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Implicit processing of prosodic information in patients with left and right hemisphere stroke

Background : Investigations of prosodic processing in brain-damaged patients have primarily focused on hemispheric specialisations in the processing of prosodic structures. Most studies of receptive prosodic abilities in brain-damaged people share a common methodological approach, i.e., they are based on explicit tasks. No attention has been paid so far to the impact of explicit or implicit experimental methodologies. Aims : The aim of this study was to compare implicit and explicit experimental paradigms of receptive prosodic processing in patients with focal left and right hemisphere lesions. Methods & Procedures : Two implicit processing paradigms were used to investigate the comprehension of turn-related prosody (turn-detection task) and of prosodic focus (phoneme-detection task) in patients with focal cerebro-vascular lesions to the left or right cerebral hemisphere. The results were compared with explicit processing data of two matched patient groups selected from a larger sample of an earlier study. Outcomes & Results : (a) In the turn-detection experiment normal subjects and left hemisphere damaged patients relied on prosodic information to detect conversational turns, whereas right hemisphere damaged patients were unable to benefit from the prosodic structure of utterances. A comparison of these results with the performance of two matched patient groups on an explicit "non-linguistic prosodic" paradigm (rating of emotional prosody) revealed the same pattern. b) In the phoneme-detection experiment prosodic focus information influenced detection latencies in both patient groups, i.e., RHD and LHD patients were able to process prosodic focus in an implicit manner. In the explicit processing of prosodic focus (identification task), RHD patients were moderately disturbed, while LHD patients showed more severe impairments. Conclusions : The finding that RHD patients showed impaired performance in three out of four tasks confirmed a substantial deficit in the processing of prosodic information. Their preserved performance in the implicit processing of prosodic focus can be explained by the presence of local cues. On the contrary, the results of the LHD subjects indicate that earlier findings of an impaired prosodic focus processing in these patients might be attributable to the explicit nature of the tasks used in most studies rather than to a primary deficit in the processing of prosodic structures.