Disordered auditory short-term memory in man and event-related potentials.

Four patients with conduction aphasia and impaired auditory but relatively preserved visual digit spans were tested in a task of short-term memory, a digit probe identification task, in both the visual and auditory modalities. Five age-matched normal subjects served as controls. Behavioural measures of response accuracy and reaction time and scalp derived event-related potentials were determined as a function of the number of items to be remembered. These measures were also recorded in a task requiring the detection of infrequent stimuli ('oddball' paradigm). The patients' performance and event-related potential wave forms for the 'oddball' task were no different from those of the controls. In the digit probe identification tasks the patients showed a reduction in amplitude of a positive component at a latency of approximately 450 ms (P450) of the event-related potential to correctly identified probes in the auditory modality. This component was within normal limits with visual testing. There was an inordinate increase in the patients' reaction time to auditory but not visual stimuli when the number of items to be remembered increased from one to three. Event-related potentials were also analysed as a function of speed of reaction time, position of the matching item in the stimulus presentation set and whether the probe was correctly identified as being in-set or out-of-set. Event-related potentials associated with fast reaction times in the auditory task when a single item was presented were no different between patients and controls, indicating that patients were capable of both performing normally and generating normal potentials on some trials. The data suggest that the deficit in auditory short-term memory in these patients occurs during stimulus classification.