Visual cues from faces provide important social information relating to individual identity, sexual attraction and emotional state. Behavioural and neurophysiological studies on both monkeys and sheep have shown that specialized skills and neural systems for processing these complex cues to guide behaviour have evolved in a number of mammals and are not present exclusively in humans. Indeed, there are remarkable similarities in the ways that faces are processed by the brain in humans and other mammalian species. While human studies with brain imaging and gross neurophysiological recording approaches have revealed global aspects of the face-processing network, they cannot investigate how information is encoded by specific neural networks. Single neuron electrophysiological recording approaches in both monkeys and sheep have, however, provided some insights into the neural encoding principles involved and, particularly, the presence of a remarkable degree of high-level encoding even at the level of a specific face. Recent developments that allow simultaneous recordings to be made from many hundreds of individual neurons are also beginning to reveal evidence for global aspects of a population-based code. This review will summarize what we have learned so far from these animalbased studies about the way the mammalian brain processes the faces and the emotions they can communicate, as well as associated capacities such as how identity and emotion cues are dissociated and how face imagery might be generated. It will also try to highlight what questions and advances in knowledge still challenge us in order to provide a complete understanding of just how brain networks perform this complex and important social recognition task.
The human brain has evolved specialized neural mechanisms for visual recognition of faces, which afford us a remarkable ability to discriminate between, remember and think about many hundreds of different individuals. Sheep also recognize and are attracted to individual sheep and humans by their faces, as they possess similar specialized neural systems in the temporal and frontal lobes for assisting in this important social task, including a greater involvement of the right brain hemisphere. Here we show that individual sheep can remember 50 other different sheep faces for over 2 years, and that the specialized neural circuits involved maintain selective encoding of individual sheep and human faces even after long periods of separation.
Faces are highly emotive stimuli and we find smiling or familiar faces both attractive and comforting, even as young babies. Do other species with sophisticated face recognition skills, such as sheep, also respond to the emotional significance of familiar faces? We report that when sheep experience social isolation, the sight of familiar sheep face pictures compared with those of goats or inverted triangles significantly reduces behavioural (activity and protest vocalizations), autonomic (heart rate) and endocrine (cortisol and adrenaline) indices of stress. They also increase mRNA expression of activity-dependent genes (c-fos and zif/268) in brain regions specialized for processing faces (temporal and medial frontal cortices and basolateral amygdala) and for emotional control (orbitofrontal and cingulate cortex), and reduce their expression in regions associated with stress responses (hypothalamic paraventricular nucleus) and fear (central and lateral amygdala). Effects on face recognition, emotional control and fear centres are restricted to the right brain hemisphere. Results provide evidence that face pictures may be useful for relieving stress caused by unavoidable social isolation in sheep, and possibly other animal species, including humans. The finding that sheep, like humans, appear to have a right brain hemisphere involvement in the control of negative emotional experiences also suggests that functional lateralization of brain emotion systems may be a general feature in mammals.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.