Abnormalities of early "memory-scanning" event-related potentials in patients with temporal lobe epilepsy

We have recorded auditory event-related potentials (ERPs) evoked by the "memory-scanning" (digit-probe identification/matching) paradigm that was originally described by Sternberg (1966), in 17 patients with complex partial seizures (temporal lobe epilepsy) and in 17 matched healthy control subjects. The patients, who had all complained spontaneously of memory difficulties, had significantly reduced scores on psychological tests of memory with relatively intact digit span and cognition. Their performance of the memory-scanning task was characterized by a higher error rate, longer reaction times and an increased slope of the reaction time/set size relationship. The associated ERPs in both patients and controls showed there were significant effects of memory load on several major components, but only a reduced amplitude of the N170 and a prolonged latency of the N290 waves distinguished the patients. In addition, the N170 wave in the patients decreased further as memory load increased. The prolonged N290 latency in the patients appeared to reflect the slowed processing time. This study has shown that ERPs generated by a short-term memory task are abnormal in patients with temporal lobe epilepsy who have neuropsychologically documented cognitive and memory deficits. Some of the significant waveform alterations occur earlier than those reported in previous ERP studies and provide electrophysiological support for the hypothesis that abnormalities of the early stages of short-term memory processing may contribute to the memory difficulties experienced by patients with temporal lobe epilepsy.