David Kemmerer (Purdue), who has been doing a lot of work on the issue of semantic representation of action, sent me a very thoughtful note on the topic in response to the recent discussion here about "somatotopic semantics". He has graciously allowed me to post his substantive points here. He's working on a chapter that will detail his analysis of the literature, including a new, up-to-date figure generated by Javier Gonzalez Castillo that summarizes the relation between motor cortex and action word processing (contact David K. directly if you'd like a preview). This will no doubt be required reading for those interested in this issue.
From David Kemmerer:
Hi Greg -
Interesting discussions on the blog lately. Very briefly, some of my current thoughts of the controversial issues are as follows.
1. I believe localizers are important, and if you look at my chapter, you'll see that several recent studies have used them and have found overlap with corresponding action verbs/sentences. But of course not all studies have found such overlap -- e.g. Postle et al. (2008). So the question is: Why the inconsistencies?
2. I've been reading Graziano's book about the motor cortex, and it's making me realize how shaky many common assumptions are. For example, somatotopy is overrated; other constraints also help shape the organization of the motor cortex. Also, the boundary between primary and premotor cortex is anything but clear; it may not even exist in any meaningful functional sense.
3. You raised an important issue when you pointed out, following Pinker (1989), that many verbs, like "pour" and "fill," are compatible with a huge range of real-world scenarios. But this doesn't imply that the meanings of all action verbs are so abstract or schematic that they couldn't possibly depend in part on motor representations. Some verbs are pretty specific - e.g., as I point out in my chapter, the class of Running verbs in English has around 125 members, many of which are distinguished from each other in rather fine-grained ways. And even for verbs with more general referential scope, monosemy is probably very difficult if not impossible to maintain (see Charles Ruhl's old book "Monosemy" for some interesting insights), and there are of course prototype- and exemplar-based approaches to characterizing their meanings. I could go on and on about this, drawing upon the substantial linguistics literature that's largely neglected by neuroscientists. But for now I'll just say that I think it's too dogmatic and simplistic to assert, especially at this early stage in the game, that it's in principle implausible that verb meanings could be partially implemented in the motor system. The challenge will be to tease apart, on the one hand, the language-specific semantic representations that reside in long-term memory (what Barsalou would call simulators), and on the other, the contextually influenced instantiations of those representations that occur in our brains during on-the-fly language processing (what Barsalou would call simulations). Basically, this is the type/token distinction.
4. I have a strong suspicion that task effects are going to end up playing a big role in all of these debates. After all, there doesn't seem to be a gold standard for determining what does and does not constitute language comprehension. It's a mercurial rather than a monolithic phenomenon. The contrast between automatic and controlled processing is probably relevant, but people seem to have philosophical differences about this. For example, I know from personal communications with Barsalou that he thinks the automatic/controlled distinction is sort of a red herring. As a result, he's not really bothered by questions about whether language-induced "imagery" falls inside or outside the realm of comprehension. Certainly other people would disagree. I'm not yet ready take a firm stand on the issue.
That's it for now. I've been thinking a lot about this stuff lately, but I'm afraid I don't have time to elaborate all of my ideas in detail here. I do hope to contribute now and then to the literature, though. And I enjoy following your blog.