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 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.
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