Auditory discrimination after left-hemisphere stroke: a mismatch negativity follow-up study. 
We sought to determine the recovery of cortical auditory discrimination in aphasic, left-hemisphere-stroke patients by using an electrophysiological response called mismatch negativity (MMN) and speech-comprehension tests.  MMN in 8 left-hemisphere stroke patients was recorded in response to duration and frequency changes in a repetitive, harmonically rich tone 4 and 10 days and again 3 and 6 months after their first unilateral stroke.  Eight age-matched, healthy persons served as control subjects.  At 4 days after stroke onset, patients' sound discrimination was impaired in their left hemisphere, as suggested by attenuated MMNs, especially to right-ear stimuli.  At 3 months after stroke, however, MMN to the right-ear duration change had significantly increased and was of normal size.  A significant change for the frequency MMN was found for left-ear stimuli between 3 and 6 months after stroke.  During the follow-up period, progressive improvement in speech-comprehension tests was also observed.  Furthermore, there was a significant correlation between the change in the duration MMN amplitude and the Boston Diagnostic Aphasia Examination speech-comprehension test from 10 days to 3 months after stroke.  These results suggest that the MMN can be used as an index of the recovery of auditory discrimination.