Mixing Memory

Of all the mirror neurons discovered in monkeys, auditory mirror neurons may be the coolest. These cells respond when a monkey performs an action, and when that monkey hears the sounds of that action being performed by another. Until recently, there was limited evidence of a corresponding auditory mirror neuron system in humans. In one study, for example, researchers contrasted the brain activity of expert pianists and nonmusicians1. In one condition, both groups of participants listened to piano music, and in another, they pressed random keys on a piano keyboard that was rigged so that it would not produce any sound. In each condition, participants brains were scanned using an fMRI machine. The results (shown below2, with pianists on the left and nonmusicians on the right) showed that when listening to piano music, pianists, but not nonmusicians, activated some of the same regions that were active while playing the piano keyboard.

Unfortunately, since this result is limited to experts, it's difficult to interpret and generalize from a mirror neuron perspective. So, in order to provide stronger evidence for the existence of auditory mirror neurons in humans, Gazzola et al., in a study published in last week's issue of Current Biology, performed a similar study with everyday actions and the sounds of those actions3. On the first day of the study, participants listened to everyday actions (see the table below for the different kinds of actions, from their supplemental material located here) along with nonaction sounds, and on the second day, they performed similar actions to those they'd heard on the preceding day. During both days, the participants were strapped into an fMRI machine.

Gazzola et al. found that the same areas, including areas in the temporal lobe (which receives signals from the ear), the parietal lobe (where sensory signals get relayed to other areas of the brain), and the premotor cortex (where mirror neurons are thought to reside), were active both when listening to the sounds of actions (but not non-action sounds) and when performing similar actions. This is exactly what we would expect to find if humans have an auditory mirror neuron system that processes actions whether they're performed or we hear someone else perform them. So, it appears that like monkeys, we have auditory mirror neurons.

Now, what purposes do these auditory mirror neurons serve? Well, one of the claims for mirror neurons in general is that they underlie empathy, by simulating the actions for example. To find evidence that auditory mirror neurons might facilitate empathy, Gazzola et al. had all of their participants complete four scales designed to measure perspective-taking levels and empathy. These scales included questions like, "I sometimes try to understand myfriends better by imagining how things look from their perspective," and, "I often have tender, concerned feelings for people less fortunate than me." Gazzola et al. then divided participants into high and low-perspective taking groups, based on their scores on these scales. Participants who scored high on the scales, indicating that they were better at taking the perspective of others and more prone to feel empathy, showed more activation in the auditory mirror neuron areas when listening to the actions of others than low-perspective taking participants. While indirect, this result is in line with what we would predict if the auditory mirror neuron system facilitated empathy.

To summarize, then, there is now significant evidence that humans have an auditory mirror neuron system that responds both when we perform actions and when we hear the sounds of those actions being performed, and that this system facilitates empathy. Despite rampant speculation, it's still not clear how this or other mirror neuron systems facilitate empathy, but given how popular mirror neurons are these days, I suspect it's only a matter of time before really good research begins to be conducted in this area and we start to answer some real questions.

¹Bangert, M., Peschel, T., Schlaug, G., Rotte, M., Drescher, D., Hinrichs, H, Heinze, H.J., & Altenmüllera, E. (2006). Shared networks for auditory and motor processing in professional pianists: Evidence from fMRI conjunction. NeuroImage, 30, 917-926.
²Ibid, From Figure 2 and 3, p. 921.
³Gazzola, V., Aziz-Zadeh, L., & Keysers, C. (2006). Empathy and the somatotopic auditory mirror system in humans. Current Biology, 16, 1824-1829.