Thursday, September 17, 2009

What mirror neurons are REALLY doing

Mirror neurons are cells in monkey frontal area F5 that respond both during the execution of action and during the perception of action. Explaining why these cells respond during action execution is easy and uncontroversial: they are motor cells in a motor area -- they respond during action execution because they are involved in the coding of actions. The perceptual response is more difficult to explain. Think first about "canonical neurons", neighbors of mirror neurons in F5. Like mirror neurons, these cells respond during action execution, e.g., grasping, and they also have sensory properties, e.g., responding to the presentation of graspable objects. The sensory responses of canonical neurons have a fairly intuitive and standard explanation: the grasping of objects needs to be informed by the shape of the object (you grasp a paperclip differently than a grapefruit) and so the sensory input is used to drive appropriate grasping gestures. Importantly, canonical neurons are not assumed to be responsible for visual recognition, they just receive relevant input from areas involved in the processing of visual features.

But what about mirror neurons? Why would the percept of someone else performing an action such as grasping a piece of food help guide the monkey's own food-grasping action? One thought is that mirror neurons support imitation, but apparently macaque monkeys don't imitate so that can't be right. So the theory that was proposed early on and completely dominates (suffocates even) thought on mirror neuron function is that these cells support action understanding. According to this view, the sensory response of mirror neurons is not relevant to the monkey's own actions, unlike canonical neurons. It is rather a mechanism for understanding what other animals are doing via motor simulation. The logic is, if I understand what I'm doing when I reach for a peanut, then if I can simulate another's peanut-reaching action in my motor system, I can understand what s/he's doing.

I have argued that the action understanding theory of mirror neurons has never actually been tested in monkeys and where it has been tested in the "human mirror system" it has been proven wrong: damage to the "mirror system" does not necessarily cause a deficit in action understanding (Hickok, 2009). I have yet to see a strong empirical refutation of the evidence I discussed, but a common response that I do hear is, "you propose no alternative theory of mirror neurons."

Although I've never been fond of it's-the-only-game-in-town arguments (a theory can be demonstrably wrong even if we don't yet have a better theory) I think the point is worth taking seriously even if it is only partially true. I did propose that mirror neurons reflected a form of motor priming, but didn't develop the idea in any detail.

In response to the only-game-in-town argument, here is what I'd like neuroscientists to do, just for fun. Rather than obsessing on the idea that the sensory response of mirror neurons has no relevance to action execution, I'd like folks to at least consider the possibility that mirror neurons, like their canonical neighbors, take sensory input for a motor purpose.

I'll start. This is speculative and unsubstantiated, but so is the action understanding theory and you have to start somewhere. Consider this a jumping off point for discussion...

Can we learn something from the behavior of dogs? If you've played fetch with a dog you may have noticed that it quickly learns to anticipate the consequences of throwing actions. For example, it is not hard to fool a naive dog who plays a lot of fetch with a fake throw. Even though the ball isn't flying through the air the dog may nonetheless take off in chase. Presumably, the animal has learned to recognize throwing actions. This is interesting because dogs can't throw and so can't have throwing mirror neurons. This is also interesting because somehow the action observation, throwing, is triggering an action execution, chasing, in the dog. This tells us that and action observation-execution sensory-motor circuit exists in the animal. There may even be "chase" cells in the dog's motor cortex that fire both during action observation and action execution. This is the same sort of circuit by which the pairing of a tone (cf., the throwing action) with an airpuff to the eye (cf., the ball flying) can eventually lead to an eyeblink response (cf., chasing) just to the presence of the tone (cf., the throwing action).

But dogs are smart. Try fake throwing without the ball, e.g., while the ball is still in the dog's mouth. You don't get much of a response (if your dog responds anyway, retry the exercise with a larger ball such that the dog can easily see whether or not you have something in your hand). This is interesting because it is kind of like pantomime and mirror neurons don't respond to pantomime. You can imagine how our non-mirror action observation-execution circuit might start to behave like a mirror neuron.

The point here is that it is not hard to imagine sensory-motor circuits that take observed actions as input and use these actions as triggers for any number of executed actions via regular old sensory-motor association. The cells underlying these circuits would probably behave like canonical neurons responding both to the execution and observation of the (non-mirror) actions.

That's all fine but how could the observation of an animal reaching for a piece of food trigger a similar action in the observer? In other words why would some actions trigger mirror actions? Here's where I need the help of any readers who know primate behavior (I certainly don't). But again reasoning from dog behavior, I've noticed that if you place a ball or toy in front of a moderately well-trained dog, it may watch you and wait (an untrained dog might just grab it). If you start to reach for the object the dog may suddenly lunge for it, trying to beat you to it. The dog isn't imitating you (in fact it can't), it learned to recognize that your reaching results in ball-possession and this triggers an action with a competitive goal. I can imagine this happening naturally with food for example, where one animal's movement towards a piece of meat triggers a competitive counteraction on the part of the dog. The trigger action could be the reach of a human, a forward movement of another dog, or even the looming flight of a bird or the mechanical action of a tool. Importantly, recognition of these actions is not being carried out in the motor system of the dog via motor simulation (at least not for the human, bird, and tool actions).

Presumably monkeys can also learn to recognize actions and respond with appropriate actions themselves. Observing an aggressive posture might trigger a flee or hit action. Observing a grasp toward a piece of fruit might trigger a competitive "mirror grasp" for the same piece of fruit. Maybe watching an experimenter reaching for a raisin that the monkey really wants triggers exactly this kind of competitive motor response and maybe this is what mirror neurons really reflect. Or maybe it is just another wrong theory about mirror neuron function.


Hickok, G. (2009). Eight Problems for the Mirror Neuron Theory of Action Understanding in Monkeys and Humans Journal of Cognitive Neuroscience, 21 (7), 1229-1243 DOI: 10.1162/jocn.2009.21189

8 comments:

Kevin Hill said...

I'm afraid that sounds a lot like the theory you are trying to place an alterative to. If a dog "recognize that your reaching results in ball-possession", isn't that just saying he understands the action? It seems that the mirror neurons are still intimately involved in the process of understanding an action in a way that pure motor neurons aren't.

At best your just moved mirror neurons from the the cause of understanding an action, to the result of an understood action, still in the absence of direct motor function.

This should be pretty easy to test however. There should be a whole host of actions where the selfish thing would be to perform a complementary action rather than the same action. (grooming vs presenting your back) So, mirror neurons should show a host of asymmetries for certain behaviors.

Greg Hickok said...

Hi Kevin,

No, no, no... there CAN'T be any mirror neuron involvement in a dog's understanding of the throwing action because dog's can't throw and so don't have any motor "throw" cells. The dog's understanding HAS to come from somewhere else.

I've never denied that dogs, monkeys or humans can understand. I just dispute the claim that it happens in the motor system via simulation.

It sounds a bit like you are confusing mirror neurons (cells that show a correspondence between the actions they respond to in observation and execution) and plain old sensory-motor cells such as canonical neurons (cells that don't show such a correspondence, i.e., don't "mirror").

Rogie said...

Hi guys,

It seems a bit silly to argue about dogs and their ability to throw a frisbee, of course they cant. I think this argument misses a major point. Mirror neurons might not be responding to 'actions' per se. Based on what I've seen, I would argue that they are mainly responding to the goals of actions. If you assume that mirror neurons are 'mirroring goals' (they respond to the same action simply because the actions reflect the same goal, eating in the case of grasping for example) it doesn't matter that a dog can't throw, what matters is that they can represent the goal of throwing. On that note, activation of the 'eat' goal by observation could certainly trigger execution of an 'eat' action, assuming the observed and executed action overlap in the goal realm, this is perfectly plausible to me.

The question is, then, how to test this idea about mirror neurons and competition. I knew a guy in the Netherlands, Rick van Baaren, who had a device called the 'silverback' he used in experiments with social mimicry. This device attached to the back of subjects like a skeleton and was used to measure postural reactions to social stimuli. I believe if people moved towards the subject, there was a natural tendency to move backwards, but there was no object in the middle that they were competing for! which could certainly alter the dynamic.

So let's propose an experiment to answer the question regararding competition and the mirror neuron system, preferably using healthy adults and fMRI. How about we compare the human MNS's response to competitive goal directed actions (a grasp towards a single food item) and non-competitive goal directed actions (maybe a grasp to one of many available foods). If the system is there to process competitive goals we might expect the response to be higher for this sort of action. Of course controlling motivation (which we know modulates MNS activity from the study on fasting) and attention (which modulates practically everything) would be a major concern here. But perhaps we can discuss this idea further.

Roger

Greg Hickok said...

Hi Roger,

You raise some interesting issues but let's focus on your point about MNs responding to goals because I have a problem with this.

First what IS the goal of a behavior where a monkey reaches for a raisin? Is the goal to REACH? GRASP? POSSESS? BRING TO MOUTH? CHEW? SWALLOW? INCREASE BLOOD SUGAR? GET A JUICE REWARD FOR CORRECT TASK PERFORMANCE? Define for me what you mean by goal and then we can actually see if goal-oriented coding makes sense.

Based on what you've said, you seem to be aligning the goal in this case with EAT: "activation of the 'eat' goal by observation could certainly trigger execution of an 'eat' action..." This would predict that a given MN should respond both to the observation of the experimenter reaching for raisin (which the monkey understands as a preamble to eating?) and to any "eating" action on the part of the monkey, which would include reaching for a raisin, peeling a banana, sucking on the juice straw, digging up ants, bobbing for apples, being fed by the investigator. Are you suggesting that MNs would respond equally well to all these actions? I don't think this would hold empirically.

Suppose it did though. Maybe any instance of eating on the observation or execution side activated said MN. Then are we still in the motor system? Or are we studying a high-level cognitive behavior? If a cell can in principle code both grasping and ordering from a menu (both preambles to eating) then we are not talking about motor representations anymore. And if we are not talking motor representations then the basis for the action understanding claim -- that of motor simulation -- has evaporated, proving that the theory is wrong.

Rogie said...

"First what IS the goal of a behavior where a monkey reaches for a raisin? "

Well, my guess is that there are are immediate goals (grasp), and ultimate goals (ingest) to say the least. You bring up the possibility for many separate subgoals that may exist as we proceed along the way to the ultimate goal. We can probably consolidate that thinking a bit if we say that certain subgoals (chew, swallow) are normally tightly linnked/sequneced such that we might be able to consider then, under some circumstances as a single entity. Research by van Schie for instance suggests separate brain mechanisms for immediate and ultimate goals at least (frontal vs. parietal for instance, MNs in different regions could have different goal-related properties). "What is a goal" is a nice question to consider though. It usually gives me a headach to consider the multiple definitions out there.


"Based on what you've said, you seem to be aligning the goal in this case with EAT: "activation of the 'eat' goal by observation could certainly trigger execution of an 'eat' action..." This would predict that a given MN should respond both to the observation of the experimenter reaching for raisin (which the monkey understands as a preamble to eating?) and to any "eating" action on the part of the monkey"

Certain neurons in monkeys do of course respond to "logically related" actions. While we don't think of these as "mirror neurons" because they code differnet executed and observed actions (i.e. experimenter grasping food and the monkey eating the food i believe is the finding), they do seem to code for a common goal to which both are related (i.e. eating). In monkeys they account for < 10% of neurons if i recall correctly, but they could be more plentiful in humans, i would suspect they are at any rate.

"Suppose it did though. Maybe any instance of eating on the observation or execution side activated said MN. Then are we still in the motor system? Or are we studying a high-level cognitive behavior? If a cell can in principle code both grasping and ordering from a menu (both preambles to eating) then we are not talking about motor representations anymore. And if we are not talking motor representations then the basis for the action understanding claim -- that of motor simulation -- has evaporated, proving that the theory is wrong. "

You know, I don't know the answer to this right now. I will go out on a limb and say that we ARE talking about motor representations most of the time (I believe there is pretty strong evidence that a lot of what we call cognition is "embodied"). Ordering from a menu involves a somewhat standardized motor protocol (okay, language has infinite possibilities, but at a restaurant, i wager word choice and combinatorial rules are somewhat limited). Even so, I agree that a single neuron is unlikely to fire for grasping (a single action) and ordering (a much more complex motor phenomenon). If a cell did code for both of these, I guess it would qualify as a 'higher-order neuron' though. Nobody is saying this would be in human mirror areas. Are there more abstract goals we can think of that are not associated/related to any motor action, after all motor simulation may not be required for all kinds of action understanding, and thus could be represented in other brain areas, my guess is yes. Gotta go, its Friday.

Greg Hickok said...

Ok, let's simplify and consider grasping with hand vs. foot vs. mouth. All have the same, let's call it, "basic level" goal: to get the raisin and eat it.

Does a typical grasp with-hand-mirror neuron also fire during the observation of grasping with foot or mouth?

If so, it is not coding motor actions because the motor programs required to grasp with the three different effectors are different. Conclusion: mirror neurons don't work by motor simulation; the theory is wrong.

If not (if the cell only fires during observation of hand grasping), then it is not coding for even the most basic level goal and the idea of abstracted goal-oriented coding is wrong.

Have a good weekend!

EddyBearr said...

I only really skimmed the article, but a lot of it seems based on the assumption that dogs are incapable of throwing because they lack "throwing cells." I think this is a bit too...for lack of a better word, conceited (for humans over dogs,) to say. It's also a logical fallacy in the form of affirming
the consequence.

I know very well, after having owned many dogs, that dogs very much understand the concept of "moving something by moving their own body." A border collie I had would move my hand over his head, using his nose, when he wanted me to pet him. A pekingese-maltese mix I have will play fetch with herself, in which she "throws" with her mouth, pushes with her nose, or "pushes" by squeezing her front paws together. I don't really know much about neuroscience (more than the average joe but less than anyone who made it their major,)but honestly, the only reason a dog can't easily throw something is because dogs lack the body parts/limbs, not the brain capacity, to throw.

I only wanted to address that one point. I didn't read the article and I'm not a neuroscience person (well, sorta. Biology major/psychology minor, with a perspective of "biology is everything, everything about our thoughts, memories, and feelings can be explained via neuroscience,) but I definitely thought this "dogs lack the brain capacity to throw" presumption was not only doubtful, but most likely wrong. I'm sure you know very well just how closely related we are, but it almost seems that you think we're further apart than we are.

janet amighi said...

I think we underestimate the amount of imitation that takes place in other animals. Sometimes it is dismissed as social facilitation-but studies now show e.g. that dogs can learn to copy the movement of other dogs, and even copy new human movements. Whats even more fascinating they can translate the movement of the human into their own bodily ability. So if a human lifts a top of the garbage bin with her hand, the dog told to copy that, will use its mouth to do the same action. This suggests considerable understanding of the action in my view.