The brain functional networks associated to human and animal suffering differ among omnivores, vegetarians and vegans

Abstract

Empathy and affective appraisals for conspecifics are among the hallmarks of social interaction. Using functional MRI, we hypothesized that vegetarians and vegans, who made their feeding choice for ethical reasons, might show brain responses to conditions of suffering involving humans or animals different from omnivores. We recruited 20 omnivore subjects, 19 vegetarians, and 21 vegans. The groups were matched for sex and age. Brain activation was investigated using fMRI and an event-related design during observation of negative affective pictures of human beings and animals (showing mutilations, murdered people, human/animal threat, tortures, wounds, etc.). Participants saw negative-valence scenes related to humans and animals, alternating with natural landscapes. During human negative valence scenes, compared with omnivores, vegetarians and vegans had an increased recruitment of the anterior cingulate cortex (ACC) and inferior frontal gyrus (IFG). More critically, during animal negative valence scenes, they had decreased amygdala activation and increased activation of the lingual gyri, the left cuneus, the posterior cingulate cortex and several areas mainly located in the frontal lobes, including the ACC, the IFG and the middle frontal gyrus. Nonetheless, also substantial differences between vegetarians and vegans have been found responding to negative scenes. Vegetarians showed a selective recruitment of the right inferior parietal lobule during human negative scenes, and a prevailing activation of the ACC during animal negative scenes. Conversely, during animal negative scenes an increased activation of the inferior prefrontal cortex was observed in vegans. These results suggest that empathy toward non conspecifics has different neural representation among individuals with different feeding habits, perhaps reflecting different motivational factors and beliefs.

Figure 1. Graph showing error bars of means and standard deviations of empathy quotient (EQ) score in the three groups of subjects.

See text for further details.

Figure 2. Within-group analysis of activations.

Cortical activations on a rendered brain from omnivore (A–H), vegetarian (I–R) and vegan (S–W) subjects during observation of pictures showing negative valence scenes of humans (A–D, I–N, S–V) or animals (E–H, O–R, Z–W) (within-group analysis, one-sample t tests, t = 3 for display purpose). Images are in neurological convention.

Figure 3. Results of the between-group comparisons of emotional (human and animal) negative valence picture views.

Results are superimposed on a high resolution T1-weighted image in the standard MNI space, at a threshold of p<0.05 corrected for multiple comparisons. Areas activated during human picture view in vegetarians and vegans vs. omnivores are shown in yellow. Activations specific for vegetarians are shown in blue. Activations specific for vegans are shown in red. A: human picture view; B: animal picture view. Images are in neurological convention.

Figure 4. Interactions between stimuli (animal/human) and groups (omnivore/vegetarian/vegan).

An interaction was found in the right amygdala (A), indicating greater increase to animal negative valence picture view in omnivores and to human negative valence picture view in vegans. An interaction between “human pictures” and “vegan group” was also found in the left amygdala (A). An interaction was found in ACC (B) between the “omnivore group” and “human pictures”, as well as between “vegetarian group” and “animal pictures”; and in the right IFG between “animal pictures” and “vegan group” (C). Foci of activations are shown on a high-resolution T1-weighted image in the standard MNI space. Plots indicate activation changes detected in the three groups during the two experimental conditions in each of these regions. Images are in neurological convention.