Abstracts

Rationale: Most patients with intractable epilepsy suffer from verbal memory deficits, which are often worse after surgical treatment. A better understanding of those areas that contribute to verbal memory storage and retrieval processes is needed, along with development of techniques to identify them prior to surgical resection. The purpose of this study is to determine which areas of the brain are activated during verbal memory tasks by analyzing EEG recordings from normal subjects and electrocorticographic (ECoG) recordings from patients with chronic subdural electrodes implanted as part of their surgical work-up for intractable epilepsy. Methods: ECoG recordings from eight epilepsy patients admitted to the epilepsy center at the University of Chicago for medically intractable seizures were studied. Silastic arrays containing 88-162 subdural electrodes chronically implanted over the frontal, parietal, and temporal lobes were placed according to the needs of the patient. The patient findings were compared to EEG and behavioral studies obtained from 14 normal volunteer subjects. Participants performed short-term and long-term verbal memory tasks using auditory presentation of words presented through a free-field speaker. For the short-term memory task the participants heard 6 words over 6 sec, followed by a probe word. They then verbally indicated whether the probe word was one of the original 6 words. Ten trials were mixed with 10 control trials. For the long-term verbal memory task the subjects listened to a list of 10 words, which they were asked to memorize. Directly following the words, the participants were distracted from rehearsal by a 1 min series of simple arithmetic problems. The subjects were then asked to state all the words from the list they could recall within 1 min. The subjects then listened to a list of 20 words, and were required to state whether each word was in the original list. Three trials of this condition were conducted. Results: The normal subjects emitted an event-related potential during the memory task which was different for new and old words. The late component at 600 msec, associated with memory retrieval for studied words was greater in amplitude (p < 0.01) and shorter in latency (p < 0.01) compared to novel probe words. The patients emitted transient bursts of high gamma activity (70-100 Hz) from motor and language areas, as well as frontal and parietal areas outside of these regions during memory storage. Conclusions: EEG recordings from a small series of normal subjects and chronic subdural ECoG recordings from epilepsy patients reveal frontal lobe cortical areas that are active during short-term and long-term memory tasks. The identification of these areas may be useful for the planning of cortical resections for the treatment of intractable epilepsy. The ability to identify the location of cortical areas that are involved in verbal memory storage and retrieval processes and their temporal dynamics may provide a better understanding of the neural substrates of language and verbal memory processes.