Tuesday, 9 August 2011

Mirror Neurons, or, What's the Matter with Neuroscience?

One of the problems I face as I try to figure out what the brain is up to, if not representing, is that I can't rely on the neuroscience literature to back me up. The problem is that, while there has been a lot of data collected over the years, very little of it has been collected within an ecological framework. Neuroscientists are looking for how the brain represents information, not how it perceives it; they're looking to see where perception and action are integrated in the brain, not how the brain-body-environment system produces stable, functional behaviour. This matters because there's no such thing as theory-free observations - all data comes from this experiment rather than that experiment, and even simply reporting a result is laden with theoretical assumptions, even when these aren't explicitly identified. So until I can find a neuroscientist interested in collecting a little data (and I would love to hear from any such person!) I'm limited to laying out the consequences of taking the ecological route and critiquing what's out there already. 

The worst offenders, in terms of theory-laden data disguised as 'merely the observed facts', are mirror neurons. They are, I think, the text book example of what's the matter with neuroscience, and I thought it was about time to talk about them a little. For those interested, the most recent exposition of what mirror neurons are and do can be found in a recent (and currently open access) issue of Perspectives on Psychological Science: there's a brief introduction (Glenberg, 2011a), a detailed Q&A paper (Gallese et al, 2011) by five main figures in the field, and a summary (Glenberg, 2011b).(Update: recent meta analysis finds mirror type activity all over the brain, including the cerebellum!)

Mirror Neurons
Mirror neurons were discovered by accident in a monkey lab in Italy (Di Pellegrino et al, 1992). While measuring single neurons from premotor cortex in macaque monkeys, someone noticed that these neurons responded both when the monkey was moving to pick up a peanut and also when the monkey was watching someone do the same action. Why would a single neuron in motor cortex respond to a merely perceptual event?

The suggested answer is that these neurons must be how the brain manages to reunite two separate processing streams (perceptual and motor). Perception is input; signals flow in from various sensors (e.g. the retina) to anatomically distinct regions of cortex, and these are processed into increasingly higher-order information as the signal is passed up at least two streams (dorsal and ventral, for visual information). Motor control is output; top-down motor commands are issued from motor cortex, generated by schemas and programmes designed to produce classes of movements. Perception and action are clearly distinct from one another, both in terms of function (input vs. output) and in terms of anatomy (cortical and bodily). We can do some interesting things, however, that seem to transcend this obvious boundary: we can copy someone else's movement, for example (imitation) and when we observe someone moving we can interpret and understand what that movement is for (action understanding). Mirror neurons seem to be the solution to the problem of how this could possibly be: if there are neurons tuned to both perception and action then there is a location in the brain where these streams cross, so to speak.

Mirror neurons solve a problem that doesn't exist
The main problem with mirror neurons is that they are solving a problem which doesn't exist; perception and action are not separate domains which must somehow come into alignment. There is, instead, the single domain of 'perception-action'; this system is how we interact functionally and successfully with the environment. The reason neuroscience looks to mirror neurons as so wonderous and amazing is that it looks to anatomy and says 'these two things are separate, we must find out how they talk to each other'. One of Gibson's many insights is that anatomy is not the final arbiter here, function is. Perception-action is the function of a broad, integrated system, not the labels of distinct entities. So 'motor' cortex responding to 'perceptual' input is only astonishing if you believe these are two separate functions of the system. 

So what are mirror neurons doing, then?
I don't dispute that Rizzolati found what he described: single neurons responding to both perceived action and the action itself. I'm disputing that this means these cells are resolving a Cartesian dualism. I'm not actually surprised there are cells that respond this way. The control of action, which 'motor' cortex is presumably involved in, also includes the ongoing perception of what the limb is up to. There is then significant overlap in the perceptual content of 'my limb moving towards a peanut' and 'your limb moving towards the peanut'; they are, in fact, two instances of the same event.

Event perception
We can categorise what goes on in the world into events. Two events are of the same type if they contain the same information, and they will only overlap in terms of information if they overlap in terms of the dynamic description of what's going on in the world. For example, a pendulum event only occurs when something is moving according to a particular equation of motion. Two instances of a pendulum event can vary in their parameters (e.g length, and thus period) but the structure of the motion will be the same. Information about these events is specific to the stable dynamic structure, not the varying parameters, and so these two events will be identifiable as being of the same type. Different events have different spatio-temporal structure, with different informational consequences, and can therefore also be identified as different on this basis. 

We are actually extra-ordinarily good at this sort of thing (Muchisky and Bingham, 2002), and from very early on, although it of course requires learning (Wickelgren & Bingham, 2001). For example, you can make a point light display of a ball being hit, accelerating then slowing down to a stop. Adults can tell the difference between the display run forwards or backwards (the velocity profiles are not symmetrical) and the backwards one simply looks incorrect (because it is - it would never occur naturally). Babies can tell the difference between the two (demonstrated via habituation) but have no preference for either one - they have yet to learn what the difference means (Wickelgren & Bingham, 2001). Biological motion perception is another fascinating example of event perception.

In the case of you or I reaching for that peanut, that same idea applies: these two motions are instances of the same underlying dynamical process, and thus overlap in their perceptual consequences. The overlap is not 100% - you can tell the difference between the two events. But they do overlap, and this overlap is meaningful - specific to the dynamics of the event. This overlap also supports an interesting feature: perceptual constancy, or view invariance. People can readily identify an event from any view which preserves access to the relevant motion profile (although some views are obviously clearer than others; Wickelgren & Bingham, 2004; Zaal et al, 2000).

Event perception is grounded in information, not mirror neurons
One of the things I struggle with a bit at the moment is that much of what I'm trying to do simply sounds like I'm redescribing an already explained phenomenon (see the final note on this post). Not here. Everything I've said explains why we might treat an instance of me moving as similar to an instance of you moving, and this explanation is based in the information created by the underlying dynamics of the event type. You don't need specialised neurons whose job it is to knit these things back together: to identify what you did as similar to what I did I simply need to perceive the informational overlap. One of the much touted jobs for mirror neurons (action understanding) becomes perception-based, and similarly, the other (imitation) becomes about moving so as to produce the underlying dynamical structure, as specified by the view-invariant information specific to that event. 

But these single neurons are up to something - so what? While there have been moves in the literature to talk about 'mirror systems', Rizzolati's original discovery was the dual sensitivity of a single neuron. However, single neurons simply aren't responsible for anything. Every neuron in our brain is actually an active participant in multiple networks; these networks ebb and flow over time in response to use, and what a single neuron's behaviour means depends entirely on it's current network context. Rizzolati's neuron might simply have been involved in event perception, and the astonishing thing isn't that it was found in supposedly 'motor' cortex. No, the astonishing thing is that we persist in calling that bit of the brain motor cortex given this data. The error, of course, comes from the unwarranted assumption that perception and action are two distinct domains.

The problem with mirror neurons is that they are nothing of the sort; they do not implement a connection between perception and action in motor cortex which grounds action understanding, imitation, empathy or lack of autism. The fact that they tend to be in 'motor' cortex only means that we've labelled that bit of cortex badly, and given the fact that all actions entail perception-action, I'm not actually surprised it is interested in event structure. 

To my knowledge, no-one has done "mirror neuron" research using event forms as controlled stimuli, although that would make an excellent beginning to an ecologically based neuroscience. Only when we have the right job description for the nervous system, however, will we understand what it's up to when we give it something to do.

Di Pellegrino, G., Fadiga, L., Fogassi, L., Gallese, V., & Rizzolatti, G. (1992). Understanding motor events: A neurophysiological study. Experimental Brain Research, 91, 176–180. Download

Gallese, V., Gernsbacher, M.A., Heyes, C., Hickock, G., & Iacoboni, M. (2011). Mirror neuron forum. Perspectives on Psychological Science, 6, 369–407. Download

Glenberg, A. M. (2011a). Introduction to the mirror neuron forum. Perspectives on Psychological Science, 6, 363-368. Download

Glenberg, A. M. (2011b). Positions in the mirror are closer than they appear. Perspectives on Psychological Science, 6, 408-410. Download

Muchisky, M.M. & Bingham, G.P. (2002). Trajectory forms as a source of information about events. Perception & Psychophysics, 64(1), 15-31. Download 

Wickelgren, E. & Bingham, G.P. (2001). Infant sensitivity to trajectory forms. Journal of Experimental Psychology: Human Perception and Performance , 27(4), 942-952. Download

Zaal, F., Bingham, G., & Schmidt, R. (2000). Visual perception of mean relative phase and phase variability. Journal of Experimental Psychology: Human Perception and Performance, 26(3), 1209-1220. Download


  1. I'd say that the problem that mirror neurons are trying to solve is not one created by a false dichotomy between perception and action in a behavioural context. Rather, they are bridging the gap between our clear ability to seamlessly perceive and act upon things in the real world in a integrated fashion, and the fairly specific dissociations that occur when parts of the brain are damaged. Where is the perceptual problem in a stroke patient who has suffered motor cortex damage? Neuropsychology, not neuroscience (or behavioural psychology) was responsible for naming the regions of our brain because of these fairly semantically straightforward deficits?

  2. When I took neuropsychology classes in undergrad, the teachers always admonished us to remember that you have to be careful when applying lessons learned on a damaged brain to normal function. (They then routinely failed to take this advice and apply like mad.) The kind of distributed damage and functional effects that show up are interesting, but the damaged brain is a qualitatively different entity that has often partially repaired some of the functional deficits by doing something atypical.

    Plus the deficits exhibited by patients are never as clear cut as they are described in the textbooks. I saw a video of a patient with no corpus callosum generate incorrect but non-random names for objects presented to his right hemisphere, and Mark has stories about DF's occasional ability to happily visually identify objects in the pub. There aren't neat modules being carved out by CO2 poisoning.

  3. Very nice post. I've always been suspicious of the whole mirror neuron business, but was never quite sure why. This may be it.


  4. "the astonishing thing isn't that it was found in supposedly 'motor' cortex. No, the astounding thing is that we persist in calling that bit of the brain motor cortex given this data"


    I have made this complaint in several other contexts, but in never occurred to me to use it for this purpose.

  5. Gavin,
    You are quite correct, but the basic labels given by neuropsychology have not been revised in decades, were based on very primitive evidence, and were motivated by many other ideas about how the brain worked that we might now like to reject. To take a brain damaged person, identify a single, obvious deficit, and declare that the function of that brain part was to do the single thing the person is obviously deficient in... well, it is a bit loony by today's standards. This is because 1) the brain is ridiculously interconnected, and 2) deficits in ability can be quite subtle, especially if you do not have pre-injury measures on the particular patient. It also assumes that we can identify what about an inability is deficient. I am reminded of the old joke:

    A biologist was interested in the jumping ability of frogs. He spent sometime training a frog to jump on command, then placed it on a table next to a ruler. "Jump frog, jump!" he commanded. The frog leapt into the air brilliantly. After checking carefully, the biologist wrote in his lab book, "Frog with four legs - jumps 35 inches." Then, he removed off one frog’s legs. Placing him back on the counter, he yelled "Jump, frog, jump!" Measuring again, he wrote, "Frog with three legs - jumps 20 inches." After removing a second leg, the experiment continued, "Jump frog, jump!" and another measure was taken, "Frog with two legs - jumps 12 inches." A quick surgery later, the frog was back in place, and the biologist once again said , "Jump frog, jump." Impressed, the scientist wrote, "Frog with one leg - jumps 8 inches." Another flick of the knife, and the investigation continued. “Jump frog, jump.” But nothing happened. “Jump frog, jump!” Again nothing. Angry now, “JUMP FROG, JUMP!” But then the scientist understood. “Frog with no legs - " he wrote carefully in his note book, "deaf.”

  6. "we can copy someone else's movement, for example (imitation) and when we observe someone moving we can interpret and understand what that movement is for"

    I am generally on board with everything you've said, but this quote made me wonder: These abilities are fairly special - imitation in particular is a rare and (almost?) exclusively human ability. I agree that we should be able to perceive the similarity between events (whether performed or observed) based on perception of the same basic spatio-temporal event dynamic. But shouldn't most animals be able to do this? And, if so, why is imitation so rare?

  7. A quick Google hunt shows that imitation does show up in other species, but in varying degrees and with various levels of evidence. I didn't see a decent review with this quick look, though.

    Susan Jones from IU had an interesting point. She's done work on imitation in infants (things like them poking their tongue out when you do, etc) and shown a) lots of it is arousal based coupling of a reflex action to something exciting in the environment and that b) imitation is learned; there's a developmental process at work, when human infants get exposed to lots of imitation early on. (And if it's learned, it should be grounded in perception, I think.)

    A quote from this paper:
    "The relevant point for the present is that imitation can be learned. This suggests that imitation may in fact usually be learned, in the usual course of development. Or put another way, some combination of behavioral experiences, expectations, awareness and knowledge of self-other physical and cognitive and motivational similarities, and other kinds of knowledge may occur or emerge in development, and may combine to enable and motivate behavioral copying. So now, what if human children actually do begin to learn to imitate in the context of thousands of vocal exchanges with their caretakers? Well then, the poor chimpanzee is at a distinct disadvantage B unless someone mitates his vocalizations from an early age. In short, I wonder if what the chimpanzee needs to develop a much stronger and more obvious imitative ability is not several million more years of evolution, but instead a large dose, early administered, of appropriate - though not typical for the species - experience."

    All that said, it's an interesting question to which I don't have a full answer. Imitation apparently does show up in other species although it's not all that common or typical. Imitation is also a hard thing to define - if I achieve the same goal with different actions, is that imitation? What if I use the same actions but mirror reversed (ie the way it looks when I watch you front on)? etc. Part of the problem may be not quite knowing what to look for.

  8. you might be interested in this: http://www.all-souls.ox.ac.uk/users/heyesc/mirror_neurons.htm

  9. imitation in animals is widespread. it tends to be studied by having a manipulandum, that you can use to obtain a food reward in several ways. Eg a lever that can be moved left or right to get food.
    A demonstrator animal is trained to make a specific action (move the lever right). An observer animal then watches the demonstrator animal, and the behvaiour of that animal is then recorded. If it makes more lever right actions than left, it is interpreted as imitation.
    A colleague has been doing some work with tortoises, where they access a food well either with their nose or their feet. She hasn't got any evidence of imitation with this task. In another task, where they can walk left or right around a clear perspex wall to get food, she has found imitation.

  10. Pam is correct, imitation is very widespread. One reason there is so much confusion about this is that imitation became, for a while, the key "why humans is special" candidate.

    Long ago, humans were the tool-using animal, then as people found out all the cool things animals could do, they tried to redefine tool-use to be exactly that thing humans do, but animals don't. Much confusion resulted, and ultimately it was abandoned. Then humans were the language using animal, until people found out all the cool things animals could do, and tried to redefine 'language' to be exactly that thing humans do, but animals don't. Much confusion resulted, and most have abandoned it. I'm sure I'm missing other stages of this game, but at some point 20 or 30 years ago, humans were declared the animal that imitates. This lead to big, silly wars in the animal behavior / comparative psychology literature - careers made and lost. As more and more evidence accumulated that all sorts of animals imitated, people tried to continuously re-define 'true imitation' to be exactly that thing humans do but animals don't. As with the other debates (tool use, language, etc.), it is clear that human and non-human animals differ, but any notion of a clean break results from the linguistic game, not from the evidence.

    P.S. For an example of the silliness of the imitation debates, many papers were rejected for publication if the researcher could not give a satisfactory answer to whether the organism was "really imitating vs. merely perceiving the affordances." Uhg!

  11. Andrew & Pam: Thanks for the info.

  12. A colleague has been doing some work with tortoises, where they access a food well either with their nose or their feet. She hasn't got any evidence of imitation with this task. In another task, where they can walk left or right around a clear perspex wall to get food, she has found imitation.
    This is actually interesting. There's probably affordance/effectivity based limits on the kinds of imitation you'd expect to be able to coax out of an animal; they'll only be perceiving certain properties of the world that are relevant to them, etc.

  13. I've been reading all your posts on representation with interest. I am strongly steeped in the representation tradition, and my work is closely related to mirror neurons, so I'm sure my comments are influenced by that perspective.

    Nevertheless, I find your description of the 'representation' approach to be a bit of a straw man. Ideas like event coding and mixing of perception & action have been around for longer than mirror neurons (read Wolfgang Prinz). And more nuanced theories that consider mirror neurons in terms of action prediction and response preparation do not carry all the extravagant untested claims that mirror neurons explain imitation and all social cognition. So while I agree with many of your points, I still think it is possible to think about things like mirror neurons within a framework of representation and information processing.

  14. So while I agree with many of your points, I still think it is possible to think about things like mirror neurons within a framework of representation and information processing.
    Sure you can - but what does it buy you? Nothing, I will suggest, except a dead end. My hunch is that if you take perception seriously (as Gibson did) you simply end up with a fundamentally different job description for the brain, and representations are simply no longer required; they go the way of the aether.

    Mirror neurons are a good example: whatever these neurons are up to, it's not connecting perception to action, which is the underlying function ascribed to them. Why not? Because there's no need, as I argue above. So I'm arguing that taking a representational, cognitivist stance actually leads to an incorrect characterisation of what the brain is doing - it's bad science, in other words.

    I'm slightly familiar with Prinz's work; it mostly strikes me as trying an attempt to re-integrate perception and action given that they have been separated by the brain. There's plenty of work like this in the cognitive literature. My issue is simply that he's struggling to solve a problem that doesn't exist, and that realising this actually simplifies a lot of things.

    Whenever I deny representations, I encounter people who can say 'well that's not what I mean by representation'. In my experience (and I'm not trying to be patronising, honest :) they often actually do mean what I say are representations, and just don't know it. But even if you do mean something different, it doesn't affect my argument: regardless of which flavour of representation you endorse, they aren't required when you start over with a more careful analysis of perception.

    I'm not trying to brush you off: this is a common counter to these arguments and it needs addressing. Right now I'm working on the assumption that I've addressed it (in various posts here, and see especially the reading group posts on Chemero, 2009) and I'm trying to move forward. Sabrina's tackling this more head on right now (here and in upcoming posts). Have a browse, I've tried to make sure posts are tagged usefully; comments, questions and challenges always welcome.

  15. Actually, let me add - critiques of my arguments by people like yourselves (doing neuroscience, framing things representationally, etc) is something I am really looking for, so thanks for reading and commenting!

  16. To add to my previous comment - I found it interesting that you say "event perception is grounded in information". That feels like it is very close to my position of seeing the brain as an information-processing device. But elsewhere I think you want to reject that perspective.

    I guess I'm just not ready yet to give up on my representation/information framework, that I feel has been and continues to be useful.

  17. I have quite a specific definition of information; patterns in (for example) light, that are extended over space and time and that are specific to the event in the world that causes the structure. (Sabrina covered the relevant chapter in Gibson (1979) here and here). The gist is that these patterns are meaningfully (lawfully, in fact) connected to the event that caused them; we detect structure in light, and therefore, by virtue of this lawful relation, perceive the event in the world.

    Information processing accounts, on the other hand, are typically about taking an impoverished stimulus (say, a retinal image) and performing computational transformations on that image to process it into something capable of supporting perception of the environment. The kind of information I'm talking about doesn't require this sort of processing; in fact, the claim is that if this sort of processing was required, perception wouldn't actually be possible - it doesn't work.

    I have seen some ecological types trying to lay claim to the term 'information processing' and use it in the context of the embodied, enactive process of detecting information and thus perceiving the world. I'm always doubtful about attempts to retool phrases with such long and specific histories, though.

  18. Indeed, the word 'information' causes trouble. If another satisfactory word could be created, it would help everyone. Alas, writing it out long-hand is not very convenient. By my understanding, anytime an ecological psychologist uses the word 'information' we should be able to substitute 'higher-order invariants that specify'

  19. Recent meta-analysis of mirror neurons in humans: still not conclusively found, but the data clusters in specific places, including the cerebellum! I read this as evidence that the concept is flawed, but of course the author's conclude "Our findings suggest a core network of human brain regions that possess mirror properties associated with action observation and execution, with additional areas recruited during tasks that engage non-motor functions, such as auditory, somatosensory and affective components.". Sigh.

  20. Any implications from this perspective for the theory (that I don't buy) that autism is a deficiency in mirror neurons?

  21. As I understand it, this idea is mostly rooted in the following logic:

    1. Imitation requires you to map perception of an action onto that action.
    2. Autistic children are impaired at imitation.
    3. Mirror neurons are required for imitation.
    4. Therefore, autistic children have problems with their mirror neurons.

    Now, given that imitation actually requires event perception (as laid out in the post), and not special neurons reuniting perception and action, then I think this logic is flawed. Second, I was involved in a project involving autistic children and as part of this did a literature review on imitation in autism. The results vary wildly and seem to depend on whether the action to be imitated is canonical (eg a salute) or made up; the former is fine in these kids. So the 'imitation' deficit is complicated.

    That said, there is some evidence children with autism are impaired at biological motion perception (which is an example of event perception). Mirror neurons are not the basis of biological motion perception, though, so whatever's wrong, it's still not mirror neurons. But asking questions about biological motion perception is a better scientific question anyway, and I'd be much more inclined to think this kind of perceptual deficit might be involved.

    TL;DR - blaming autism on mirror neurons is wishful thinking and a distraction from better, more fruitful lines of enquiry, I think.

  22. Spoke too soon: no clear visual motion processing deficit in autism, although the low IQ autistic participants were worse (that could be due to several factors). So actually, this is more evidence against any kind of mirror neuron hypothesis.

  23. phrenology 2.0 ... neuroscience

  24. I agree with many of the criticisms here, but I think it is possible to deal with them in the context of an information-processing approach. The key is to focus on what Cecilia Heyes calls the "correspondence problem" -- the problem of matching an observed action with the corresponding self-generated action.

    The "standard theory" says that mirror neurons implement a hard-wired solution to the correspondence problem, but to anybody familiar with computational neuroscience that's impossible to believe. The problem is way too hard.

    My own theory is essentially the reverse of the standard theory. Rather than hard-wiring a solution to the correspondence problem in order to enable imitation, I believe that imitation generates a teaching signal that allows children to learn the solution to the correspondence problem. Kevin Laland and William Hoppitt have published papers expressing a very similar idea, and implemented it in a neural network simulation.

    The background for all this is way too complex to cover here, but I've written a chapter about it for a book that I have been writing, and I've placed a version of it on my web site, at http://weskaggs.net/?p=102, if anybody is interested.

    Regards, Bill Skaggs

    1. Thanks for the comment, Bill. A quick thought:

      imitation generates a teaching signal that allows children to learn the solution to the correspondence problem
      The question remains, what is the content of this teaching signal and why does it correspond to anything? I still think the answer is that the content comes from the dynamics and the correspondence comes from the fact that the two events (moving and watching someone else move) overlap in these dynamics.

      You have to trace it back this one extra step, we argue, or else your theory isn't anchored to a plausible mechanism. In your account, what is it about the teaching signal that allows it to inform learning about the correspondence?

  25. I think this is a good article. I also wrote a similar one before. Considering consciousness model, I think it will be complemented.

    I found some objection in the comments. Actually counterexample may be found. However they did not seem to have their conscious model. I would like to propose episodic memory (which cause consciousness and also mirror neuron phenomenon) is the base of all (that is equal to consciousness).