Looking at Pain in the Mirror: pain mirror neurons found in rats.

How is it that we get overwhelmed when watching someone cry, or feel a coating of inspiration when we see a face filled with hope? And, are we the only animal to exert an empathic response to those emotional cues? A study published last week by Carrillo and colleagues uncovers the potential mechanism that allows for emotional sharing in both the brains of rats and humans.

The common assumption among humans regarding more “primitive” animals, like a rat, is that they must not be able to experience emotion, or share an emotional experience with their counterparts, the way we do. This assumption is born-out from a long-held attitude of human exceptionalism: “we’re at the top of the animal kingdom, therefore, no other species can have the mental faculties that we do.” We like to feel special and our thinking is biased by that. Yet, just as the old assumption that held we must be at the centre of the universe, is now debunked, this new form of exceptionalism will, too, yield to scientific observation.

Chimpanzees have been observed to risk their own lives in attempts to rescue their fellow beings from drowning. Studies showed rats exerting a dumbfounding level of effort to rescue their fellow comrades from a trap. Rats have also been shown to be able to detect fright in their fellow beings. Give one rat a mild but sudden electric shock and it freezes all movement: a defence mechanism meant not to alert themselves to any prowling predators. Interestingly, nearby rats who witness this instantaneous freeze of another rat mimic this response: also displaying fear. Fear, then, can be transported from rat-to-rat, just like our own emotional displays usually trigger the same response in those around us. These studies laid-out the red carpet for inspecting the brains of rats and humans in order to investigate what mechanism makes it possible for our emotions to become contagious – and whether these mechanisms are continuous across species.

In humans, the potential mechanisms of emotional contagion have been studied using functional magnetic resonance imaging (fMRI) – a device that tracks the blood flow within the brain to gauge how active a region is when performing a certain task – with experiments generally using two conditions for comparison: in the first condition, the participant is exposed to a stimulus that invokes an emotion; the second condition, involves a participant observing that same emotion in another.

For instance, the first condition could involve a participant receiving an electric shock to a finger to see what brain regions show activation. What previous studies have shown is that the anterior cingulate cortex (ACC) becomes activated during pain perception. Patients with damage to their ACC, moreover, do not exhibit the same emotional response that is usually associated with pain. This evidence has led researchers to term the ACC as the “hub” of emotional pain.

The second condition would involve seeing whether the same region fired-up in response to the participant observating someone else experiencing this pain. And this is exactly what happens. The ACC of a participant activates to the same extent as it would if this observed pain were their own. This observation, that the same region is involved in experiencing pain and witnessing it in others, led to the postulation of “mirror neurons:” brain cells that assimilate the pain of others onto our own experience of pain. But this mirror neuron hypothesis still lacked empirical support.

Carrillo and her colleagues, at the Netherlands Institute of Neuroscience, via recording individual ACC neurons, have found supporting evidence for the existence of pain mirror neurons. The researchers conditioned rats to associate a tone with the likely induction of a shock. Within the rats’ cingulate cortices, groups of neurons responded to the pain inducing shock, but did not respond to the fear invoking tone. These neurons, then, appear to be integral for a rat’s own sense of pain. Importantly, this same cluster of pain perceptive neurons also accelerated their activity when a rat witnessed another in the same pain. Just as the mirror neuron hypothesis predicted, cells within the rat cingulate cortex make the pain of another rat relatable to an observant rat – as the band Title Fight would say: “your pain is mine now.”

The rats ACC pain neurons were quick to respond to the viewing of a fellow rodent’s pain, suggesting the mimicking of pain is similar to an uncontrollable reflex, rather than to a process of mentalisation and perspective taking. Carrillo’s study also showed this mirroring process of pain, in both rats and humans, happens within the cingulate cortex, which suggests both species share the same biological mechanisms that make emotional contagion possible.

Again, we discover we’re not as exceptional as we once seemed, and we’ve learnt not to underestimate the complexities of those animals we deem “lesser.” It turns out rats can do empathy, too.

Study: https://doi.org/10.1016/j.cub.2019.03.024

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