# Quick notes on “mirror neurons”

1\. Two definitions of “mirror neurons”
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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.[^3y1cjifsm5u]

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
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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 *anti*social behavior! For more on that, see [Symbol grounding & human social instincts](https://www.lesswrong.com/posts/5F5Tz3u6kJbTNMqsb/intro-to-brain-like-agi-safety-13-symbol-grounding-and-human), Section 13.5 on empathetic simulation.

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

![](https://39669.cdn.cke-cs.com/rQvD3VnunXZu34m86e5f/images/ad25efb94855163048a24af615c33303b7108bf47caa17c4.png)

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

[^3y1cjifsm5u]: 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).