The relationships between morphological and phonological errors in aphasic speech: data from a word repetition task.

Current theories of single-word processing predict that in some cases brain damage should selectively impair morphological processes, resulting in the selective occurrence of morphological errors. However, such a selective pattern of errors has never been documented, and the available case studies report the systematic association of morphological and phonological (segmental) errors in the same subject. The number of relevant case reports is very small, however. To better understand the relationship between morphological and phonological processes, we analyzed the repetition performance of 26 subjects who produced morphological errors in a screening battery for aphasia. Although the results confirm that subjects who make morphological errors also invariably make phonological errors, the probabilities of the two types of errors are not (quantitatively) correlated. Furthermore, the relationship between morphological and phonological errors was asymmetrical: although all subjects who produced morphological errors also produced phonological errors, some aphasics who produced phonological errors did not also produce morphological errors. The one-way relationship between morphological and phonological errors could result either from the anatomical proximity of the structures involved in morphological and phonological processes, or from the functional inseparability of the two processes. The fact that phonological errors can occur either in isolation or in association with morphological errors could be attributed to the functional heterogeneity of phonological errors, and to the separability of the neural substrates involved in processing various aspects of phonological information.