Treatment of sentence comprehension and production in aphasia: is there cross-modal generalisation?

Exploring generalisation following treatment of language deficits in aphasia can provide insights into the functional relation of the cognitive processing systems involved. In the present study, we first review treatment outcomes of interventions targeting sentence processing deficits and, second report a treatment study examining the occurrence of practice effects and generalisation in sentence comprehension and production. In order to explore the potential linkage between processing systems involved in comprehending and producing sentences, we investigated whether improvements generalise within (i.e., uni-modal generalisation in comprehension or in production) and/or across modalities (i.e., cross-modal generalisation from comprehension to production or vice versa). Two individuals with aphasia displaying co-occurring deficits in sentence comprehension and production were trained on complex, non-canonical sentences in both modalities. Two evidence-based treatment protocols were applied in a crossover intervention study with sequence of treatment phases being randomly allocated. Both participants benefited significantly from treatment, leading to uni-modal generalisation in both comprehension and production. However, cross-modal generalisation did not occur. The magnitude of uni-modal generalisation in sentence production was related to participants' sentence comprehension performance prior to treatment. These findings support the assumption of modality-specific sub-systems for sentence comprehension and production, being linked uni-directionally from comprehension to production.

Training-induced improvement of noncanonical sentence production does not generalize to comprehension: evidence for modality-specific processes.

The presence or absence of generalization after treatment can provide important insights into the functional relationship between cognitive processes. The aim of the present study was to investigate the relationship between the cognitive processes that underlie sentence comprehension and production in aphasia. Using data from seven participants who took part in a case-series intervention study that focused on noncanonical sentence production [Stadie et al. (2008). Unambiguous generalization effects after treatment of noncanonical sentence production in German agrammatism. Brain and Language, 104, 211-229], we identified patterns of impairments and generalization effects for the two modalities. Results showed (a) dissociations between sentence structures and modalities before treatment, (b) an absence of cross-modal generalization from production to comprehension after treatment, and (c), a co-occurrence of spared comprehension before treatment and generalization across sentence structures within production after treatment. These findings are in line with the assumption of modality-specific, but interacting, cognitive processes in sentence comprehension and production. More specifically, this interaction is assumed to be unidirectional, allowing treatment-induced improvements in production to be supported by preserved comprehension.

Can the resource reduction hypothesis explain sentence processing in aphasia? A visual world study in German.

Resource limitation has often been invoked as a key driver of sentence comprehension difficulty, in both theories of language-unimpaired and language-impaired populations. In the field of aphasia, one such influential theory is Caplan's resource reduction hypothesis (RRH). In this large investigation of online processing in aphasia in German, we evaluated three key predictions of the RRH in 21 individuals with aphasia and 22 control pparticipants. Measures of online processing were obtained by combining a sentence-picture matching task with the visual world paradigm. Four sentence types were used to investigate the generality of the findings, and two test phases were used to investigate RRH's predictions regarding variability in aphasia. The processing patterns were consistent with two of the three predictions of the RRH. Overall, our investigation shows that the RRH can account for important aspects of sentence processing in aphasia.

Variability in sentence comprehension in aphasia in German.

An important aspect of aphasia is the observation of behavioral variability between and within individual participants. Our study addresses variability in sentence comprehension in German, by testing 21 individuals with aphasia and a control group and involving (a) several constructions (declarative sentences, relative clauses and control structures with an overt pronoun or PRO), (b) three response tasks (object manipulation, sentence-picture matching with/without self-paced listening), and (c) two test phases (to investigate test-retest performance). With this systematic, large-scale study we gained insights into variability in sentence comprehension. We found that the size of syntactic effects varied both in aphasia and in control participants. Whereas variability in control participants led to systematic changes, variability in individuals with aphasia was unsystematic across test phases or response tasks. The persistent occurrence of canonicity and interference effects across response tasks and test phases, however, shows that the performance is systematically influenced by syntactic complexity.

A Computational Evaluation of Two Models of Retrieval Processes in Sentence Processing in Aphasia.

Can sentence comprehension impairments in aphasia be explained by difficulties arising from dependency completion processes in parsing? Two distinct models of dependency completion difficulty are investigated, the Lewis and Vasishth (2005) activation-based model and the direct-access model (DA; McElree, 2000). These models' predictive performance is compared using data from individuals with aphasia (IWAs) and control participants. The data are from a self-paced listening task involving subject and object relative clauses. The relative predictive performance of the models is evaluated using k-fold cross-validation. For both IWAs and controls, the activation-based model furnishes a somewhat better quantitative fit to the data than the DA. Model comparisons using Bayes factors show that, assuming an activation-based model, intermittent deficiencies may be the best explanation for the cause of impairments in IWAs, although slowed syntax and lexical delayed access may also play a role. This is the first computational evaluation of different models of dependency completion using data from impaired and unimpaired individuals. This evaluation develops a systematic approach that can be used to quantitatively compare the predictions of competing models of language processing.

Unambiguous generalization effects after treatment of non-canonical sentence production in German agrammatism.

Agrammatism is-among others, characterized by a deficit in producing grammatical structures. Of specific difficulty is the utilization of complex, non-canonical sentence structures (e.g. object-questions, passives, object-clefts). Several studies have documented positive effects when applying a specific treatment protocol in terms of increasingly correct production of target complex sentence structures with some variance in generalization patterns noted across individuals. The objective of this intervention study was to evaluate an intervention program focussing on the production of non-canonical sentences. Hypotheses about the occurrence of treatment effects were formulated on the basis of syntactic complexity, referring to the amount of syntactic phrase structures necessary to generate specific German sentence structures. A multiple single case study with seven agrammatic participants was applied, each participant receiving training in the production of object-relative-clauses and who-questions. The investigation was designed to unambiguously evaluate for each individual, structure specific and generalized learning effects with respect to the production of object-relative-clauses, who-questions and passive sentences. Results showed significant improvements for all sentences types. This outcome is considered within methodological issues of treatment studies. Theoretical and clinical implications are discussed.

Evaluation of lexically and nonlexically based reading treatment in a deep dyslexic.

The aim of the single case study was to evaluate two different treatment procedures to improve reading skills with a German-speaking deep dyslexic. Generally, in treatment studies for deep dyslexia, retraining of grapheme-phoneme correspondences is described, but hardly any treatment focuses on reactivating residual functions of the semantic-lexical route. This strategy was explored here with an experimentally presented priming paradigm, to implicitly strengthen residual skills of lexical access with semantically/phonologically related primes (lexically based treatment). In contrast, grapheme-phoneme associations and blending were explicitly relearned during a nonlexically based treatment. Stimuli were controlled for part of speech, word length, and frequency. A cross-over design to identify item- and treatment-specific effects for both procedures was applied. Results indicate positive outcomes with respect to treatment-specific effects for both procedures, generalization to untrained items, and a transfer task after the nonlexically based procedure. All effects remained stable in the follow-up assessment. Implications for theoretically/empirically generated expectations about treatment outcomes are discussed.