Tuesday, July 23, 2013

Rodd, Davis, and Johnsrude: "The Neural Mechanisms of Speech Comprehension" (2005)

This paper reports on two fMRI experiments which contrast ambiguous speech to unambiguous speech, and unambiguous speech to speech-like noise.

Blob One and Blob Two

By comparing pictures taken in each of these conditions, extracting significant differences, this design gives an indication of where in the head ambiguity is sorted out. The conclusion is that two areas in particular seemed to be disproportionally active when the experimental subjects listens to ambiguous speech:
The results of two fMRI experiments show that when volunteers listen to sentences that contain semantically ambiguous words, activity increases in both temporal and frontal brain regions. This confirms the involvement of these regions in the semantic aspects of sentence comprehension (i.e. activating, selecting or integrating word meanings). (p. 1266)
Roughly speaking, the areas in question were the bit of the brain behind the ears (on both sides of the head), and the bit behind the eyebrow (on the left side only).

Anatomical drawing of a brain from a 1918 textbook.
The parts of the brain discussed in the text are roughly located behind
the lower part of the temple and the lower left side of the forehead.

Decoding Efforts vs. Selection Efforts

The study did not include any distinctions more fine-grained than ambiguous/unambiguous. In particular, it did not contrasts skewed and balanced ambiguity; this is significant, since reading a word used in one of its less frequent meanings involves inhibition which may require cognitive effort.

Consider for instance the following example sentence from the paper:
  • the cymbals/symbols were making a racket/racquet
There are 67 results for cymbal(s) in the BNC; there are about 3000 for symbol(s). If I read this sentence aloud for you and took a picture of your brain while you listened, I would see a lot of activity in some regions; but this might be best interpreted as a trace of the force you exert in order to suppress the dominant but irrelevant meaning of the sound /ˈsɪmbəɫ/.

This process of suppressing a loud noise may or may not be a different from choosing between two competing alternatives, but we can't say on the basis of this experiment.

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