Mirror Neurons On Trial

Decades of research have suggested that certain “mirror neuron” groups are activated not only when we perform an action, but also when we see it being performed, or even when we hear it being performed in another room. But other studies have raised strong criticisms about what this data actually means – and now the dispute is heating up more than ever.

Two neuroscientists debate the relative merits of each others' stances on mirror neurons.

In this month’s issue of the journal Perspectives on Psychological Science, a group of brain research veterans debate the evidence for and against the mirror neuron hypothesis. Their consensus is that mirror neurons do seem to play a role in our ability to understand and imitate the actions of others, but that the contributions of these cells are a lot more subtle and complex than previously thought.

The article focuses on three main categories of abilities in which some neuroscientists think mirror neurons take part: understanding speech, understanding others’ actions, and understanding others’ minds. The researchers on a few different sides had plenty to say about each of these categories, so let’s break ’em down.

1. Actions
The role of mirror neurons in understanding visually perceived actions was the first to be researched. In the 1990s, a team led by Giacomo Rizzolatti at the University of Parma were studying the neurons of macaque monkeys, when they realized that certain groups of these monkey neurons (located in the ventral premotor cortex) responded in very similar ways whether the monkey picked up a piece of food or watched someone else pick it up.

A lot of subsequent research confirmed that these firing patterns were definitely happening – but what they meant was far from clear. Some scientists, such as UC Irvine’s Gregory Hickock, argued that supposed “mirror” activity might just reflect the normal behavior of the monkeys’ overall motor system, rather than being direct evidence for imitation or theory of mind.

Also, new evidence contradicts some major predictions about the role mirror neurons were thought to play in understanding actions:

The repeated finding that increased experience executing actions is associated with decreased, not increased, activation in putative MN [mirror neuron] regions, lies completely opposite that predicted … [One prominent MN researcher] predicts that executing these more familiar actions … should produce greater activity in mirror areas. However, increased experience executing actions is not associated with increased activity in putative MN regions.

So, at the very least, it seems that we need to take another look at how (or if) mirror neurons come into play when we imitate an action that we’ve seen or heard.

2. Speech
When it comes to our ability to understand spoken language, the evidence seems to strongly suggest that mirror neurons aren’t nearly as important as some of their supporters have claimed. Although our motor areas can be activated when we listen to someone speaking, no one’s been able to demonstrate a direct correlation between mirror neurons and our ability to understand speech. Not to mention that individuals who have no working motor system whatsoever can still understand spoken words:

High levels of speech perception ability have been demonstrated in (a) patients with severe motor speech deficits and damage to the mirror system; (b) individuals who failed to develop a functional motor speech system due to neurological disease; (c) infants who have not yet developed motor speech control; and (d) even chinchilla and quail, which don’t have the biological capacity for speech.

Mirror neurons might help us figure out what someone’s saying when it’s hard to hear, or under conditions where we have to lip-read or otherwise guess some of the speech’s content. But the part they play in actual speech comprehension seems to be minor, at best.

3. Minds
Last but not least, let’s talk about the idea that mirror neurons might help us understand the thoughts, feelings, and intentions of others. Some researchers have claimed that these neurons are activated when we (or monkeys) try to understand the motivations and thoughts behind another’s action – but just about all those papers cite the same four studies in support of this, and three of those are Rizzolati’s studies from the ’90s. Even in those studies, Rizzolati and his teams stressed that the mirror neuron system they observed was probably responsible only for linking observation and imitation – not for any sort of abstract comprehension.

Loads of debate has also centered around the question of whether autistic individuals might have a malfunctioning mirror neuron system – an idea known as the “broken mirror hypothesis.” Two of this view’s biggest supporters are UCLA’s Marco Iacobini and Mirella Dapretto, who have argued for several years that there’s a link between autism and decreased activation in sites “with mirror properties.” However, other studies have failed to replicate these results – and even more importantly, a growing body of evidence suggests that the study’s whole premise may have been flawed:

Much larger and more firmly established bodies of data contradict predictions made by MN theory. For example, it has been repeatedly demonstrated that autistic persons of all ages (from preverbal children to mature adults) have no difficulty understanding the intention of other people’s actions.

So, although mirror neurons may have something to do with our ability to imitate the actions of others, it doesn’t seem particularly likely that they’re an essential component of our ability to understand others’ minds.

As you can tell, a lot of these researchers are still far from waving a white flag – and there are  plenty more interesting questions about mirror neurons that are begging to be researched – for instance, what role their firing patterns might play in the process of learning, and whether the human brain even has a direct equivalent of the macaque mirror neuron system at all. The more research we perform on mirror neurons, the more conundrums they seem to reflect back at us – and it seems they’ll continue to provide us with intriguing puzzles.

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