Working memory impairment in aphasia: The issue of stimulus modality

The presence of short-term/working memory deficits in aphasia has been well-established in the relevant literature. Moreover, recent studies have demonstrated such deficits in both the verbal and visuospatial modalities in patients with aphasia following a left hemisphere stroke. The present study aims to investigate the prevalence and possible lesion correlates of modality-dependent and modality-independent short-term/working memory impairment in aphasia. For this purpose, 54 individuals with post-stroke aphasia were recruited. Aphasic disturbances were assessed with the Boston Diagnostic Aphasia Examination (BDAE) and memory deficits with the digit span and Corsi block-tapping tasks. Structural imaging data were obtained for each patient and lesion sites were identified and coded by two independent neuroradiologists for 16 predetermined left hemisphere areas. Results clearly showed that visuospatial deficits were relatively common, even though verbal deficits are more frequent, and further illustrate the existence of distinct performance patterns within each modality. Analyses revealed comparable lesion extent and locations between patients demonstrating modality-independent and patients demonstrating modality-dependent deficits. Hierarchical regression analyses indicated that deficient performance on digit span forward, indicating impaired verbal short-term memory, was associated with lesions encompassing the inferior frontal gyrus and the inferior parietal cortex. Similar analyses for digit span backward, and the two conditions of Corsi did not yield significant results. However, behavioral data indicate a dissociation between forward and backward visuospatial span, which could be explained by selectively impaired primacy and recency effects. The emergence of modality-dependent and modality-independent deficits is further discussed in relation to hemispheric lateralization during the course of human evolution.