Abstracts

Rationale: Electrocorticography (ECoG) is primarily used clinically to localize the epileptogenic zone with excellent spatial resolution. This modality is also employed in translational research in which task-related augmentation of high gamma activity (50-150Hz) is used for functional mapping of the cortical surface. Various modalities, including fMRI, FDG-PET, and rTMS, have revealed mental calculation to consistently involve the left inferior-parietal lobule and inferior-temporal gyrus. The objective of this study was to evaluate the localization of specific mental calculation function by high gamma augmentation using ECoG while contrasting with an auditory naming task. Methods: Nine patients, aged 9 to 28 years, hospitalized to enable extra-operative ECoG evaluation of intractable epilepsy were included in this study. Six patients had left hemispheric recording and three had right hemispheric recording. Each patient was auditorily provided 96 simple calculation questions, addition and subtraction only, and 85 naming questions. Sites showing significant gamma augmentation during mental calculation or auditory naming were defined as calculation-related or naming-related sites, respectively. We compared calculation-related sites with naming-related ones to determine sites specifically related to mental calculation.Results: Eight patients, among which two had right hemispheric recording, showed a double dissociation with two sites differentially showing calculation- or naming-specific gamma-augmentation. In these patients, calculation-specific gamma-augmentation was noted in the left middle-temporal gyrus in two patients, left inferior-temporal gyrus in one patient, and left inferior-parietal lobule in two patients. Calculation-specific gamma-augmentation was also noted in the frontal lobe on either hemisphere. Such calculation-specific gamma-augmentation took place after auditory stimuli but prior to responses. Gamma augmentation commonly elicited by both tasks involved the superior-temporal gyri while listening to auditory stimuli, and the pre- and post-central gyri during responses. Conversely, naming-specific gamma-augmentation was noted in widespread regions, bilaterally. Conclusions: This study externally validated the previous observations which localized mental calculation within the left inferior-parietal lobule and left inferior-temporal gyrus. Based on the temporal characteristics, such calculation-specific gamma-augmentation was not associated with auditory perception or sensorimotor functions of the mouth. Further, we observed additional regions of specificity for mental calculation in the left middle-temporal gyrus. These results suggest that the left middle-temporal gyrus may also possess regions specifically related to mental calculation.