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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