Capacity Theory

Just and Carpenter (1992) put forward a theory dealing with some of the constraints on sentence comprehension. Their capacity theory focuses on working memory, by which they mean "the part of the central executive in Baddeley's theory that deals with language comprehension" (Just & Carpenter, 1992, p. 123; see Chapter 6). Within the theory, working memory is used for both storage and processing during comprehension. Storage and processing demands can be heavy, and it is assumed that working memory has a strictly limited capacity. Thus, the storage demands during language processing need to be held to manageable proportions. For example, each word is processed thoroughly when first encountered (this is known as the immediacy assumption), instead of storing it for future processing.

The central assumptions made by Just and Carpenter (1992) are that there are individual differences in the capacity of working memory, and that these individual differences have substantial effects on language comprehension. Working memory capacity is assessed by the reading-span task (Daneman & Carpenter, 1980). On this task, participants read a number of sentences for comprehension, and then try to recall the final word of each sentence. The largest number of sentences from which a participant can recall all the final words more than 50% of the time is defined as his or her reading span. It is assumed that the processes used in comprehending the sentences require a smaller proportion of the available working memory capacity of those with a large capacity, and thus they have more capacity for retaining the last words of the sentences.

Reading span is a useful measure. It typically correlates about +0.8 with the ability to answer comprehension questions about a passage, and it correlates about +0.6 with verbal intelligence (see Just & Carpenter, 1992). In addition, those with high reading spans read hard portions of a text much faster than those with low reading spans. Theoretically, Daneman and Carpenter (1980) assumed that reading span measures fairly specific processes relating to reading and other verbal tasks.

Experimental evidence

Capacity theory has been applied to issues considered earlier in the chapter, such as whether the initial syntactic parsing of a sentence is affected by meaning. Just and Carpenter (1992) studied reading times for sentences such as, "The evidence examined by the lawyer shocked the jury", and "The defendant examined by the lawyer shocked the jury". "The evidence" (an inanimate noun) is unlikely to be doing the examining, whereas "the defendant" (an animate noun) might well be. Accordingly, the actual syntactic structure of the sentence should come as more of a surprise to readers given the second sentence. However, if meaning does not influence initial syntactic parsing, then the gaze duration on the critical phrase "by the lawyer" should be the same for both sentences.

Just and Carpenter (1992) found that the reading times of participants with a low reading span were unaffected by the animate/inanimate noun manipulation (see Figure 12.2). In contrast, participants with a high reading span made use of the cue of inanimacy, and so their initial parsing was affected by meaning. Presumably only those participants with a high reading span had sufficient working memory capacity available to take the animacy/inanimacy of the subject noun into account. Thus, individual differences need to be taken into account when considering whether meaning affects initial syntactic parsing.

Another area of controversy relates to the processing of sentences containing syntactic ambiguity. One possibility is that those encountering such ambiguity try to retain both (or all) interpretations until disambiguating information is provided. Alternatively, people might select a single interpretation, and retain it unless or until there is invalidating information. Just and Carpenter (1992) discussed a study in which sentences were presented in a self-paced, word-by-word moving window paradigm. Some sentences were

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