A couple of comments on the Gallese et al. 1996 paper (Brain, 119:593-609) and Rizzolatti, Fogassi, & Gallese 2001 review paper (Nature Reviews Neuroscience, 2: 661-670.
The Gallese et al. paper is an empirical report, and as such, a good one. Have a look if you want to know about the functional properties of mirror neurons. One interesting tidbit: five different hand actions were assessed, grasping, placing, manipulating, hand interaction, and holding. Most cells, by far, are partial to grasping. Of 92 cells sampled, 30 were selective for grasping (next highest selective cell count was placing cells & manipulating cells, 7 cells each), and 39 responded to grasping plus some other action. So grasping is represented in 75% of sampled cells. This probably just means monkeys grasp a lot, but maybe it's worth noting.
There is some commentary in the discussion section that's worth examining because a clear definition of action understanding is provided. "By this term [understanding], we mean the capacity to recognize that an individual is performing an action, to differentiate this action from others analogous to it, and to use this information in order to act appropriately" (p. 606).
This is a strong position. If we flip the statement around, it implies that without MNs the animal cannot "recognize that an individual is performing an action" or "differentiate this action from others analogous to it" or "use this information in order to act appropriately".
This view seems to rule out the possibility of observational learning. Now, I'm way outside my area of expertise, so I'm going to resist stepping too far out on a limb. I am aware however, that observational learning occurs in many species. E.g., I saw a TV science show once where an octopus learned to get into a jar by watching another octopus open the jar. (I TOLD you I was outside my area of expertise!) Anyway, the point is, making a claim that rules out observational learning seems to a lay person like me, the wrong theoretical move to make (see below).
We looked that Rizzolatti, Fogassi, & Gallese (RFG) review paper because it was continually cited in the Rizzolatti & Craighero paper after statements regarding other mechanisms for action understanding. This is important, because if there is another way to break into the system, you might get out of some of the circularity problems we noted previously with respect to imitation and action understanding, as well as this related problem with observational learning. I was hoping we'd get a bit of clarification on the question of these other mechanisms, and we did.
RFG are admirably strong in their claims regarding the importance of MNs in action understanding. They're certainly not hedging any claims: "We understand action because the motor representation of that action is activated in our brain" (p. 661).
So what about those pesky STS cells that also respond to the perception of action? Well, RFG provide a nice summary of these cells. STS cells respond not only to simple actions, but also appear to combine action-related bits of information. For example, the firing of some action responsive cells in STS is modulated by eye gaze information: the cell fires only if the actor is looking at the target of the action, not if the actor is looking away.
RFG summarize, "The properties of these neurons show that the visual analysis of action reaches a surprising level of complexity in the STSa" (p. 666). A level that seems to be much more sophisticated than that found in F5, I would interject. RFG continue, "But the existence of these neurons and, more generally, of neurons that bind different types of visual features of an observed action, is not a sufficient condition for action understanding per se" (p. 666). I completely agree. You can't make logically necessary inferences about function on the basis correlated neural activity. But the same is true of mirror neurons.
I think, though, that the point RFG are trying to make here is that unlike in pure sensory perception (in their view), the "semantics" of an action is somehow inherent in the action itself. Again, evidence from aphasia leads me to question this position. How is it that there are patients who can perfectly reproduce an action (repeat speech verbatim) yet fail to understand the meaning? But let's move on.
So how does STS acquire these complex response properties? To the best I can tell, RFG hold that the motor system endows the STS with its action perception abilities.
"We argue that the sensory binding of different actions found in the STSa is derived from the development of motor synergistic actions. Efferent copies of these actions activate specific sensory targets for a better control of action. Subsequently, this association is used in understanding the actions of others" (p. 666).
So it sounds like the STS action recognition system fully inherits its action recognition/understanding abilities from the motor system. How it inherits "a surprising level of complexity" from a much simpler system is not clear. But RFG's position is clear: action understanding is a function of the motor system. Action understanding can be achieved via other systems, such as the STS, but this is only because the motor system has endowed such systems with its action understanding ability via previous association.
Correct me if I'm wrong, but from this collection of assumptions, namely, >
1. "We understand action because the motor representation of that action is activated in our brain." RFG p. 661.
2. "We cannot claim that this is the only mechanism through which actions done by others may be understood." RC p. 172.
3. These other mechanism acquire their action understanding properties via associations with the motor system. (My paraphrase of above quotes from RFG p. 666.)
it follows that an action cannot be understood unless it has already been executed by the perceiver. From this it follows that observational learning should be impossible in creatures that use their mirror neurons to understand actions. I don't know about monkeys, but I believe (without knowledge of the literature) that humans are good at observational learning.
If all of these assumptions are correct, we've just proven that the mirror neuron theory of action understanding is wrong.
So let's discuss the problems with my assumptions. Anyone have any ideas?