Dealing with the evidence

3.3.5 Dealing with the evidence

  To conclude this review of resources in (a) aspects of language processing and (b) parsing in particular, we will briefly return to the resource theories outlined in 3.2.2 Resource theories (p. 56) and re-examine each in light of current evidence.

  Carpenter, Daneman, Just, Varma and colleagues: Single resource theory The theory of Carpenter and colleagues, an interactionist, symbolic account, is

  consistent with studies of lexical ambiguity, centre-embedded sentence processing, main-verbreduced relative ambiguity processing, and relative clause attachment: it hypothesizes that the DC RST processing-and-storage task taps the same resources as those used in primary sentence interpretation. Where those resources vary with the DC RST score, these researchers expect a corresponding variability in the resources available for primary sentence interpretation. The best evidence for this hypothesis comes from the three-way Span by Clause Type by Region interaction reported by MacDonald, Just and Carpenter (1992) and Pearlmutter and MacDonald (1995), which pinpoints the influence of processing load. Though Waters and Caplan (1996a) failed to replicate this finding, they applied rather different methodological techniques (see p. 86 in this thesis).

  It should also be noted that CC-READER, a detailed implementation of this hypothesis, has successfully modelled all of the findings presented by Carpenter and colleagues by assuming multiple syntactic representations in higher-span readers. It has also launched similar models. For example, a successor architecture to the 3CAPS framework maps sentence comprehension onto cortical function (Cortical Capacity- Constrained Collaborative Activation-Based Production System, or 4CAPS). Results from an fMRI study (Just, Carpenter, Keller, Eddy Thulborn, 1996) are consistent with the prediction that more brain activation would be associated with object-relative sentences than subject-relative sentences. There are also a number of 3CAPS models that address various levels of comprehension (Goldman Varma, 1995; Haarmann, Just Carpenter, 1997; Thibadeau, Just Carpenter, 1982; Varma Goldman, 1996).

  One of the theoretical weaknesses of the approach is that, although experience is seen to have a role in processing, and is acknowledged as a factor in comprehension, the CC-READER model does not incorporate any learning mechanism. Thus, for Just and colleagues, the role of experience, though a component of their verbal description (see

  Just Varma, 2002, p. 56), is unspecified in CC-READER 1 . This contrasts with the

  connectionist approach.

  Within the realm of a single resource, research on stragetic resource allocation (e.g. Vos, Gunter, Schriefers Friederici, 2001; Friedirici, Steinhauer, Mecklinger Meyer, 1998) proposes that reading differences between higher- and lower-span readers may lie with differences in reading strategy. Friedirici et al.’s ERP study, for example, compared subject-relative and objective-relative sentences that were disambiguated late, earlier or immediately. The only significant effect of span was shown in late- disambiguating object-relatives. The authors interpret this as higher-span readers committing to a single analysis (subject-relatives) and having to recompute (to an object-relative structure), whereas lower-span readers maintain multiple representations and experience less recomputation cost. While strategic resource allocation may prove a worthwhile addition to the single resource account (see Vos et al., 2001), current evidence for the use of strategies remains unconvincing (for example, a number of studies suggest that higher-span readers processs multiple information sources, not commit to single analyses; see 3.3.3 The role of reading span in discourse processing, p. 74). Additionally, it is unclear how one may predict reading strategy (so far, suggestions have been limited to post hoc examinations of materials and their fillers; e.g. Vos et al., 2001).

  Waters, Caplan and colleagues: Separate sentence interpretation resource theory The modular, symbolic account of Waters and colleagues predicts that the processes

  measured by the DC RST have no relationship with syntactic processing because the latter takes place within the primary psycholinguistic module while the former employs

  a nonpsycholinguistic module (or modules). Waters and colleagues have been successful in raising concerns with studies that attempt to document this relationship (e.g. King Just, 1991); for example, they highlight the importance of the Span by Clause Type by Region interaction (Waters Caplan, 1996b, c; 2001; Caplan Waters, 2002).

  However, their own theory has shortcomings. Firstly, the partition of unconscious (interpretive) processing and conscious (postinterpretive) processing renders the account

  1 Just and Carpenter (1992) note that CC-READER does incorporate lexical frequency information (the higher the frequency, the faster the activation). However, this information does not vary between higher-

  and lower-span readers.

  vulnerable to Navon’s (1984) vacuity objection. That is, Waters and colleagues have yet to indicate ‘the boundaries and nature of the on-line language comprehension process’ (Waters Caplan, 1996a). Evidence for pragmatic expectations (Trueswell et al., 1994) and lexical effects in on-line processing (MacDonald, Pearlmutter Seidenberg, 1994) are problematic for SSIR Theory. Waters and Caplan go on to say that ‘even if the language-processing system is not encapsulated with respect to its input, it may be domain-specific in terms of its output’ (p. 770). As Just et al. noted, if a module processes ‘psycholinguistic’ and ‘nonpsycholinguistic’ information together, its status as an encapsulated process must be questioned.

  In addition to their criticisms of the two other approaches, Waters, Caplan and colleagues offer neuropsychological evidence of dissociations together with studies that do not find span effects. For example, patients exist who have extremely limited verbal short-term memory spans but retain the ability to use complex forms of syntax (e.g. Martin, 1993). However, this may be a limitation of the STS phonological loop, not the CE itself (Waters Caplan, 1996c). Negative evidence for SSIR Theory comes from Kane, Conway and Engle (1999). They administered a battery of working memory tasks that measure both processing and memory. They tested alphabet span, backward digit span, missing digit span, subtract-2 span, running item span and the WC Reading Span Test. While reading span correlated highest (but insignificantly) with measures of on- line processing efficiency, none of the working memory tasks showed a significant relationship with reading comprehension.

  Another aspect of their theory that has been subject to some criticism (e.g. Just Varma, 2002) is the conscious-unconscious distinction between postinterpretive and interpretive processing. This may be a misleading dichotomy because aspects of language behaviour seem to fall along a continuum of conscious-unconscious processing (Just et al., 1996).

  MacDonald, Pearlmutter, Christiansen and colleagues: A connectionist account In common with theory of Carpenter and colleagues, the interactionist, non-symbolic

  Connectionist account is largely consistent with the evidence presented so far. However, it has some aspects that may be regarded as improvements upon the former. For example, Daneman and Carpenter’s (1980) original verbal description of improved efficiency with exposure, while consistent with the connectionist claims, includes a prediction (e.g. Daneman, 1988) that vocabulary size will be an important predictor of Connectionist account is largely consistent with the evidence presented so far. However, it has some aspects that may be regarded as improvements upon the former. For example, Daneman and Carpenter’s (1980) original verbal description of improved efficiency with exposure, while consistent with the connectionist claims, includes a prediction (e.g. Daneman, 1988) that vocabulary size will be an important predictor of

  MacDonald and Christiansen (2002) interpret correlations between working memory resources and language processing as reflective of common connectionist networks that underpin them all. On this account, reading span, like any other language task, will correlate with reading because they both require similar mechanisms. However, this position is not obviously compatible with the correlation between the DC RST and grossly-similar symbolic working memory tasks (such as the math-processing plus storage tasks) as reported by Daneman and Merikle’s (1996) in their meta-analysis. A recent study by Roberts and Gibson (2002) also indicated that, though an N-back task and a variant of the DC RST both correlated with a measure of sentence complexity comprehension, the N-back task and the DC RST did not correlate with one another. These findings are more consistent with the symbolic resource theories.

  One key aspect of the connectionist approach is the importance of phonological representations in sentence processing. MacDonald and Christiansen (2002) argue that studies of extrinsic memory load are, in fact, studies of phonological interference (cf. Navon’s, 1984, comments on capacity, p. 60). However, other researchers have expressed doubt that phonological representations are as crucial to sentence comprehension as the authors suggest. Caplan and Waters (2002), who are thorough on these matters, examined the studies that MacDonald and Christiansen cited in support of their phonological interference hypothesis: the studies make a good case for the existence of phonological codes in lexical access (e.g. Pollatsek, Lesch, Morris Rayner, 1992) but not syntactic processing.