Tuesday, August 26, 2008

How much can a motor representation contribute to action word meaning?

I'm guest editing a special issue of Brain and Language on mirror neurons so I've been poking through more of the mirror neuron-and-speech literature recently. Most of this literature concerns the semantics of action words. There's no shortage of papers showing that motor cortex activations to action words follow some degree of somatotopic organization. I don't think there much evidence at all suggesting that these activations go beyond simple association -- the meaning of the word kick is associatively linked to foot actions -- but I want to raise a more general issue. Namely, even if we admit that motor representations are part of an action word's semantics, how much of the meaning of these words is actually explained by movement?

Let's take the sample items from a prominent study by Friedemann Pulvermuller et al. (2005, Functional links between motor and language systems. European Journal of Neuroscience, 21: 793-797). This is a TMS study that found faster lexical decision RTs to hand/arm words when hand/arm areas were stimulated, and faster RTs to leg words when leg areas were stimulated, etc.

So here are the sample arm words: fold, beat, grasp.

What does the motor code for the action FOLD look like? It depends on what you're folding. Motor codes associated with folding an empty sugar packet are going to be rather different from those associated with folding a bed sheet. And the meaning of fold is not restricted to hand/arm actions. I can fold my tongue, I can fold paper with my feet, I can fold paper by feeding it into a machine, and proteins can fold without my help at all. Clearly the meaning of fold is not dependent on any specific hand/arm actions.

The verb beat is no better. I can beat an egg with a fork or a hand-held blender, and I can beat an attacker with any number of actions (punching, hitting with a bat, kicking, sitting on).

Likewise, grasping can be achieved with a hand or a tool or a mind, as in "Language within our Grasp." Consider as well, that if i reach out to grasp a glass, but the glass is damaged and shatters in my grip, I did not grasp the glass, but instead crushed the glass. So the same motor action can lead to different conceptual action.

The situation isn't much better for the leg action example words: kick, hike, step. Hike in particular is odd in that motorically it is identical to walk, the difference being in the purpose of the excursion not the motor codes at all.

In other words, a specification of motor codes is not going to get you very far in capturing verb meaning, even for these cherry-picked examples.


Anonymous said...

Namely, even if we admit that motor representations are part of an action word's semantics, how much of the meaning of these words is actually explained by movement?

Does it really matter though, how much of the 'meaning' of the word is explained, i.e. the fact that a word has multiple meanings does not really seem important.

Greg Hickok said...

It has nothing to do with multiple meanings. Take hike. What is the motor representation of that action? Is it distinguishable from the motor representation of walk? If not, the semantic representation affording by the motor code can't even distinguish these two verbs. Therefore, we haven't explained much.

Anonymous said...

Surely some words have more specific motor codes than others, in the same way that words referring to objects can be rated as more or less imageable/concrete (e.g. wine glass vs. container). It could also be the case that even if a motor code is indistinguishable between two similar verbs (hike/walk), other aspects of meaning (e.g. visual knowledge) jointly disambiguate them.

Greg Hickok said...

Right, so now we get to the issue: how much is actually encoded? My point is that if you start looking carefully at the how much meaning can actually be represented in a motor code, the answer is not so much.

Anonymous said...

how much of the meaning of these words is actually explained by movement?

Part of the meaning of a verb is its argument structure. Would something be explained if there was a strict relation between motor activation and argument structure ?
Unclear that hike and walk actually differ in this regards.

Anonymous said...

Unclear to me what 'explaining the meaning' refers to ?

If I follow the discussion, finding a distinct motor activation for every distinct verb in a language L (hike vs walk) WOULD constitute an 'explanation'?? How so ?

Let me entertain a different possibility. Part of a verb meaning is its argument structure. If a relation between motor activation and argument structure could be found would this go further than 'mere association' and constitute an 'explanation' of the 'meaning' ?

Bottom line, I just would like to hear more about what could count as an explanation in this case.

Greg Hickok said...

That is an excellent question: what does it mean to say that the semantics of action words are encoded or grounded in the motor system? How about we look through some of the papers promoting this position and see if we can figure out what they mean?

Anonymous said...

so we can figure out what they mean?