Wednesday, May 18, 2011

Two new ways the mirror system claim is losing steam

Next week Giacomo Rizzolatti will give the Keynote Address at the 23rd annual meeting of the Association for Psychological Science. According to the published abstract of the talk
... he will discuss the limits of the mirror mechanism in understanding other people. He will stress that the parieto-frontal mirror mechanism is, however, the only mechanism that allows a person to understand others’ actions from the inside, giving the observing individual a first-person grasp of other individuals’ motor goals and intentions.

As is clear from this quote and from recent publications by Rizzolatti and colleagues (e.g., Rizzolatti & Sinigaglia, 2010), the Parma group is reining in their original claim that mirror neurons are the basis of action understanding, period. Now the mirror system has a much more restricted role in which the system allows understanding "from the inside". It's still not at all clear to me that this concept actually does any work, but even granting this point, it is worth noting that mirror neurons are only responsible for understanding actions that the observer knows how to perform. This is a highly restricted domain of function when considering the range of actions that one needs to understand yet has no experience executing. For example, I've never actually punched someone in the face, but I need to be able to recognize and understand such an action should I see it (and I believe that I can do so). Similarly, I've never coiled, but I can understand the intensions of a coiled snake. I've never reached for a holstered gun, but if saw such an action, I'd know what it meant. So what does understanding from the inside buy us? Not much in addition to what the regular action recognition system can already do, I would argue. Or put differently, not much that one couldn't pick up via pure sensory learning. (Side note relevant to this last point: motor knowledge and sensory learning are rather confounded. The more experience we have performing an action, the more opportunities we have to learn the sensory consequences.)

I think there is a more serious theoretical concern with the direction that mirror neuron theory is going, however. There is a shift away from the idea that mirror neurons code particular movements and toward the idea that they code motor goals or intentions. So mirror neurons don't do their magic via motor simulation, but by activating the goal or intention directly. This sounds like a profound insight, but in fact it pushes mirror neurons right out of the motor system and into the dreaded cognitive system that Rizzolatti and colleagues so wish to avoid: "Because the observers are aware of the outcome of their motor acts, they also understand what the others are doing without the necessity of an intermediate cognitive mediation" (another quote from the APS abstract).

Why is this the case? Because the high level goals or intentions, the things we are trying to understand, are inherently non-motoric. Now, movements can have goals, to bring my hand in proximity to a raisin, but being able to predict the trajectory of another's reaching movement is not the kind of understanding that Rizzolatti is talking about. He's arguing that mirror neurons code intentions, i.e., to possess the raisin, to eat the raisin. These are not motor goals. I can just as easily achieve this goal by grasping the raisin and putting it on my tongue, bending over and slurping it directly with my mouth, or asking someone to put it in my mouth for me. The goal is sensory (taste the raisin, satiate hunger) or cognitive (possess the raisin). The motor system is just a set of options for achieving the sensory/cognitive goal. Think of any action you like and you end up with same conclusion regarding the nature of the goal. The goal or intention is not the movement itself, it is the consequences of the movement. It's no wonder one doesn't need a motor system to understand the goals or intentions of actions: the goals and intentions are not motor!

The progression of mirror neuron theory was predictable from what happened to the motor theory of speech perception. It started out quite strong and therefore interesting, but when it was discovered that strict motor simulation did not disrupt speech perception, the "motor representations" that were used for speech perception became more abstract. It was no longer the actual motor gestures, but the intended motor gestures that were critical. This pushed the critical representations right out of the motor system and into the "cognitive" system, or as I and others would argue, straight back into the sensory system.

The mirror system is part of a larger system designed to for action control, not action understanding (Hickok & Hauser, 2010). As Rizzolatti has pointed out himself, the actions of others are clearly relevant to selecting and controlling one's own actions, just as the shape of objects are relevant to selecting and controlling actions. The idea that mirror neurons support action understanding is an interesting hypothesis, but one that has been thoroughly vetted. It's time to move on.

Hickok G, & Hauser M (2010). (Mis)understanding mirror neurons. Current biology : CB, 20 (14) PMID: 20656198

Rizzolatti, G., & Sinigaglia, C. (2010). The functional role of the parieto-frontal mirror circuit: interpretations and misinterpretations Nature Reviews Neuroscience, 11 (4), 264-274 DOI: 10.1038/nrn2805


JKwasniak said...

I have commented on this excellent posting at

Anonymous said...

Dear Prof. Hickok,

I just can’t understand how it is possible that the Mirror Neuron Theory may enjoy still so much success. That this happens in Italy (where I live) it is comprehensible, but that this also happens in the United States it is not (see Association for Psychological Science). But do people read the literature? You have so clearly shown the weaknesses of Rizzolatti’s theory. Famous neuropsychologists as Caramazza, Gopnik, Heyes etc. are skeptical about the real importance of the mirror neurons. It has happened to me listening to some seminars by Rizzolatti & Co. No a word about the criticisms turned to their theory. Their seminars invariably end with thunderous applause and Rizzolatti & Co. look pompous. Recently, I read the Italian translation of the book by L. D. Rosenblum "See What I'm Saying. The Extraordinary Powers of Our Five Senses. " The author talks about mirror neurons in an absolutely enthusiastic way. And what about Ramachandran? I am completely puzzled by all this.

There is one last thing that I would like to ask you. Could you clarify the difference between the following concepts: “ mirror neurons enable action understanding via strict motor simulation” and “mirror neurons code the goals of the action". They do not say the same thing?


Greg Hickok said...

Scientific opinion is both empirically and sociologically driven. The mirror neuron position is more sociological and empirical at this point.

Strict motor simulation refers to the idea that when an action is observed the movement itself is simulated rather directly (grasping with the right hand using a particular grip is simulated with a right hand grasp and the same grip). Coding or activating the goals of the movement is abstracted from the movement itself. So that same movement would not be simulated in terms of the details of the movement but in terms of some higher level goal (e.g., to possess the object) which can be achieved by any number of motor programs.