Monday, August 4, 2008

New paper by Corianne Rogalsky on the anterior temporal lobe and sentence processing

Congratulations to Corianne Rogalsky, who successfully defended her dissertation here in the TB West lab last month. Corianne will be moving up the road to begin her post-doc in the Damasio lab at USC starting in the Fall. Corianne's dissertation work focused on the neural basis of sentence comprehension, and included studies (i) on the role of anterior temporal regions in syntactic vs. combinatorial semantic processes, (ii) on the relation between working memory and sentence comprehension in Broca's area, & (iii) on the relation between sentence comprehension and melody perception.

The first of these experiments has just been published in Cerebral Cortex (Rogalsky & Hickok, 2008, Selective attention to semantic and syntactic features modulates sentence processing networks in anterior temporal cortex, advance e-pub). The goal of the study was to try to use fMRI to distinguish between two competing hypotheses for the role of the ATL in sentence processing, one that it is involved in syntactic computations (of some undetermined sort), and two that it is involved in combinatorial semantic computations. These are difficult to separate because manipulations of sentence structure affects combinatorial semantics and vise versa. So we decided to leave the sentences alone and ask the subjects either to attend to structure (monitoring for occasional syntactic agreement errors) or to sentence-level meaning (monitoring for occasional semantic implausibilities). The idea is that attention to one or another aspect of the sentence will boost the gain of that process, which will be reflected in greater activity in the neural networks supporting said process. We focused on the ATL region that has previously been shown to show relatively selective responses to sentences compared to non-sentence stimuli. To do this we ran a "localizer" where we contrasted passive listening to sentences with passive listening to lists of words. This picked out a region in the ATL bilaterally (see black outline in the figure below). So is this region modulated more by attention to syntax, suggesting a syntactic function? Or attention to sentence semantics, hinting at a more compositional semantic function?

As it turns out it wasn't an either-or result. A small portion of the left ATL "sentence ROI" was modulated by attention to sentence semantics but not syntax (see anterior blue shade region inside the black outline). Most of the left ROI was equally modulated by both tasks, that is, activation was higher in during the attentional conditions (equally so) than during the passive listening condition that defined the ROI. The attention tasks did not modulate activity at all in the right hemisphere "sentence ROI": activation was no different in the attention tasks than in the passive listening condition.

What does this mean? Given that all of the left hemisphere sentence ROI was sensitive to the semantic attention task, I think we can conclude that the region isn't performing some "pure" syntactic computation. Rather, it seems to be involved in some kind of syntactic/semantic integrative function, at least in the left hemisphere. Of the two competing hypotheses, then, our result seems to favor the combinatorial semantic view.

1 comment:

David Poeppel said...

Congrats, Corianne. Really interesting results. Tackling the issue of combinatorics in language is pretty damn tough, and this adds important new constraints on how to think about this. I hope lots of people read the paper.