The Brain Functional Networks Associated to Human and Animal Suffering Differ among Omnivores, Vegetarians and Vegans

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Introduction

Social cognition includes mental processes necessary to understand and store information about the self and other persons, as well as interpersonal norms and procedures to navigate efficiently in the social world [1]. Basic abilities underlying social cognition include the perception and evaluation of social stimuli, the integration of perceptions with contextual knowledge, and finally the representation of possible responses to the situation. One of the hallmarks of social cognition in humans is the ability to understand conspecifics as beings like oneself, with intentional and mental lives like one's own [2]. Accordingly, human beings tend to identify with conspecifics and attribute mental states to them. Such abilities rely on the activity of several brain regions, including the frontal lobes (orbitofrontal cortex, medial prefrontal cortex, and cingulate cortex), the temporal lobes (including the amygdala), the fusiform gyrus, and the somatosensory cortices[3], [4], [5]. The majority of these regions is also critically involved in the processing of emotions [6]. This suggests that the merging between emotions and feelings experienced by oneself and those perceived in other individuals may be a key ingredient of social understanding, and it may play a major role in promoting empathy, prosocial behaviours, and moral norms [1], [3]. Moreover, empathic responses can be modulated by the subjective attitude held toward suffering individuals [7], as well as by personal experience [8]. Several functional magnetic resonance imaging (fMRI) studies showed that observing the emotional state of another individual activates a neuronal network involved in processing the same state in oneself, whether it is pain, disgust, or touch[3], [4], [5]. Empathy toward another person, which can be defined as the ability to share the other person's feeling in an embodied manner, has been related to recruitment of a network mostly including the somatosensory and insular cortices, limbic regions and the anterior cingulate cortex (ACC). Whereas cognitively inferring about the state of other person (known as theory of mind) has been associated with recruitment of medial prefrontal regions, the superior temporal sulcus and the temporo-parietal junction[4].

A few investigations have also assessed whether affective links between people modulate their brain empathic responses to others, such as when these are loved ones or strangers[9], or when they are believed to be fair or unfair persons [7], [9]. The majority of previous studies attempting to characterize empathy-related responses did not separate empathy towards humans from that towards animals. Furthermore, in some studies, scenes showing animals were treated as a neutral condition. However, a recent study [10] that compared stimuli depicting human and non human animal targets demonstrated higher subjective empathy as the stimuli became closer in phylogenetic relatedness to humans (mammalianvs. bird stimuli), thus indicating that empathic response towards humans may generalize to other species.

In this study, we postulated that the neural representation of conditions of abuse and suffering might be different among subjects who made different feeding choice due to ethical reasons, and thus result in the engagement of different components of the brain networks associated with empathy and social cognition. In details, we tested the hypothesis that the neural processes underlying empathy in vegetarians and vegans may not only operate for representations about humans but also animals, and thus vary between them and omnivore subjects. Vegetarians and vegans, who decided to avoid the use of animal products for ethical reasons, have a moral philosophy of life based on a set of basic values and attitudes toward life, nature, and society, that extends well beyond food choice. The earliest records of vegetarianism as a concept and practice among a significant number of people was closely connected with the idea of nonviolence towards animals and was promoted by religious groups and philosophers. The term veganism, which was coined from vegetarianism, acknowledges the intrinsic legitimacy of all sentient life and rejects any hierarchy of acceptable suffering among creatures. Veganism is a lifestyle that seeks to exclude the use of animals for food, clothing, or any other purpose [11]. The central ethical question related to veganism is whether it is right for humans to use and kill animals. Due to these differences of believes and behaviours, we also hypothesized that, in addition to a common shared pattern of cortical processing of human and animal suffering, vegetarians and vegans might also have functional architecture differences reflecting their different motivational factors and believes.