Post by Flora Moujaes
What's the science?
Why do we wince when we see someone else in pain? Neuroimaging studies have shown that one of the brain regions active in humans experiencing pain, the anterior cingulate cortex (ACC), is also active when they see another in pain. Activity in this area of the brain is also stronger in more empathic individuals and reduced in psychopaths. It has been hypothesized that increased ACC activity in response to both experienced and observed pain may be caused by ‘mirror neurons’, a type of neuron first discovered in monkeys that activates both when a monkey observes someone else picking up an object and when they themselves pick up an object: these neurons mirror the action of another. Thus, mirror neurons may play a key role in empathy and explain why we wince when seeing another’s pain.
We still don’t know what the exact function of mirror neurons is and whether mirror neurons even exist in humans, as it is not possible to record the activity of individual brain cells in humans. This week in Current Biology, Carrillo and colleagues show for the first time that the rat ACC contains mirror neurons that respond both when a rat experiences pain and witnesses another rat in pain.
How did they do it?
Researchers examined four main questions: (1) does the ACC contain mirror neurons (neurons that show overlap between observed and experienced pain) (2) are the mirror neurons in the ACC specific to pain (i.e. they don’t code for other emotions such as fear) (3) do these mirror neurons use the same code (or firing pattern) to signal both experienced and observed pain, and (4) is the ACC necessary for experiencing another’s pain?
To address these questions 17 rats participated in three experimental conditions. In the first, they observed other rats experiencing painful electric shocks. In the second, they experienced pain themselves (triggered by a heat laser). In both the first and the second condition the intensity of the pain was varied. In the third condition the rats experienced fear using a conditioned stimulus, as they were subjected to a tone that had previously been paired with an electric shock. The researchers used various cell recording methods to examine neurons in the ACC, an area implicated in pain empathy in humans. Finally, to see whether the ACC is necessary for experiencing another’s pain, they injected a drug that deactivated the ACC into 6 rats, as well as injecting saline into 8 control rats. They then repeated the experiment to see whether deactivating the ACC changed the rats’ freezing response to observing pain or to experiencing fear first-hand. The freezing response is used as a measure of fear, as when rats are scared their natural reaction is to freeze to avoid being detected by predators.
What did they find?
Researchers found that the rat ACC does contain mirror neurons: they found a number of neurons in the rat ACC that showed overlapping activity between observed and experienced pain. They showed evidence that a large number of the neurons that showed overlap were not also activated by fear. This indicates that the ACC contains mirror neurons that may be somewhat specific to pain and not encode for salient emotions such as fear. A decoding scheme trained using the spike count to decode the intensity of another rat’s experience was able to decode the intensity of the rat’s own pain experience. This shows that mirror neurons use the same code (or firing pattern) to signal both observed and experienced pain. The researchers also found a small population of neurons that responded to observed pain and the experience of fear, but not the experience of pain. This suggests a more complex understanding of mirror neurons in the ACC, as it seems the ACC may map the distress of another animal onto a mosaic of pain- and fear-sensitive channels in the observer. Finally, they found that deactivating the ACC did decrease the freezing response of rats to observing pain but not to experiencing fear first-hand. This indicates that the ACC is necessary for experiencing another’s pain.
What's the impact?
This is the first study to show that mirror neurons, which were originally shown to mirror physical actions, are also involved in how mammals process another’s pain. This is also the first study to show that after deactivating the ACC rats show reduced distress when witnessing another in pain. This indicates that mirror neurons may also play an important role in empathy. Many disorders such as psychopathy, are characterized by a lack of empathy, and therefore understanding the neural basis of empathy could have huge implications for understanding such disorders. More research is needed to confirm whether the mirror neurons in the ACC are selective for pain, as while they were not shown to respond to fear, they could still code for numerous other non-painful but equally salient emotional stimuli.
Carrillo et al. Emotional Mirror Neurons in the Rat’s Anterior Cingulate Cortex. Current Biology (2019). Access the original scientific publication here.