Why can we feel other people's pain? A recent study in rats investigates.
From imaging studies of human brains, it was already clear that a region called the anterior cingulate cortex (ACC), which is active during the experience of pain, can also become active when observing pain in others.
These studies have also shown that, when observing another's pain, this region is more active in people with high levels of empathy and less active in people with psychopathy.
However, those earlier investigations did not clarify the underlying mechanisms or the cells involved.
Now, by studying rats, researchers at the Netherlands Institute for Neuroscience (NIN) in Amsterdam have identified that the ability to feel the pain of others has to do with "mirror neurons" in the ACC.
Scientists have found mirror neurons before, but they were concerned with observing movement.
Brain studies of monkeys observing other monkeys have revealed that cells in the motor system become active as though the observing monkeys were the ones performing the actions. This activity occurs in addition to the processing of visual information.
In a Current Biology paper, the authors of the new study describe how mirror neurons in a rat's ACC become active not only when the animal itself is in pain but also when it observes another rat in pain.
Testing theories of empathy
"What is most amazing," says lead study author Christian Keysers, who is a professor at the NIN, "is that this all happens in exactly the same brain region in rats as in humans."
Prof. Keysers and his team see their findings as an important advance in understanding how lack of empathy arises in certain psychiatric conditions.
In their study paper, they recap two reports from previous investigations that suggested that "some ACC neurons respond to the observation and experience of pain." One was an observation in a single human patient, and another was in a mouse study.
Given these findings and the fact that mirror neurons already feature in the brain's motor system, they decided to test two theories. The first was that there are mirror neurons in the ACC "that trigger our own feeling of pain and are reactivated when we see the pain of others," and the second was that "this is the reason why we wince and feel pain while seeing the pain of others."
As it is not possible either to map the activity of individual neurons or to stimulate changes in the ACC in the human brain, the researchers decided to tests these theories in rats.
One of the reasons why they chose rats as the model was because part of their ACC has a similar cell structure and connectivity to the ACC area that has implications in pain empathy in humans.
Brain changes while observing others
The study comprised a series of experiments in which the scientists measured changes in the brain and behavior of rats, both when the animals received a "mild shock" and when they then observed other rats receiving the same unpleasant stimulus.
The results showed that the rats' behavior and brain changes in response to seeing other rats in pain were the same as those that occurred when the observer rats experienced the pain themselves. Both responses involved the same ACC neurons.
The researchers then repeated the empathy experiment, but this time, they suppressed the activity of the ACC neurons that had been active before. When the treated rats observed pain in other rats, they no longer showed the same empathic behavior.
Pain is a complex experience that has sensory and emotional components. Many studies of pain tend to use definitions that derive from people's descriptions and behavior patterns in animals.
However, to be sure that it is pain and not some other negative experience that is at work in the brain, it is important to take into consideration signals in the neural system.
Decoding of pain intensity 'same as in self'
Prof. Keysers and his team take up this point in their study paper. Scientists have argued, they write, that a "vicarious response" that is specific to pain, as opposed to fear, for example, needs to show two features in the brain.
The first feature is that the brain response "must be selective." The second feature is that the circuit has to have the same way of coding pain in the self as in others.
Their results met both of these requirements. They showed that most of the observer rats' ACC mirror neurons did not activate in response to another negative emotion, such as fear. In addition, the observing rats could "decode pain intensity in the self from a pattern decoding pain in others."
Prof. Keysers concludes that the study sheds light on how lack of empathy might arise in psychopathy and other disorders. He notes, "It also shows us that empathy, the ability to feel with the emotions of others, is deeply rooted in our evolution."
"We share the fundamental mechanisms of empathy with animals like rats."
Prof. Christian Keysers