The neural architecture of discourse compression
Introduction
According to current linguistic views discourse processing is hierarchical (Graesser et al., 1994, Greene et al., 1992, Kintsch and van Dijk, 1978). Microstructure is formed through the initial extraction of propositions which are organized into a multileveled network, operating at the level of word meanings, syntactic rules, and local cohesion within sentences through mechanisms such as substitution and ellipsis. Memory constraints dictate that only a subset of propositions is retained, forming a macrostructural representation of the message. Refinement of the macrostructural representation occurs through processes such as deletion, generalization and constitution, and most important in the present context, can be facilitated by re-telling (Bloom, 1994).
The mental representation of discourse is not simply a function of the text itself, but is shaped and determined by a variety of cognitive, sociolinguistic, and situational factors including repetition (Johnson-Laird, 1983, Van Dijk and Kintsch, 1983, Xu et al., 2005). Re-telling progressively produces a more compact and concise narrative. This is seen as a decrease in speaking time and word count, as well as the duration and distribution of pauses across repeated cycles (Goldman-Eisler, 1968). These changes will be referred to here as the compression effect. A similar effect is seen on tasks of discourse comprehension where silent reading of the same passage of text is associated with a decrease in reading time (Levy & Burns, 1990). In theoretical terms, refinement through repetition reflects the progressive loss of direct links between the microlinguistic aspects of the text or language output and the overarching mental representation. Failure to compress a message with repetition might be general marker of inefficient discourse representation, as suggested by the absence of a compression effect on re-telling in neurologically impaired individuals, such as those with temporal lobe epilepsy (Field, Saling, & Berkovic, 2000).
The cerebral organization of discourse compression is of considerable interest. To the extent that compression is a marker of effective representation, the existence of a neural network that is progressively recruited on repeated engagement with narrative, either in re-listening or re-telling, could be hypothesized. We are aware of only one neuroimaging study that throws some light on this issue. While not designed to isolate a cerebral network underlying the compression effect, a PET activation study on comprehension and memory of discourse conducted by Maguire, Frith, and Morris (1999) showed recruitment of the left middle frontal gyrus and medial parietal cortices when a second hearing of a story was compared with a first hearing.
The aim of the present study was to study the functional neuroanatomy of compression using functional magnetic resonance imaging (fMRI), and an activation task in which subjects listened to a story presented in on three consecutive occasions. Our major contrast of interest was the change in blood oxygen level dependent (BOLD) signal between the first and final presentations of the story. To establish if the regions identified in this way were specifically associated with discourse changes on re-telling, we conducted correlational analyses between fMRI data and performance on an established, out-of-scanner measure of the compression effect (Goldman-Eisler, 1968).
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Participants
Twenty healthy volunteers (10 male; 1 left handed), who were native English speakers and had begun or completed a tertiary education, participated in the study. The mean age of the participants was 29.9 years (SD 1.5 years; range 18–48 years), and performance for all subjects on standard language testing was within normal limits: the mean number of words generated on the Controlled Oral Word Association Test (COWAT) (Spreen & Strauss, 1991) was 42.2 (SD = 7.4; range 31–55), and the mean score on
Contrasts between each presentation of the story and rest
There was a striking difference in the pattern of activation associated with the first and subsequent presentations of the same story relative to rest (Fig. 1). The first presentation of a story relative to rest was associated with a highly significant increase in BOLD signal in perisylvian cortex bilaterally. The second and third presentations of the same story relative to rest were also associated with bilateral activation of perisylvian areas. In addition, activation extended to frontal,
Discussion
Our findings point to the existence of a functional neuroanatomical substrate underlying the compression effect, parts of which are common to the effects of re-telling and re-listening. The study demonstrated a neural network that was elicited and modified in response to repeated engagement with narrative. On each presentation of the story relative to rest, perisylvian cortex was activated bilaterally. The second and third presentations of the same story relative to rest activated the same
Conclusion
Our findings reveal a neurofunctional basis for the compression effect, a phenomenon only previously described in the psycholinguistic literature. The compression network involves regions beyond the classical left hemisphere perisylvian language axis encompassing the left and right middle frontal gyri as well as the right inferior parietal region. The right inferior parietal region might represent a key co-ordinating hub in a wider discourse processing network since it not only responds to
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