Mismatch negativity and the n2b component elicited by pure tones and speech sounds in anomic aphasia: a case study

BackgroundThe auditory processing impairments frequently observed in aphasia are being slowly clarified by using eventrelated potentials (ERPs), a method that allows brain processes to be observed at high temporal resolution. Mismatch negativity (MMN) and the N2b amplitude reflect aspects of echoic memory, attention, and phonological representation. This study evaluates the auditory processing of speech and pure tones in an anomic aphasia subject 6 years after a stroke, and investigates whether ERPs can detect possible neurophysiologic sequelae after recovery and rehabilitation.Material and MethodsA recovered subject with anomic aphasia, 6 years post-stroke, was compared with 6 healthy controls. Event-related potentials (MMN, N1, N2b) were obtained during two auditory oddball paradigms, one using pure tones and the other consonant–vowel (CV) stimuli.ResultsWhen compared to healthy subjects, the anomic aphasia subject had reduced MMN amplitude across the frontocentral electrode sites, particularly for speech stimuli. Average deviant waveform analysis revealed poor morphology of N2b to speech stimuli, which might relate to deficits in phonological representation.ConclusionsIn the presented case the neurophysiologic brain activity for processing of phonologic representations had not fully recovered 6 years post-stroke. MMN and N2b are highly sensitive ERPs for evaluating impairments in auditory processing and can be registered in the absence of attention and with no task requirements, features which makes it particularly suitable for investigating aphasic subjects.