Towards empirical dimensions for the classification of aphasic performance

Presented at the ISCA Tutorial and Research Workshop on Experimental Linguistics, Athens, Greece, 28–30 August 2006

Athanassios Protopapas,1 Spyridoula Varlokosta,2 Alexandra Economou, 3 & Maria Kakavoulia4
1 Institute for Language & Speech Processing / Athena
2 Department of Mediterranean Studies, University of the Aegean
3 Department of Psychology, University of Athens
4 Department of Communication, Media, and Culture, Panteion University

The term aphasia refers to a wide range of language disorders following brain damage. Patterns of breakdown in aphasia can be highly informative about the human cognitive system of language. Classical neurological and aphasiological taxonomy use localization and clinical criteria to distinguish, for example, among expressive and receptive subtypes, and between structural and semantic deficits. These distinctions have important implications for the conceptualization and description of language ability and use, and imply that distinct dimensions of skill underlie observed variance in language performance.

Recent cross-linguistic evidence has led to re-evaluation of certain fundamental assumptions on which aphasia subtyping is typically based. In addition, the different opportunities for linguistic analysis and performance breakdown patterns offered by different languages have made cross-linguistic research indispensable in aphasia. In Greek, the rich verbal morphology allows study of functional categories in situations of controlled structural complexity and in relation to more global assessments such as fluency and severity. An inclusive approach to participant selection permits objective comparisons on the basis of performance patterns rather than a-priori categorization potentially leading to selection bias.

Here we report studies assessing production fluency, utterance length, grammaticality judgment, and sentence completion (with a variety of elicited verb forms). In an initial group of 7 Greek patients diagnosed with aphasia (as compared with a control group matched on age, sex, and years of education), all of these measures were found to correlate along a single dimension of “severity.” Sets of test sentences were constructed using verbs controlled for phonological properties, regularity (in aspectual formation), and frequency (estimated via subjective familiarity). Additional testing included standard picture description and a brief interview. Examination of performance in verb production and reception, broken down by functional category, revealed that aspect was most vulnerable whereas subject-verb agreement was most resistant. Verb frequency was found to affect functional category production but not grammaticality judgment. Verb regularity did not affect performance. These findings were complemented by a subsequent study of an additional group of patients (and matched controls), in which the relationship between oral fluency and utterance length was examined and related to grammaticality of production, based on an interview and two picture-naming tasks.

These data are interpreted and evaluated in relation to the major neurolinguistic approaches, including structural and processing accounts. We consider the role of measures assessing patient performance on a range of tasks that tap theoretically distinct abilities, and the importance of cross-linguistic opportunities to inform aphasia research. We conclude that experimental approaches to aphasia are an indispensable tool in the study of language and of aphasia itself. Functional dimensions, such as fluency and severity, already well known in clinical practice, can provide valuable insights into the mechanisms underlying linguistic performance when systematically related to structural assessments of particular theoretical linguistic significance. Finally, a priori subtyping should be re-examined in light of performance patterns, and only accepted when patient clustering is empirically derived and theoretically meaningful.