Abstract
BACKGROUND: Children with specific language impairment (SLI) show particular difficulty comprehending and producing object (Who did the bear follow?) relative to subject (Who followed the tiger?) wh-questions. AIMS: To determine if school-age children with SLI, relative to children with typical development (TD), show a more distinct unevenness, or asymmetry, in the comprehension of these questions. In addition, this study examined whether the sustained left-anterior negativity (LAN) in event-related potentials (ERP) could be used as a marker for atypical processing of these questions in children with SLI. The LAN effect signals the greater working memory processes for maintaining in memory the dislocated object in object wh-questions and reflects working memory capacity in adults. It was predicted that the amplitude of the LAN would be greater in children with SLI, reflecting the characteristic low working memory capacity in this population. The concomitance of these behavioural and electrophysiological effects would suggest that the subject-object asymmetry in SLI should be investigated in relation to poor working memory skills. METHODS & PROCEDURES: Groups including 13 children with SLI, 17 same-age TD children and 18 normal adults completed an auditory sentence comprehension task requiring button responses while continuous electroencephalography (EEG) was collected. Accuracy for subject and object questions was calculated. The mean amplitude values of the ERP data for the wh-questions were examined to identify differential processing of subject and object questions. OUTCOMES & RESULTS: TD children demonstrated asymmetrical comprehension of subject and object wh-questions, whereas children with SLI comprehended both question types poorly and adults did not show subject-object asymmetry. ERP waveforms spanning the wh-dependency revealed a large and widespread sustained anterior positivity for object relative to subject questions in the TD group, indicating differential processing of these questions. This effect was attenuated and non-significant in the SLI group. The adults' grand average waveforms showed the expected LAN effect, which was opposite in polarity relative to the children, although it only approached significance. CONCLUSIONS & IMPLICATIONS: The finding of less differential processing of subject and object wh-questions in SLI relative to TD children suggests inefficient maintenance of sentential information in working memory for object questions in SLI. Whereas behavioural methods did not identify subject-object asymmetry in SLI, the more fine-grained method of ERPs elucidated this effect. Further analysis of working memory as the basis for the subject-object asymmetry in SLI is critical for identifying appropriate intervention targets for this population.