Empathy, the ability to share the feelings of another individual, plays a large role in human development and is an important feature in social behaviour and relationships. It is one type of ‘prosocial’ behaviour, meaning that the motivation for action or emotion is to benefit others, and not the self. Decreased empathetic abilities or the absence of empathy is a feature of some psychiatric conditions and can include impairments in other traits such as callousness (not caring about others), and antisocial behaviours (Decety & Moriguchi, 2007). In these conditions, the ability to share vicarious emotions is dampened, and thus the motivation to help others is reduced. The key to managing and treating diagnosed psychiatric conditions comes from understanding the likely causes of the behaviour. Prosocial behaviours such as empathy, and other altruistic efforts, used to be considered an exclusively human trait. When limited to a single species, research on the neuroscience (what is happening in the brain) behind this type of behaviour was limited. However, animal models that are commonly used in research, such as primates and rodents, have been found to be capable of prosocial behaviours to some degree (Meyza et al., 2017). Expanding the number of species able to be studied has allowed for a greater collection of knowledge on what is responsible for creating empathy in the human brain.

It is one thing to study the behavioural output of a particular emotion in humans, but it is quite another to determine the area of the brain and associated neurons responsible for causing that behaviour. New technology for imaging the human brain such as functional magnetic resonance imaging (fMRI) has provided ideas about what areas may be involved, however imaging studies are rife with limitations, and it is impossible to conclusively determine the mechanism (system of linked actions that result in a particular outcome) responsible (Meyza et al., 2017).  This is where rodent models of behaviour are especially useful. By using rats and mice, prosocial behaviour can be observed and the brain areas thought to be involved in producing the behaviour can be manipulated. Based on early work using fMRI in humans, the anterior cingulate cortex (ACC) was found to be active when observing someone else’s negative emotions (Wicker et al., 2003). The ACC is known to receive sensory input from other brain regions and change its neuron activity as a result. In rodents, the region has also been implicated as a region that can regulate emotional responses to pain and plays some understudied role in the social behaviours of fear (Jeon et al., 2010). Later work continued this investigation using rodents and suggests that mirror neurons in the ACC influence prosocial behaviours.

Mirror neurons are a particular type of neuron that is activated by directly experiencing an event or performing an action, as well as by observing someone else’s experience or actions. For example, watching someone grasp an object with their hand. In this way, the firing of one person’s neurons “mirrors” the firing of other people’s neurons, which is sometimes called “vicarious activation”. Mirror neurons are important in child development, as they allow for learning through imitation (Bonini, 2017). Recently, it has been suggested that mirror neurons in the ACC of rats are what is responsible for emotional contagion, or the ability to share in the emotional states of others when responding to pain, but not to fear (Carillo et al., 2019). Although previous research suggests that rats who observe a peer in distress will act to lessen the distress of the peer (Jeon et al., 2010), it remains unclear how strong the experience of emotional contagion is through mirror neurons.

Hernandez-Lallement and colleagues (2020) designed an experiment to investigate the degree of aversion a rat would have to seeing another rat of the same age and sex, known as a conspecific, experience pain. To do this, they built a square box with one lever on the right side, and one lever on the left side. At first, the rat could push either of the levers and receive a treat. This was referred to as the training phase, in which a baseline number of how many times the rat was willing and motivated to press the lever was recorded. In this case, since there were two levers, the researchers also noted which one the rat preferred, or pressed more on.  Once training was completed, a second rat was placed into an adjacent box, and was visible and audible to the original rat. However, this box did not contain levers for treats. Instead, the floor inside was a metal grid that could apply a shock to the feet of the new rat. This shock was applied to the adjacent rat when the original rat pressed on the lever that had been previously determined to be its preference. While pressing the lever shocked the second rat, the original rat still received their treat. In this way, the researchers were able to see whether the original rat would avoid pressing on their preferred lever to get a treat, if it observed that it was hurting another rat, or if they would continue to press the lever because it was of personal benefit.

One limitation in behavioural research is the presence of something called “individual differences”. This means that the effect attempting to be measured must be strong enough to be detected above and beyond any differences that arise simply because everyone will always be a little bit different from others. This is true of both humans and rats! In the case of this research, even though the rats had a large variety of individual differences in how they responded to the lever press that delivered a foot shock to their neighbour, the effect of harm aversion was above and beyond. The original rats would switch from using their preferred lever when it shocked the adjacent rat, to the extent where they would use a lever that was harder to press, or even received fewer treats, to avoid causing the other rat harm. It didn’t matter if the shocked rat was known or unknown to the original rat; they reduced their lever-pressing in both cases. In another application of this experiment, some of the lever-pressing rats were made to experience a foot shock before the introduction of the neighbour rat, while some were not. The researchers hypothesized that those who knew what it was like to be shocked would be more likely to avoid pressing the lever to deliver a shock to another rat compared to those who had never experienced a shock before. This was found to be true, and thus prior experience with pain made the original rats more sensitive to the pain of the other rat and more likely to switch to a different lever.

Based on the previous research supporting the role of the ACC in regulating prosocial behaviours like empathy, the researchers wanted to see if deactivating the area would influence the lever-pressing behaviours of the original rat. To do this, they administered muscimol, a compound that leads to decreased neuronal firing, directly to the ACC, and then repeated the same test paradigm. There was no difference in baseline levels of lever pressing between the ACC deactivation group and the normal control group. However, once the lever pressing caused a shock to the nearby rat, the control group (the rats whose ACC had not been deactivated) switched away from the shock-delivering lever significantly more than the deactivated ACC group, again supporting the theory that the ACC is central to the formation of prosocial behaviours.

One drawback to answering this question using rats is that it’s not possible to say that rats experience empathy. Human empathy has an altruistic component wherein the witnessing individual relates and wishes to lessen the pain of the experiencing individual through no benefit to themselves. There are two types of empathetic responses documented in human research where people’s desires to help are more or less selfishly driven (Batson et al., 1983). In one case, the motivation to help another is because the helping person feels personal distress at the state of the person needing help. Thus, their motivations are selfishly driven because it is with the goal of reducing their own personal discomfort. Other people report more altruistic motivations wherein they do not need to experience the other person being in distress to be willing to help them, which is referred to as a “truly other regarding” type of motivation (Batson et al., 1983).  Since the rats can’t tell us what they’re thinking, we have no way of knowing what their personal motivations are, and therefore cannot project human emotions and interpretations onto them.

All in all, rats tend to avoid causing harm to a peer. These results show promising directions for future research. The successful design of a methodology to measure the reluctance to harm another is novel and opens new possibilities for study of possible sources of antisocial behaviours. While the current study did find effects when the ACC was deactivated, they were unable to pinpoint the exact stage of the task that it was relevant to, and thus this will require further investigation. Ultimately, the results suggest that there are neural causes of the antisocial and psychotic behavioural traits typically seen in certain psychiatric disorders. This study is an excellent foundation for future research on harm aversion and prosocial motivation behaviours, with a focus on brain regions that potentially regulate these actions. While we still don’t know exactly how empathy is formed, we do have a better idea about where it’s coming from.

References

Batson, C.D., O’Quin, K., Fultz, J., Vanderplas, M. & Isen, A.M. (1983). Influence of Self-Reported Distress and Empathy on Egoistic Versus Altruistic Motivation to Help. Journal of Personality and Social Psychology, 45(3), 706-718.

Bonini, Luca. (2017). The Extended Mirror Neuron Network: Anatomy, Origin, and Functions. The Neuroscientist, 23(1), 56-67. 

Carillo, M., Han, Y., Migliorati, F., Liu, M., Gazzalo, V., Keysers, C. (2019). Emotional Mirror Neurons in the Rat’s Anterior Cingulate Cortex. Current Biology, 29, 1301-1312. 

Decety, J., & Moriguchi, Y. (2007) The empathetic brain and its dysfunction in psychiatric populations:implications for intervention across different clinical conditions. BioPsychoSocial Medicine, 1(22).

Decety, J., Bartal, I.B., Uzefovsky, F., Knafo-Noam, A. (2016). Empathy as a driver of prosocial  behaviour: highly conserved neurobehavioural mechanisms across species. Philosophical Transactions B, 371: 20150077.  

Hernandez-Lallement, J., Attah, A.T., Soyman, E., Pinhal, C.M., Gazzola, V., & Keysers, C. (2020). Harm To Others Acts as a Negative Reinforcer in Rats. Current Biology, 30, 949-961.

Jeon, D., Kim, S., Chetana, M., Jo D., Ruley, H.E., Lin, S.., Rabah, D., Kinet, S., &  Shin, H. (2010). Observational fear learning involves affective pain system and Cav1.2 Ca2+ channels in ACC. Nature Neuroscience, 13(4), 482-490.

Meyza, K.Z., Bartal, I.B., Monfils, M.H., Panskepp, J.B., & Knapska, E. (2017). The roots of empathy: Through the lens of rodent models. Neuroscience and Biobehavioural Reviews, 76, 216-234.

Wicker, B., Keysers, C., Plailly, J., Royet, J., Gallese, V., & Rizzolatti, G. (2003). Both of Us Disgusted in My Insula: The Common Neural Basis of Seeing and Feeling Disgust. Neuron, 40(3), 655-664.

Thumbnail image obtained from “Created with BioRender.com”