Different patterns of spoken and written word comprehension deficit in aphasic stroke patients

This study presents neuropsychological evidence for differences in the semantic representations underpinning spoken and written word comprehension. Potential modality-based discrepancies in the semantic system were examined by testing whether spoken word (auditory-verbal input) and written word (visual-verbal input) comprehension exhibited the same effect profile on variables typically used to distinguish so-called access and storage disorders (e.g., response consistency, sensitivity to item frequency). The study was based on the premise that damage to a common set of semantic representations should have an equivalent impact upon comprehension performance irrespective of input modality, whereas damage to partially dissociable semantic representations may give rise to different qualities of deficit (access/storage) in the comprehension of stimuli presented in different input modalities (spoken/written). The study involved two patients with global aphasia following left middle cerebral artery stroke (F.B.I. and H.O.P.). The two patients showed matched performance on conventional tests of single word comprehension with clear evidence of semantic impairment for stimuli presented in both the spoken and written input modalities. However, in H.O.P., spoken and written word comprehension was affected in the same way by variations in stimulus category, frequency, and multiple stimulus presentations, whilst in F.B.I., there were clear differences between input modalities with all three variables. More specifically, F.B.I.'s written word comprehension was significantly affected by category (living > nonliving) and frequency (high > low) but not multiple presentations (single = multiple), more consistent with degradation of stored representations (storage deficit). By contrast, his spoken word comprehension was unaffected by category (living = nonliving) and frequency (high = low) but was affected by multiple presentations (single > multiple; serial position effects), more consistent with impaired access to stored representations (access deficit). These spoken/written input modality differences were observed on tasks matched closely for output modality, stimulus identity, and executive control requirements. It is argued that subtle differences in comprehension performance for stimuli presented in different input modalities may reflect damage to multimodal representations, which are intermediate between unimodal and amodal representations on a continuum of convergence within the semantic system. These ideas are discussed in the context of existing "distributed-only", "distributed-plus-convergence", or "distributed-plus-hub" models of conceptual knowledge.