Neat!The study - in basic terms - investigates this hypothesis by placing subjects into a functional Magnetic Resonance Imaging (fMRI) machine and looking at the "activation" of different brain areas as subjects view a randomized series of pictures. The pictures used for this study included neutral scenes and an even share of scenes depicting various kinds of animal and human suffering.
The first main finding of this study is that, compared to Omnivores, Vegans and Vegetarians show higher activation of empathy related brain areas (e.g. Anterior Cingular Cortex and left Inferior Frontal Gyrus) when observing scenes of suffering; whether it be animal or human suffering.
Further, pictures of animal suffering (in contrast to pictures human suffering) recruited specific brain regions in Vegans and Vegetarians that were not differentially recruited by Omnivores. These were areas which are thought to be associated with higher-order representations of the self and self values (e.g. medial Prefrontal Cortex).
don't worry, i won't let them eat you
In addition to generally higher activations in the above mentioned areas, a second main finding of this study is that there are certain brain areas which only Vegetarians and Vegans seem to activate when processing pictures of suffering. In particular, when viewing pictures of human suffering, Vegetarians in this study recruited additional brain areas thought to be associated with bodily representations that distinguish self from others. (Notably these areas were particularly active when mutilations were shown).