1. Two definitions of “mirror neurons”

I have noticed the term “mirror neurons” being used in two ways:

  • (A) Any set of neurons that activates both when I am doing / feeling / relating to X and when I am watching (or imagining) someone else doing / feeling / relating to that same X.
  • (B) Same as (A), but where X is more specifically an action concept (e.g. “grasp”), and where the neurons in question are more specifically in certain specific frontal-lobe brain areas including (parts of) premotor cortex, primary motor cortex, and Broca’s area.

The (B) usage is the original one, and much more common in the technical neuroscience literature. By contrast, (A) seems quite common among non-neuroscientists, and shows up at least occasionally even in the technical neuroscientist literature.[1]

Anyway, regardless of what we call it, I strongly believe that (A) is a real neuroscience phenomenon. If I watch you juggle, versus if I juggle myself, some of the same juggling-related neural circuits are active in both cases. That seems perfectly obvious to me. After all, how else could I possibly generalize from one to the other? How could I even know that they have anything to do with each other??

By contrast, my sense is that (B) is a giant dumpster fire of nonsense and badly-interpreted experiments. I recommend the excellent book The Myth of Mirror Neurons by Gregory Hickok for that whole story.

2. Mirror neurons and kindness

There’s a pop science trope that mirror neurons explain love and kindness and so on. I don’t think that (A) in itself explains human prosocial behavior. I do think that (A) is probably an ingredient in the explanation of human prosocial behavior. And I also think that (A) is probably an ingredient in the explanation of human antisocial behavior! For more on that, see Symbol grounding & human social instincts, Section 13.5 on empathetic simulation.

3. Why I myself don’t use the term “mirror neurons”

Why don’t I personally use the term “mirror neurons” for (A)? First, because it would be arguably an incorrect usage, or at least ambiguous—see above. Second, because I find “mirror neuron” a somewhat misleading term anyway.

For example, as above, take some set of temporal-lobe neurons which collectively represent the concept “juggle”. These neurons are (I claim) active both when I’m juggling and when I’m watching you juggle. Should we call those neurons “mirror neurons”? I say no. It’s much more helpful and sensible to call them “neurons that represent the concept ‘juggle’”, right??

Likewise, consider neurons in the anterior cingulate cortex which activate both when I’m disgusted and when I see a disgusted face. Should we call those “mirror neurons”? I don’t think so; in my mind, it’s much clearer to call them “disgust-reaction neurons, and oh by the way, fun fact, those neurons also tend to activate during empathetic simulation of someone else experiencing a disgust-reaction”.

So my policy is to avoid the term “mirror neuron” altogether when trying to describe how things actually work in the brain.

“A neuron wistfully gazing into the mirror” (DALL-E 2)
  1. ^

    Examples of people using the word “mirror neuron” to refer to (A) are Meyza et al. 2017 and van der Gaag et al. 2007. I can also report personal communication with an eminent neuroscientist who was under the impression that the definition of “mirror neuron” was (A). 

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One of the problems I've encountered with certain people who like to talk about mirror neurons is that they tend to think of them as being, in themselves, the causal explanation for empathy, etc., rather than just neurons that happen to be involved in broader behavior-modeling circuits. Sort of a "built by evolution, run by magic" way of thinking about them. As though they were sensory neurons, tapping into "telepathic energies" or whatever.

So I strongly agree that moving away from talking about mirror neurons like they're some special class of neurons is important for deconfusing what is actually going on in the brain. Instead, the focus should be on figuring out what sort of brain circuitry is necessary for learning common neural representations that can associate self-behavior with other-agent-behavior.

That's smart! When I started graduate school in psychology in 2013, mirror neurons felt like, colloquially, "hot shit", but within a few years, people had started to cringe quite dramatically whenever the phrase was used. I think your reasoning in (3) is spot on.

Your example leads to fun questions like, "how do I recognize juggling", including "what stimuli activate the concept of juggling when I do it" vs "what stimuli activate the concept of juggling when I see you do it"?, and intuitively, nothing there seems to require that those be the same neurons, except the concept of juggling itself. 

Empirically I would probably expect to see a substantial overlap in motor and/or somatosensory areas. One could imagine the activation pathway there is something like

visual cortex [see juggling]->temporal cortex [concept of juggling]->motor cortex[intuitions of moving arms]

And we'd also expect to see some kind of direct "I see you move your arm in x formation"->"I activate my own processes related to moving my arm in x formation" that bypasses the temporal cortex altogether.

And we could probably come up with more pathways that all cumulatively produce "mirror neural activity" which activates both when I see you do a thing and when I do that same thing. Maybe that's a better concept/name?

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