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Sound and syllable discrimination in stroke patients with severe comprehension disabilities as reflected in mismatch negativity (MMN)

Mismatch Negativity (MMN) is an ERP component usually interpreted as a trace of auditory stimulus discrimination without the participants attending to it. It is thought that MMN reflects the earliest, and "low level" processes, e.g. frequency, time, and phonemic differences. Some studies have shown that shorter latency, longer duration and larger peak amplitude implied better discrimination. Absent or reduced MMN amplitude is often recorded in aphasic patients, especially those with severe comprehension deficits. Many studies show that absence or reduction of MMN is associated with lesions involving temporal lobe. The aim of this study is to study changes in MMN at different levels of auditory processing. For that purpose two oddball experiments have been done on patients with severe comprehension deficits (n=10) and their age controls (n=10). The first experiment varied between a frequent tone (1000 Hz, 50 ms duration), a rare tone with a longer duration (75 ms) and a rare tone of a higher frequency (1100 Hz). The second experiment varied between a frequent syllable ('ba') a rare syllable ('ga') and a rare syllable ('pa'). One rare syllable thus differed in the place and the other in the manner of articulation. In both experiments the ratio between the frequent and the rare stimuli was 70:15:15. Results have shown that both groups of participants produced MMN in the first experiment, with a more pronounced MMN in duration condition than frequency condition. Peak amplitude was lower, duration was shorter and the scalp distribution different (more frontal and right) in the aphasic group. In the second experiment only control participants elicited MMN and just for the 'pa' condition (manner of articulation). There were no MMN in aphasic group, neither for stimuli that differ in the place nor for the stimuli that differ in the manner of articulation. These results indicate that the comprehension deficit at the "higher" level of language processing could be explained by some specific phonemic or phonological deficit at the "lower" end of linguistic processing while the auditory, non-linguistic mechanisms might not be involved.

aphasia ; stroke ; comprehension disabilities ; sound and syllable auditory discrimination ; ERP ; MMN

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