± 0.158; Z= 3.30, P < 0.001), which in turn did not differ from one another (Z = 0.97, P = 0.33). Moreover, MEP amplitudes were lower for distaste expressions (0.122 mV ± 0.080) than for positive facial expressions (0.142 mV ± 0.091; Z= 2.73, P = 0.006) and the face block baseline (0.138 mV ± 0.090; Z= 2.92, P = 0.004) which in turn did not differ from one another (Z = 0.28, P = 0.78).

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This tM1 cortico-hypoglossal suppression was specific for visual stimuli associated with oral disgust, as Friedman ANOVAs (χ²₂ = 0.43, P = 0.81) and Wilcoxon tests (all Z < 0.45, P > 0.65) performed on the invertebrate block were not significant. Moreover, tM1 suppression was comparable in the two groups of participants tested at the University of Bologna and the University of Queensland (Supplementary Materials).

To estimate the suppression effect induced by pictures of disgusting food, we computed a MEP contrast index as the difference between the tongue MEP amplitude recorded during disgusting food pictures and the mean MEP amplitude recorded during pleasant food pictures and the food block baseline. A similar MEP contrast index was computed for disgusted facial expressions (distaste expressions minus mean of pleasant faces and the face block baseline) and invertebrates (disgusting minus pleasant invertebrates and the invertebrate block baseline). A Friedman ANOVA on these contrast indices was significant (χ²₈= 7.00, P = 0.03). Wilcoxon tests showed that MEP contrast indices for disgusting foods (−0.012 mV ± 0.008) and facial expressions of distaste (−0.015 mV ± 0.016) were comparable (Z = 0.72, P > 0.47) and lower than the MEP contrast index for disgusting invertebrates (0.014 mV ± 0.048; all Z > 2.29, all P < 0.022).

The same analysis performed on MEP contrasts computed from the ECR data showed no significant effects (Friedman ANOVA: χ²₂ = 0.46, P = 0.79; Wilcoxon tests: all Z < 0.17, all P > 0.86).

To test whether inter individual differences in disgust sensitivity (DS-R) predict the magnitude of tM1 inhibition during observation of oral-related disgust pictures (indexed by the mean MEP contrast for disgusting foods and facial expressions of distaste), a regression analysis was performed. Because the two variables were normally distributed (Shapiro-Wilk test: P > 0.24) a parametric analysis was used. The regression model was significant and it evidenced a negative relation with a large effect size (_adjR² = 0.31, β = −0.60, F_1,12 = 6.83, P = 0.023; ƒ² = 0.45) that was confirmed using non-parametric correlation (Spearman ρ = −0.54, P = 0.047). The model strongly improved after the removal of one outlier with standard residual > 2 SD (_adjR² = 0.55, β = −0.77, F_1,11 = 15.82, P = 0.002; ƒ² = 1.24; Figure 3).

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The negative relationship indicated that participants with greater disgust sensitivity tended to show stronger reduction in tM1 excitability when exposed to pictures related to oral disgust. The same regression analysis performed on the MEP contrast calculated from the ECR was not significant (_adjR² = 0.03, β = −0.0002, F < 1, P = 0.55; Spearman ρ = −0.13, P = 0.67; no outliers in the dataset).

Discussion

To establish the cortico-motor correlates of imaginative and social observational aspects of disgust, we measured tongue MEPs as an index of tM1 cortico-hypoglossal output during the observation of pictures related to gustatory disgust and revulsion—namely, rotten foods and facial expressions of distaste, as well as pictures eliciting non-oral disgust (i.e. repulsion, originating from disgusting invertebrates like worms) and control stimuli.

In line with our hypotheses, we detected a modulation of tM1 excitability for the food and face picture categories, which possess clear associations with the tongue, but not for the invertebrate category. In contrast, no modulations of aM1 cortico-spinal excitability were detected across stimulus categories. These findings hint at somatotopic specificity when processing visual signs of gustatory disgust. They suggest that tM1 cortico-hypoglossal projections might be sensitive not only to disgust and the imaginative aspects of disgust that are likely evoked by the sight of disgusting foods (Supplementary Tables S1 and S2)—and that may require inhibition of preparatory tongue activity—but also to the perception of social signals of gustatory disgust in others. In particular, we found that exposure to pictures of rotten foods brought about a consistent reduction in the amplitude of MEPs recorded from the tongue. Tongue MEPs in this condition were lower than when participants observed pictures of fresh food or a fixation cross (baseline). A similar reduction was found when watching pictures of faces expressing distaste, compared with pictures of happy faces and the baseline. No similar modulations were found when seeing disgusting invertebrates that are more associated with repulsion and physical withdrawal/not wanting to touch or be touched. This selectivity suggests that tM1 cortico-hypoglossal excitability is specifically affected by images related to gustatory revulsion but not by non-oral repulsion.

We propose that the suppression of tM1 cortico-hypoglossal excitability while seeing pictures of disgusting foods might reflect an implicit avoidant-defensive mechanism of motor inhibition to prevent the ingestion of potentially harmful contaminants. In this sense, one might interpret this physiological phenomenon as an index of anticipatory action inhibition that is triggered by the sight of potentially threatening foods, and may have the ultimate role of preventing any toxin swallowing and contamination (Rozin et al., 2000^; Oaten et al., 2009).

The idea that detecting potential threats taps into motor inhibitory mechanisms is in line with influential models of emotion perception (Frijda 1986^, 2009^; Lang and Bradley, 2010). Although behavioral studies have reported generalized freezing effects, like the reduction of body sway when viewing aversive pictures (e.g. Azevedo et al., 2005^; Facchinetti et al., 2006^; Hagenaars et al., 2014), TMS evidence has documented specific inhibitory defensive mechanisms. For example, reduced M1 cortico-spinal excitability was found during administration of noxious stimulation to a specific body part (e.g. Farina et al., 2001^; Urban et al., 2004). Moreover, suddenly approaching objects (Makin et al., 2009) or sounds (Serino et al., 2009^; Avenanti et al., 2012) occurring near the body (i.e., within the peripersonal space; Serino et al., 2011) were found to trigger body-part specific cortico-spinal inhibition. More recently, it was shown that observing signals of potential threats specifically reduces the excitability of hand motor circuits controlling approaching movements like grasping (Borgomaneri et al., 2015c). However, all these studies have limited their investigation to upper limb motor excitability or postural sway and none of them have specifically investigated the emotion of disgust. Our data expand previous evidence by showing that observation of potentially threatening spoiled foods selectively inhibits the observers’ tM1 cortico-hypoglossal pathway, possibly to prevent the organism from toxin contamination via oral ingestion.

These findings complement those of a previous study in which tobacco-addicted patients were exposed to craving-inducing visual cues (i.e. pictures of cigarettes) (Vicario et al., 2014). In those conditions, increased tongue MEPs was found. Although preliminary, the report of Vicario et al. (2014) and the present study suggest that changes in the excitability of the tM1 cortico-hypoglossal pathway may reflect a somatomotor marker of oral approach/avoidance.

In a similar fashion, the reduced tM1 excitability during exposure to faces expressing distaste might be accounted for by a mirroring phenomenon that associates the observation of disgusted expressions with a specific somato-gustatory self-experience (i.e. distaste) and the consequent motor response (i.e. inhibition of preparatory tongue activity to avoid swallowing). This is in agreement with the notion of shared neural representations for self-experience of an emotion and the observation of the same emotion in others (Wicker et al., 2003^; Avenanti et al., 2005^; Oberman and Ramachandran, 2007^; Niedenthal et al., 2010^; Gallese and Sinigaglia, 2011^; Lamm and Majdandžić, 2015^; Rütgen et al., 2015a, b).

Strong evidence of shared neural mechanisms in the human motor system comes from studies showing that the sight of an action facilitates those motor circuits involved in making the same action (Fadiga et al., 2005^; Avenanti et al., 2013a^; Koch et al., 2010^; Mukamel et al., 2010). Notably, this mirroring might occur at a relatively abstract level. Studies indicate that the observer's motor system reflects the encoding of the distal goal of the action, irrespective of the specific movements performed to achieve it (Cattaneo et al., 2009^, 2013^; Jacquet and Avenanti, 2015). A similar proposal has been put forward for the tendency to imitate others’ emotional expressions (i.e. facial mimicry, Dimberg et al., 2000), based on the evidence that people mimic an interpretation of others’ emotions and not the actual expressive movements they observe (Magnée et al., 2007^; Hess and Fischer, 2014^; Fino et al., 2016). This is relevant for our study, where the actors' disgust was signaled by gaping mouth and protruded tongue, i.e. an active movement of the tongue. Previous studies have found that perception of tongue movements may facilitate tongue MEPs (Fadiga et al., 2002^; Sato et al., 2010^; D’Ausilio et al., 2011). Remarkably, however we found tongue MEPs inhibition rather than facilitation for observed expressions of distaste. This reduction hints at the vicarious activation of tongue motor programs that mirror those activated by imaginative aspects of gustatory disgust (i.e. elicited by the sight of rotten food pictures), rather than the observed actor’s tongue movements. Thus, we expand previous evidence by showing that tM1 cortico-hypoglossal projections are modulated in a similar mirror-like fashion when one is exposed to stimuli potentially eliciting revulsion within oneself (pictures of spoiled foods) and when exposed to facial expressions of distaste. Moreover, our study supports the aforementioned notions that motor mirroring may reflect high-level aspects of observed behavior (i.e. the inferred action goal/emotion), rather than the low-level motor pattern (i.e. the specific movement) being observed (Cattaneo et al., 2009^, 2013^; Hess and Fischer, 2014^; Jacquet and Avenanti, 2015).

Finally, we found that the suppression of tM1 excitability was predicted by DS-R scores. That is, participants who showed higher sensitivity to disgust in everyday life also showed greater reductions in tM1 excitability when seeing oral-related disgust (pictures of disgust-eliciting foods and facial expressions of distaste). These findings are in line with studies demonstrating that interindividual differences in disgust sensitivity modulate disgust-related neural activations (Schienle et al., 2005^; Calder et al., 2007^; Schäfer et al., 2009^; Borg et al., 2013). They are also in keeping with studies showing that the magnitude of vicarious activations is predicted by inter individual differences in personality and social attitudes (Dapretto et al., 2006^; Bastiaansen et al., 2009^; Avenanti et al., 2009b^, 2010^; Borgomaneri et al., 2015a). Importantly, these correlational data further demonstrate a specific link between observational tM1 modulations and the experience of disgust.

Previous investigations have shown that observation of negative scenes affects the excitability of cortico-spinal projections to upper limb muscles (e.g. Schutter et al., 2008^; Coombes et al., 2009^; Coelho et al., 2010^; Borgomaneri et al., 2014), although none of the previous studies have specifically tested disgust-related scenes. In the current study, we did not find changes in ECR MEPs (which reflect aM1 cortico-spinal excitability) for any of the three stimulus categories. This result supports our hypothesis of somatotopic specificity and suggests that pictures signaling oral-related disgust do not suppress motor excitability in general, but specifically in the tM1 cortico-hypoglossal pathway, which is a key sector of the motor system involved in avoiding ingestion of contaminants.

Our study has potential limitations that should be discussed. First, the sample size of our study is not large and the experiment was carried out in two laboratories. However, control analyses indicate comparable effect sizes in the two datasets (Supplementary Materials), suggesting the reliability of the findings. Second, it is well established that MEPs recorded from the tongue reflect mainly tM1 cortico-hypoglossal excitability (Muellbacher et al. 1994^; Sato et al., 2010), yet stimulation of the tM1 can activate other descending cortico-bulbar pathways (i.e. the cortico-pharyngeal and cortico-vagal pathways that regulate swallowing) which may affect tongue MEPs due to volume-conducted potentials. Although this influence is expected to be minimal, future studies might simultaneously monitor the dynamics of different cortico-bulbar pathways involved in ingestion during imaginative and observational social aspects of distaste. Third, although the experimental stimuli were evaluated in two pilot studies, participants in the main TMS experiment did not provide subjective ratings of the visual stimuli or the magnetic pulses. This prevented us from exploring the relations between disgust judgments, TMS discomfort and motor excitability. Thus, future studies could test whether subjective ratings of disgust closely parallel tongue MEPs, in a way that is similar to the results of the regression analysis (Figure 3) reported in the present study. Lastly, while the lack of ECR MEP modulation speaks against a generalized effect of gustatory disgust on motor excitability, a null finding should be interpreted with caution. In keeping with the notion that emotions prime the body for action (Frijda 1986^, 2009^; Lang and Bradley, 2010^; Vicario and Newman, 2013) one may expect that threatening stimuli requiring active avoidance may trigger activity in the observer’s motor system. The lack of modulation in the present experiment might have resulted from the general emotional qualities of the stimuli or the minimal functional role of our control muscle in approach/avoidance movements. Indeed, we do not rule out that other sectors of the motor system may be modulated by disgusting stimuli. For example, observing highly disgusting pictures associated with strong repulsion and arousal (e.g. spiders, dirty toilets, mutilations) could be expected to modulate several upper limb motor representations. Future studies will directly test these possibilities.

In sum, our study shows that pictures of revulsion-eliciting foods and facial expressions of distaste selectively reduce the excitability of tM1 cortico-hypoglossal projections, and this tM1 suppression is predicted by inter individual differences in disgust sensitivity. Our study suggests an avoidant-defensive mechanism in human cortico-hypoglossal circuits, and its vicarious activation during social perception of others’ distaste. These findings support the notion of shared neural representations of disgust and suggest that inter individual dispositions to experience disgust strongly affect such neural representations. Thus, these findings support a role for the motor system in emotion-driven motor anticipation and social cognition.