Mapping mental calculation systems with electrocorticography

• Calculation and naming tasks differentially elicited augmentation of gamma activity.
• Calculation-specific gamma activity involved the left parietal, temporal and frontal lobes.
• The timing of calculation-specific gamma activity differed across the regions.

ObjectiveWe investigated intracranially-recorded gamma activity during calculation tasks to better understand the cortical dynamics of calculation.MethodsWe studied 11 patients with focal epilepsy (age range: 9–28 years) who underwent measurement of calculation- and naming-related gamma-augmentation during extraoperative electrocorticography (ECoG). The patients were instructed to overtly verbalize a one-word answer in response to auditorily-delivered calculation and naming questions. The assigned calculation tasks were addition and subtraction involving integers between 1 and 17.ResultsOut of the 1001 analyzed cortical electrode sites, 63 showed gamma-augmentation at 50–120 Hz elicited by both tasks, 88 specifically during naming, and 7 specifically during calculation. Common gamma-augmentation mainly took place in the Rolandic regions. Calculation-specific gamma-augmentation, involving the period between the question-offset and response-onset, was noted in the middle-temporal, inferior-parietal, inferior post-central, middle-frontal, and premotor regions of the left hemisphere. Calculation-specific gamma-augmentation in the middle-temporal, inferior-parietal, and inferior post-central regions peaked around the question offset, while that in the frontal lobe peaked after the question offset and before the response onset. This study failed to detect a significant difference in calculation-specific gamma amplitude between easy trials and difficult ones requiring multi-digit operations.ConclusionsAuditorily-delivered stimuli can elicit calculation-specific gamma-augmentation in multiple regions of the left hemisphere including the parietal region. However, the additive diagnostic value of measurement of gamma-augmentation related to a simple calculation task appears modest.SignificanceFurther studies are warranted to determine the functional significance of calculation-specific gamma-augmentation in each site, and to establish the optimal protocol for mapping mental calculation.