Abstracts

Rationale: Planning for epilepsy surgery in parietal cortices often requires functional mapping prior to resection. However, no standard battery of tasks is widely used for functional parietal lobe mapping. Studies have shown that parietal cortices are recruited when performing mental imagery tasks and that localization and task difficulty both depend on the type of task performed (whether involving imagined rotation of an object or of oneself). Subdural electrodes are implanted for seizure localization, and task-related increases in electrocorticographic (ECoG) high gamma (65-150 Hz) activity recorded from these electrodes have been used to detect cortical activation. We hypothesized that ECoG mapping would discriminate the cortical representations of two mental rotation tasks, requiring either imagined rotation of the subject or an object, although the visual transformations underlying these two tasks are identical. We also hypothesized that subjects would find the object rotation task to be more difficult than the self rotation task.Methods: We studied 2 epilepsy patients (ages 15 and 28 years, both female) in whom subdural electrodes were implanted over parietal cortex for epilepsy surgery planning. Patients underwent ECoG mapping of imagined rotation of oneself around an object (SR) and of an object rotating in space along the transverse plane (OR). We performed time-frequency analyses of ECoG signals using a matching pursuit algorithm and detected statistically significant increases in event-related high gamma activity (defined as ECoG+ sites). ECoG+ sites were localized by co-registration of post-surgical CTs to pre-surgical volumetric MRIs and assigned appropriate Brodmann areas.Results: One subject performed more accurately on the self rotation task than the object rotation task (70% correct versus 52% correct, respectively) but responded more quickly during the object rotation task. The second subject performed poorly on both tasks (27% correct on OR and 34% correct on SR) and responded more quickly during SR. Of 164 total electrodes tested, 14 were ECoG+ during object rotation (4 during OR only), 16 during self rotation (6 during SR only), and of these, 10 during both self and object rotation. ECoG+ electrodes localized to frontal cortex (BA4, 6, 10, and 46), middle and poster temporal regions (BA21 and 37), visual cortex (BA18), and to associative visual cortex (BA19) and supramarginal gyrus (BA40) in the parietal lobe. More ECoG+ electrodes were seen in parietal cortex during self rotation (8) compared to object rotation (2). In fact, all ECoG+ electrodes during OR were also ECoG+ during SR.Conclusions: ECoG high gamma mapping revealed distinct and shared ECoG+ regions during object and self mental rotation tasks. More ECoG+ sites were identified during self rotation compared to object rotation within parietal regions. These findings suggest that administration of a self rotation task may have benefit over using an object rotation task in mapping parietal cortex. The study also demonstrates the utility of ECoG functional mapping during mental rotation.