Abstracts

Rationale: Identification of expressive and receptive speech areas and other eloquent cortex is an important part of the surgical work-up for intractable epilepsy. We have found that these areas can be identified through analysis of local changes in electrocorticographic (ECoG) patterns obtained during simple motor and language tasks. The goal is to create a functional cortical map through passive recordings from subdural electrodes without utilizing direct electrical stimulation of cortex.Methods: Twelve epilepsy patients (age range 12-44 y) admitted to the epilepsy center at the University of Chicago were studied during their invasive work-up for medically intractable seizures. The patients were tested after seizures had been recorded and characterized using ECoG video monitoring from as many as 152 subdural electrodes chronically implanted over the frontal, parietal and temporal lobes. The testing usually took place 24-48 h before their resective surgery. Two language tasks were performed during a recording session lasting approximately 30 minutes. During the first task, patients listened to a series of 50 words preceded by warning tones and were simply asked to repeat each word. During the second language trial of 100 tones followed randomly by 50 words from the first trial and 50 new words, patients were asked to repeat the word only if it was a new word, and remain silent if they heard the word during the first task. Auditory stimuli were presented through open field speakers at a comfortable intensity level. Subdural ECoG was recorded from 63 subdural electrodes, with a 64th channel recording the patient's voice. ECoGs were recorded using Neuroscan Synamps (500 Hz/channel, 1-100 Hz bandpass), with triggers marking tone onset, word onset, and voice onset time. Results: Localized, task-specific changes in gamma activity (70-100 Hz) related to primary auditory processing, expressive and receptive speech, were observed over Broca's and Wernicke's areas (t = 2.77, df = 11, p < .02) and distant areas in the frontal, parietal and temporal cortex. The location of eloquent cortex was confirmed by direct electrical stimulation and analysis of the distribution of the subdural average auditory and somatosensory evoked potentials. Evidence for suppression of the response to one's own voice was found in superior temporal regions.Conclusions: The results indicate that receptive and expressive speech-related areas can be identified by analysis of the location of high-frequency gamma activity. This broad-spectrum activity extended higher than the so-called 40 Hz band, and appeared to extend beyond than the 100 Hz limit of our recording paradigm. It appears that this procedure might relegate direct electrical stimulation of cortex to map speech areas to a confirmatory role. Supported by R01 NS40514, The Brain Research Foundation and The Susman & Asher Foundation.