Abstracts

Rationale: The potential complications following epilepsy surgery include temporary or permanent language deficits. Postoperative development of language deficit may require speech therapy and may delay the return to home, school or work. Surgeries of the lateral left hemisphere generally pose the greatest risk of such deficit. That is, unless essential language function is reorganized to the right hemisphere, as most commonly seen in left-handed epileptic patients with early-onset neocortical lesions on neuroimaging. To minimize the risk of postoperative deficits, investigators try to achieve accurate localization of eloquent cortices prior to tissue resection. Studies have shown that language-related augmentation of gamma activity on electrocorticography (ECoG) often detects language-related sites defined by electrical stimulation. Methods: We validated the utility of auditory-language-related gamma-augmentation as a biomarker of underlying language function, by analyzing a consecutive series of 74 epileptic patients (age: 4 - 56 years). First, in left-handed patients with left-sided seizure focus and early-onset left-sided neocortical lesions, essential language function was estimated to reorganize to the right hemisphere. In the remaining patients, the left hemisphere was estimated to contain essential language function, regardless of handedness. We determined the effects of seizure onset zone, electrode location, and patient age upon gamma-augmentation elicited by an auditory-naming task. We determined if resection of sites showing gamma-augmentation predicted a language deficit. Results: Significant gamma-augmentation was less frequently elicited within seizure onset sites compared to the other sites. Regardless of age, patients showed gamma-augmentation, most often involving the 80-100 Hz frequency band. Gamma-augmentation initially involved the superior-temporal regions, bilaterally, followed by the middle-temporal, medial-temporal, inferior-frontal, dorsolateral-premotor, and medial-frontal, and concluded with bilateral inferior-Rolandic involvement. Sites showing gamma-augmentation outside of superior-temporal and inferior-Rolandic regions were more extensive in the left hemisphere with considerable temporal overlap. Compared to younger patients, those older than 10 years had a larger proportion of left dorsolateral-premotor sites showing gamma-augmentation. The incidence of a post-operative language deficit requiring speech therapy was best predicted by the number of resected sites with significant gamma-augmentation in the superior-temporal, inferior-frontal, dorsolateral-premotor, and inferior-Rolandic regions of the left hemisphere estimated to contain essential language function (r-square=0.651; p=0.001; Odds Ratio: 8.9 [95%CI: 2.6-31.0]). The prediction performance of this logistic regression model incorporating gamma-augmentation remained significant after controlling for the extent of resection or electrical stimulation data. Conclusions: Auditory-language-related gamma-augmentation on ECoG can provide additional information useful in the localization of primary language areas.