Figure 123

Differences in reading time per word between ambiguous and unambiguous sentences as a function of part of the sentence. Adapted from Just and Carpenter (1992).

The capacity theory has been applied to brain-damaged patients suffering from aphasia, in which language abilities are impaired. According to the theory, what is common to all aphasic patients is a reduction in the working memory resources involved in language comprehension. Caspari, Parkinson, LaPointe, and Katz (1994) studied various patients suffering from aphasia. Performance on a listening-span task correlated +0.82 with an independent measure of text comprehension in these aphasic patients.

Carpenter, Miyake, and Just (1994) discussed a simulation model based on the capacity theory. This model is called CC READER, with the initials standing for Capacity Constrained. According to this model, activation underlies all the activities taking place in working memory: "The constraint on capacity is operationally defined as the maximum amount of activation that the system has available conjointly for maintenance and processing purposes" (Carpenter et al., 1994, p. 1091). Carpenter et al. (1994, p. 1093) argued that this model can readily be applied to aphasia: "The maximum amount of activation available for the storage and processing of linguistic information is far more severely limited in the aphasic system than in the normal system." Haarmann, Just, and Carpenter (1997) used the computational model to observe the effects of reducing activation and working memory resources on comprehension performance. The error rates of the model across various types of sentences resembled those of aphasic patients.

Probably the greatest strength of the capacity theory put forward by Just and Carpenter (1992) is the assumption that there are substantial individual differences in the processes used in language comprehension. Some individuals have more processing resources available than others, and so can carry out forms of processing that those with fewer processing resources cannot. This approach has shed important new light on some major controversies (e.g., the role of meaning in the initial parsing of sentences).

On the negative side, it is not clear how to interpret the high correlation between reading span and language comprehension. Just and Carpenter (1992) argued that the efficiency of working memory (as measured by reading span) determines the efficiency of language processing and comprehension. Ericsson

Evaluation and Kintsch (1995) and Ericsson and Delaney (1998) argued for the opposite position, namely, that language processing efficiency determines the efficiency of working memory as assessed by reading span. It is hard to distinguish between these possibilities. According to Ericsson and Delaney (1998, p. 111), "Individual differences in working memory capacity during reading do not reflect innate, fixed capacity differences, in contrast with traditional theories (Just & Carpenter, 1992; Daneman & Carpenter, 1980). Instead, such individual capacity differences primarily reflect differences in the relevant knowledge and acquired memory skills that support encoding of the text." There is also the issue of whether reading span reflects reading-related processes as claimed by Daneman and Carpenter (1980), or whether it reflects more general processing resources (Turner & Engle, 1989).

Waters and Caplan (1996) argued that there are theoretical reasons for doubting whether the processes involved in maintaining the last word of each sentence in the word-span task are the same as those involved in sentence comprehension. According to Waters and Caplan (1996, p. 769):

[the word-span task] requires conscious retrieval of items held in memory, to an extent not found in processing sentence structure...the memory load that is imposed in the [word-span] task is unrelated to the computations of that task, whereas the verbal material that is stored in sen tence processing is relevant to the ongoing computations required by that task.

Another issue concerns the importance attached to reading span within capacity theory. As Towse, Hitch, and Hutton (1999, p. 111) pointed out, there are concerns about relying on reading span, "a task requiring sentence comprehension, as the vehicle to explain sentence-comprehension processes."

Finally, the theory emphasises working memory capacity rather than the specific processes involved in comprehension. Thus, Just and Carpenter (1992) do not provide us with a detailed account of comprehension processes. As a result, capacity theory does not provide a comprehensive account of language comprehension in normals or aphasic patients.

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