Wednesday, March 13, 2013

Assessing the Armada: Language Comprehension and the Motor System

In the wake of the discovery of mirror neurons an armada of studies on the role of the brain’s motor system in language processing has appeared over the horizon the past decade. We review some of this work here. Behavioral studies have shown interactions between reading and motor tasks and brain-imaging studies have shown that (pre)-motor areas of the brain are active during the processing of action words and action sentences.

Some researchers have taken mirror neuron theory and these results to mean that the motor system plays a central role in language comprehension, whereas others are downright skeptical about the role of the motor system.

In our own behavioral studies, we have found interactions between language comprehension and motor actions. Although one can draw limited conclusions from such experiments, they do suggest that motor resonance is modulated by sentence context. You can observe interactions between reading and action only when the focus of the sentence is on the action and even when the target word is not an action verb.

In a recent fMRI study (well, recent...I actually had the idea for this study in 2007), we again found evidence that motor resonance is modulated by sentential context. We presented Dutch subjects with Dutch sentences (somehow this made more sense to us than presenting them with Mongolian ones) that contained a subordinate clause. In main clauses, Dutch is, like English, a subject-verb-object (SVO) language. However, in subordinate clauses, Dutch uses a SOV order, which means that the verb is at the end of the sentence.

We made use of this feature of the Dutch language because we wanted to examine the effect of sentence context on the motor activation elicited by action verbs. To this end, we contrasted literal and nonliteral sentences. Here are two examples.

1. Iedereen was blij toen oma de taart aansneed. (Everyone was happy when grandma the cake cut.)
2. Iedereen was blij toen oma een ander onderwerp aansneed. (Everyone was happy when grandma a different topic broached.)

So here we have the same target verb (aansneed) at the end of the sentence. In one case it refers to a manual action and in the other to a mental/verbal one (thank God grandma stopped grandpa from telling that boring fishing story for the zillionth time). If motor activation is verb-driven, then the target verb should elicit similar amounts of (pre)-motor activation for literal and nonliteral sentences. However, if motor activation is modulated by sentence context, then there should be more motor activation elicited by literal sentences than by nonliteral ones.

There were more components to this study (for example concerning somatotopy) but I just want to focus on the literal/nonliteral comparison. We found more motor activation in structurally defined motor areas BA4 (the primary motor cortex) and BA6 (the premotor cortex) for literal than for nonliteral sentences. In other words, we found that sentence context modulates motor activation. Other studies have found similar patterns and they are discussed in our paper.

So motor resonance seems to be driven by conceptual combination rather than by action verbs themselves. I am working on a theoretical account for this and for related findings, which I will describe in a later post and/or paper, but I want to look at a different question here.

Virtually all of the research on language and motor resonance has focused on individual words or sentences. Using these “textoids” might yield a very skewed view of language comprehension. Specifically, in this case it might lead one to overestimate the role of the motor system in language processing.

However you want to slice the cake, even if the (pre)motor cortex reliably responds to every occurrence of the word kick in a story and even if you can establish a causal role for motor activation, what does this tell us about the role of motor activation in discourse comprehension?

If you look even at simple stories like The Ugly Duckling, you’ll find that verbs denoting simple actions are just not very common. Stories—let alone expository texts—tend to be about bigger things than kicking a ball, handing over a pizza, or screwing in a light bulb.

Some years ago, my then student Larry Taylor and I wrote a paper on language comprehension as fault-tolerant processing. We argued that language can be understood at different levels. A schematic understanding can be had by combining cues from grammar and the closed-class elements of a sentence (function words, suffixes such as –ed). A deeper understanding requires the establishment of causal links between the events described in a sentence, a situation model.

A yet deeper understanding presumably involves a first-person mental simulation of the described events, such that not only the causal connection between the events is established but also the manner in which this connection is formed. We give a concrete example of this in the paper.

According to this reasoning, the role of motor activation is limited to the deep understanding of simple events. Given the small role that simple actions play in narrative and nonnarrative discourse, my current view is that the motor system plays a supportive role in discourse comprehension. It helps “flesh out” representations of simple actions.

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