By Christian Jarrett

It is not too long ago that mirror neurons were touted as one of the most exciting discoveries in neuroscience (or most hyped, depending on your perspective). First discovered in monkeys, these brain cells fire when an individual performs a movement or when they see someone else perform that movement. This automatic neural mirroring of other’s actions was interpreted by some scientists as the seat of human empathy. The cells’ most high-profile champion, US neuroscientist Vilayanur Ramachandran, described them as “the neurons that shaped civilisation” and, in 2000, he (in)famously said they would do for psychology what DNA did for biology. Nearly 20 years on, what evidence do we have that mirror neurons provide the basis for human empathy? According to a new meta-analysis and systematic review released as a preprint at PsyArXiv, the short answer is “not a lot”.

The research team, led by Soukayna Bekkali and Peter Enticott at the University of Deakin, searched the literature for all English language studies conducted in humans that had investigated whether mirror neuron activity was correlated with empathy.

This search led to the identification of 52 relevant papers involving over 1000 participants. These studies had used a range of methods to measure mirror neuron activity (including fMRI brain scanning, EEG recording of brain activity and TMS – magnetic stimulation typically applied to the motor cortex, to check how much it had already been activated by witnessing movement by others). There were also various measures of empathy: mostly self-report questionnaires, but also measures of “motor empathy” (the automatic mimicry of other people’s movements) and the recognition of other’s facial emotions.

There was a notable lack of objective measures of empathy, such as looking at participants’ heart rate or skin conductance (the involuntary production of sweat as part of an emotional response). Overall, participant samples were small and there was great inconsistency in methods.

Bekkali and her colleagues broke down the literature according to whether the papers had investigated motor empathy, emotional empathy (feeling what someone else is feeling), and/or cognitive empathy (knowing what someone else is feeling or thinking).

Regarding motor empathy, the accumulated evidence pointed to no association with mirror neuron activity. Regarding emotional empathy, there was no evidence for a link with mirror neuron activity in one key brain region where these cells are thought to reside (the inferior parietal lobule; IPL) and only weak evidence for an association with postulated mirror neuron activity in another key brain area, the inferior frontal gyrus (IFG). “It remains unclear how the mirror neuron system relates to the experience of emotional empathy,” the researchers said.

In terms of cognitive empathy, there was no evidence for a mirror neuron link in the IPL and weak evidence of a link with activity in the IFG. “The current results suggest mirror neuron activity may play a role in cognitive empathy,” the researchers said, adding that “the mirror neuron system may potentially be one neurophysiological mechanism subserving cognitive empathy.”

One problem with interpreting the existing evidence base is that the results tended to vary according to the methods used, not only in terms of how mirror neuron activity and empathy were measured, but also according to the stimuli presented to participants, such as whether they viewed real images of people performing actions, or cartoons, and whether the images were dynamic or static. There is no currently accepted “gold standard” measure of mirror neuron activity in humans, the review authors also noted, and some evidence of a file-drawer problem in the literature, in which positive results are more likely to be published.

Finally, although this thorough investigation of the existing evidence base uncovered some modest evidence that mirror neurons may be involved in empathy – especially cognitive empathy (or understanding other people’s perspectives) – to date there is no evidence that mirror neurons play a causal role in empathy or are necessary for empathy.

When excitement about mirror neurons was at its peak, it was common for newspaper columnists and media commentators to reference these cells with confidence when discussing empathy. “It’s thanks to these cells that we are willing to help strangers in need”; “the best rom coms trigger our mirror neurons”; “altruists have more mirror neurons”, and so on. We were getting ahead of ourselves. After more than two decades of research, this new review provides a sober reminder that our understanding of the neural basis of empathy, and especially the role played by mirror neurons, remains far from complete.

—Is the Putative Mirror Neuron System Associated with Empathy?A Systematic Review and Meta-Analysis [this paper is a preprint meaning it has not yet been subjected to peer review and the final published version may differ from the version that this report was based on]

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest