Characterization of High-Gamma Activity in Electrocorticographic Signals

IntroductionElectrocorticographic (ECoG) high-gamma activity (HGA) is a widely recognized and robust neural correlate of cognition and behavior. However, fundamental signal properties of HGA, such as the high-gamma frequency band or temporal dynamics of HGA, have never been systematically characterized. As a result, HGA estimators are often poorly adjusted, such that they miss valuable physiological information.MethodsTo address these issues, we conducted a thorough qualitative and quantitative characterization of HGA in ECoG signals. Our study is based on ECoG signals recorded from 18 epilepsy patients while performing motor control, listening, and visual perception tasks. In this study, we first categorize HGA into HGA types based on the cognitive/behavioral task. For each HGA type, we then systematically quantify three fundamental signal properties of HGA: the high-gamma frequency band, the HGA bandwidth, and the temporal dynamics of HGA.ResultsThe high-gamma frequency band strongly varies across subjects and across cognitive/behavioral tasks. In addition, HGA time courses have lowpass character, with transients limited to 10 Hz. The task-related rise time and duration of these HGA time courses depend on the individual subject and cognitive/behavioral task. Task-related HGA amplitudes are comparable across the investigated tasks.DiscussionThis study is of high practical relevance because it provides a systematic basis for optimizing experiment design, ECoG acquisition and processing, and HGA estimation. Our results reveal previously unknown characteristics of HGA, the physiological principles of which need to be investigated in further studies

Facephenes and rainbows: Causal evidence for functional and anatomical specificity of face and color processing in the human brain

Neuroscientists have long debated whether some regions of the human brain are exclusively engaged in a single specific mental process. Consistent with this view, fMRI has revealed cortical regions that respond selectively to certain stimulus classes such as faces. However, results from multivoxel pattern analyses (MVPA) challenge this view by demonstrating that category-selective regions often contain information about “nonpreferred” stimulus dimensions. But is this nonpreferred information causally relevant to behavior? Here we report a rare opportunity to test this question in a neurosurgical patient implanted for clinical reasons with strips of electrodes along his fusiform gyri. Broadband gamma electrocorticographic responses in multiple adjacent electrodes showed strong selectivity for faces in a region corresponding to the fusiform face area (FFA), and preferential responses to color in a nearby site, replicating earlier reports. To test the causal role of these regions in the perception of nonpreferred dimensions, we then electrically stimulated individual sites while the patient viewed various objects. When stimulated in the FFA, the patient reported seeing an illusory face (or “facephene”), independent of the object viewed. Similarly, stimulation of color-preferring sites produced illusory “rainbows.” Crucially, the patient reported no change in the object viewed, apart from the facephenes and rainbows apparently superimposed on them. The functional and anatomical specificity of these effects indicate that some cortical regions are exclusively causally engaged in a single specific mental process, and prompt caution about the widespread assumption that any information scientists can decode from the brain is causally relevant to behavior.National Institutes of Health (U.S.) (Grant DP1HD091947

Left hippocampectomy in an epilepsy patient with right hemisphere language dominance

Background: The left hippocampus is believed to play an important role in memory function. Thus, left hippocampectomy should be carefully performed. However, the left hippocampus may be less important for memory in patients with right hemisphere language dominance. We performed left hippocampectomy in a patient with epilepsy and right hemisphere language dominance. Here we present the findings of our functional mapping and examine the relationship between memory and language dominance. Methods: The patient was a 32-year-old left-handed female implanted with subdural electrocorticography (ECoG) grids over both hemispheres to locate the epilepsy focus. Language dominance was determined by the Wada test. Functional localizations were identified with ECoG and electrocortical stimulation (ECS) mapping. The Wechsler Memory Scale-Revised (WMS-R) was used to evaluate the patient’s memory before and after surgery. Results: The epilepsy focus was located in the left mesial temporal lobe and hippocampus. The Wada test revealed right hemisphere language dominance. High gamma activity (HGA) mapping was consistent with ECS mapping in that both showed language-related activity in the right precentral gyrus. The WMS-R revealed improved memory after left hippocampectomy, without neurological deficits. The patient fully recovered without further epileptic seizures. Conclusion: Left hippocampectomy in a patient with epilepsy and right hemisphere language dominance did not cause memory dysfunction. Thus, memory function is possibly related to the language-dominant hemisphere. Furthermore, preoperative mapping and determining a patient’s language dominance can help to preserve language and memory in epilepsy surgery

Integration of functional brain information into stereotactic irradiation treatment planning using magnetoencephalography and MR-axonography

Purpose: To minimize the risk of neurologic deficit after stereotactic irradiation, functional brain information was integrated into treatment planning. Methods and materials: Twenty-one magnetoencephalography and six magnetic resonance axonographic images were made in 20 patients to evaluate the sensorimotor cortex (n = 15 patients, including the corticospinal tract in 6), visual cortex (n = 4), and Wernicke's area (n = 2). One radiation oncologist was asked to formulate a treatment plan first without the functional images and then to modify the plan after seeing them. The pre- and postmodification values were compared for the volume of the functional area receiving ≥15 Gy and the volume of the planning target volume receiving ≥80% of the prescribed dose. Results: Of the 21 plans, 15 (71%) were modified after seeing the functional images. After modification, the volume receiving ≥15 Gy was significantly reduced compared with the values before modification in those 15 sets of plans (p = 0.03). No statistically significant difference was found in the volume of the planning target volume receiving ≥80% of the prescribed dose (p = 0.99). During follow-up, radiation-induced necrosis at the corticospinal tract caused a minor motor deficit in 1 patient for whom magnetic resonance axonography was not available in the treatment planning. No radiation-induced functional deficit was observed in the other patients. Conclusion: Integration of magnetoencephalography and magnetic resonance axonography in treatment planning has the potential to reduce the risk of radiation-induced functional dysfunction without deterioration of the dose distribution in the target volume

Proceedings of the Seventh International Workshop on Advances in Electrocorticography.

The Seventh International Workshop on Advances in Electrocorticography (ECoG) convened in Washington, DC, on November 13-14, 2014. Electrocorticography-based research continues to proliferate widely across basic science and clinical disciplines. The 2014 workshop highlighted advances in neurolinguistics, brain-computer interface, functional mapping, and seizure termination facilitated by advances in the recording and analysis of the ECoG signal. The following proceedings document summarizes the content of this successful multidisciplinary gathering

Role of surgery for optic pathway/hypothalamic astrocytomas in children

Optic pathway/hypothalamic pilocytic astrocytomas in children are usually treated with chemotherapy following a surgical biopsy. In this report, we retrospectively considered the role of surgical intervention. In a series of 25 patients without neurofibromatosis type 1, the median age at initial treatment was 3.1 years (range, 0–15 years). Twenty cases were verified by histology, and five cases were diagnosed by MRI findings. Twenty-three patients received chemotherapy. All patients were alive at median follow-up of 66 months. Aims of surgery at the initiation of treatment were biopsy in 12 cases (1 stereotactic and 11 craniotomies) and debulking in 7 cases. The 11 open biopsies revealed pilocytic astrocytoma; however, noticeable complications occurred in five children after the biopsies. Review of preoperative MRIs showed that all had typical findings indicating pilocytic astrocytoma. The open biopsy offered no noteworthy benefit for the patients despite surgical risk and delay of chemotherapy. The extent of the seven resection surgeries was 70% or less removal, and postoperative adjuvant therapy was needed for six of the seven patients. The remaining six children who did not undergo surgery obtained remission with chemotherapy alone. After relapse in nine patients, 15 bulk-reduction surgeries were performed. Surgical resection was not curative in any patient. In five patients, mostly older children, cystic expansion of tumor was partially resected, resulting in additional remission. In conclusion, considering the risk of open surgery and the effectiveness of chemotherapy, the role of surgical intervention is restricted to bulk-reduction surgery only when it is inevitable, especially at relapse after chemotherapy