Wednesday, May 28, 2008

Mirror neurons: what ARE they doing?

Our mirror neuron course is wrapping up soon. For the final meeting (next monday) we decided that we would all go to the literature and pick our favorite mirror neuron paper and report back to the class on what we learn. I know some students are looking into papers talking about language evolution, others are looking at clinical applications (e.g., autism), so it should be interesting. If any TB readers would like to summarize a favorite paper, please feel free to post a comment!

So what did we learn in this course? One thing I learned is that it is completely unclear what real (i.e., macaque) mirror neurons are doing. There is no evidence whatsoever that mirror neurons in macaque support action understanding. In fact, Rizzolatti has stated that this hypothesis is impossible to test using standard methods (lesion). Might MNs support imitation? Well, macaques are supposed to lack the ability to imitate. If this is true -- and given recent reports the jury may still be out on this -- MNs cannot support imitation. I suggest they simply reflect good old-fashioned sensory-motor associations.

On the human front, we also learned that there is no evidence that the "mirror system" (in quotes because the response characteristics of this system are different from real mirror neurons) supports action understanding, and in the speech domain there is, in fact, good evidence against the view that the "mirror system" is the basis for speech recognition. The "mirror system" may support gesture imitation in humans, which need not involve understanding. But this is just another way of saying that the brain can establish sensory-motor associations, which is certainly not news.

So if there is NO evidence supporting the most flashy claim regarding mirror neurons (action understanding), why has this system gotten so much attention? And why has it been so widely and blindly accepted as truth? I don't know, but I can speculate:

1. The idea has intuitive appeal. Since we all understand our own actions (or at least think we do -- I bet this is debatable), it seems reasonable that we might understand other people's actions by relating them to our own. Be wary of hypotheses with intuitive appeal: they require much less empirical support to convince people that they are true.

2. The idea simplifies a complex issue. Semantics is complex. (See our previous blog discussions.) If we can explain how we "understand" via the response properties of a small population of cells in Broca's area, this avoids all kinds of messy semantic complications.

3. There is a cellular grounding. If the only data regarding the mirror system came from human functional imaging studies, I bet no one would believe it. The fact that a cell in the monkey F5 shows "mirror" properties provides a neurophysiological anchor for human-related speculation, and this goes a long way towards lending credibility.

4. There is a cognitive grounding. The motor theory of speech perception provided an independently motivated theory (even if it is wrong) to back up the general concept of a motor theory of action understanding. It's no accident that the motor theory was referred to in the earliest empirical MN papers.

5. It is easily generalizable. If we understand actions of others by relating them to our own, we might understand speech, emotions, etc., in the same way. Now we have a cellular basis for all sorts of complex systems ranging from speech to empathy, and potential explanations for complex disorders such as autism.

You put all these plusses together and you have a theory that people are willing to impulse-buy without evidence, and without thinking too hard about it.

So the next time you review a paper that states, "This [mirror neuron] research has shown that areas of the brain which subserve motor action production are also involved in action perception and comprehension." (Saygin et al., 2004, Neuropsychologia, 42:1788-1804, p. 1802), note in your review that this claim is speculative and unsupported by empirical evidence. If the authors want to include such a statement, this is fine, but it should be prefaced by "Although there is no evidence to support the theory, mirror neuron research has been used as a basis to speculate that..." Otherwise, the innocent reader will assume that there is an empirical backing to the claim and the speculation will continue to be propagated in the literature as fact. If we take this attitude, the field will be better situated to test the hypothesis rigorously, and if the evidence is supportive, the theory can win over even the skeptics like me. If it is not supported, then we can figure out what this system might be doing. Either way, we make progress.

1 comment:

Michael said...

Very nice summary Greg - Certainly this area of research (MNs) is far from being irrelevant or dead. As this course has shown, better science and skepticism needs to be at the forefront of this research and not, perhaps, a researcher's own benevolent zeal or passion for widely creative possibilities.