Abstracts

Rationale: Recent studies have suggested that recording of event-related gamma-oscillations on electrocorticography (ECoG) may be useful to localize language-related cortical areas in presurgical evaluation for patients with intractable focal epilepsy (Sinai et al., 2005; Brown et al., 2008; Towle et al., 2008). Here, we determined how gamma-oscillations were modulated by three distinct word-association tasks. Methods: The study was performed on a 17-year-old right-handed male with a diagnosis of intractable focal epilepsy. The Wada test suggested left hemisphere dominance for speech. The patient underwent chronic implantation of subdural electrodes on the left hemisphere, in order to determine the presumed epileptogenic zone and eloquent cortical regions. During extraoperative ECoG monitoring, he completed three word-association tasks. He was presented a total of 40 names of common foods (e.g.: apple) during each task. He was instructed to overtly verbalize the color (e.g.: red) of each given food during the first association task, the taste (e.g.: sweet) during the second task, and the texture (e.g.: crunchy) during the third task. Each ECoG trial was transformed into the time-frequency domain, and we determined ‘when’ and ‘where’ gamma-oscillations were augmented (cortical activation) or attenuated (cortical deactivation). Results: The areas showing gamma-augmentation commonly elicited by all three word-association tasks included: (i) the superior temporal gyrus immediately following auditory presentation of the name of food, (ii) the inferior temporal gyrus and inferior frontal gyrus prior to verbalization, and (iii) the pre- and post-central gyri immediately prior to and during verbalization. On the other hand, the food-texture association task differentially elicited gamma-augmentation in the supramarginal gyrus. Ictal subdural ECoG recording showed the seizure onset arising from the left medial temporal region and the patient underwent left anterior temporal lobectomy while language-related cortex indicated by event-related gamma-oscillations was preserved. He has been seizure-free without apparent speech deficits (follow-up: five months). Conclusions: This preliminary study generated the hypothesis that word-association tasks can also localize the language-related cortical regions. A possible explanation for differential gamma-augmentation in the supramarginal gyrus is that the food-texture association task required more workload, compared to the remaining two tasks. (Supported by NIH grants NS47550 & NS64033 to E. Asano)