Identification of target tones and speech sounds studied with event-related potentials: Language-related changes in aphasia

Abstract

Background: Although frequently observed in aphasia, the contribution of speech sound processing impairments to auditory comprehension deficits has not been clarified. Event-related brain potentials can be used to investigate the neural processes underlying speech sound processing in aphasia, thereby adding information to investigations exclusively based on behavioural assessments. Aims: Against the background of previous findings of reduced amplitudes in an aphasic group during a syllable identification task, three main research questions are pursued in this study: whether possible ERP alterations in the aphasia group (1) are specific for language processing or can also be observed in non-language processing, (2) are also found in brain-injured persons without aphasia, and (3) correlate with auditory comprehension deficits. Methods & Procedures: Here 10 aphasic participants were compared with 10 non-aphasic participants who had right hemisphere lesions and with 18 healthy controls. Event-related brain potentials (ERPs) were obtained during two oddball paradigms in which the participants had to identify: (i) target tones differing in pitch and (ii) target syllables differing in the initial consonant. Outcomes & Results: Overall the aphasic participants performed the task successfully, but responded correctly to fewer targets and had prolonged response times. When compared to healthy controls, N1 amplitude was reduced in both brain-damaged groups; in response to syllables, this attenuation was more pronounced in the aphasic participants. The aphasia group demonstrated significantly reduced N1 amplitude in response to standard and target tones and target syllables, while the right hemisphere group had significantly reduced N1 amplitudes in response to tones only (both standard and target). In both brain-injured groups, N1 amplitudes were larger over the contralesional hemisphere. This hemisphere difference was significantly larger for syllables than tones in the aphasic participants. Furthermore this group had delayed N2 and P3 latencies, whereas N2 amplitude reduction was found in the right hemisphere group. Conclusions: The present results indicate reduced early attentional processing and delayed subsequent processing in aphasia. In addition to disturbed non-linguistic auditory processing, the present results suggest impairments in speech sound related processing in aphasia occurring as early as at 100 ms post-stimulus. Topographical distribution patterns in the aphasia group suggest a larger right hemisphere involvement in the processing of syllables than for tones.