Localization of Expressive Language Cortex in a 2-Year-Old Child Using High-Gamma Electrocorticography

Cortical stimulation mapping is the gold standard for presurgical language mapping; however, it cannot be reliably performed in very young patients. Language mapping using noninvasive modalities is also challenging in very young patients. Although utility of language mapping using power of high-gamma in electrocorticographic recordings was demonstrated in adults and older children, there is a gap of knowledge in the ability of this procedure for localizing language-specific cortex in very young patients. We describe a case of a 2-year-old patient who, to our knowledge, is the youngest person to undergo successful high-gamma electrocorticographic presurgical language mapping for localization of the expressive language cortex (Broca area). The surgical plan was to resect a cortical tuber within the left inferior frontal gyrus and there was a strong concern about postoperative language deficit after resection. Presurgical language mapping using noninvasive modalities were attempted without success. Cortical stimulation mapping was not feasible in this patient. Therefore, high-gamma electrocorticography was the only viable option for language mapping, and it successfully localized the expressive language cortex. The patient underwent surgery for resection of the IFG tuber based on results of high-gamma electrocorticography and had no postoperative language deficit. High-gamma electrocorticography can be used for localizing language-specific cortex, especially Broca’s area, in very young patients.

Deep Learning provides exceptional accuracy to ECoG-based Functional Language Mapping for epilepsy surgery

The success of surgical resection in epilepsy patients depends on preserving functionally critical brain regions, while removing pathological tissues. Being the gold standard, electro-cortical stimulation mapping (ESM) helps surgeons in localizing the function of eloquent cortex through electrical stimulation of electrodes placed directly on the cortical brain surface. Due to the potential hazards of ESM, including increased risk of provoked seizures, electrocorticography based functional mapping (ECOG-FM) was introduced as a safer alternative approach. However, ECoG-FM has a low success rate when compared to the ESM. In this study, we address this critical limitation by developing a new algorithm based on deep learning for ECoG-FM and thereby we achieve an accuracy comparable to ESM in identifying eloquent language cortex. In our experiments, with 11 epilepsy patients who underwent presurgical evaluation (through deep learning-based signal analysis on 637 electrodes), our proposed algorithm made an exceptional 23% improvement with respect to the conventional ECoG-FM analysis (∼60%). We obtained the state-of-the-art accuracy of 83.05% in identifying language regions, which has never been achieved before. Our findings have demonstrated, for the first time, that deep learning powered ECoG-FM can serve as a stand-alone modality and avoid likely hazards of the ESM in epilepsy surgery. Hence, reducing the potential for developing post-surgical morbidity in the language function.

Technical and Practical Aspects of Invasive Recordings and Brain Stimulation

This chapter discusses the technical and practical issues involved in invasive recording and cortical stimulation mapping in patients with drug-resistant epilepsy. It reviews the way in which EEG signals are generated, circumstances when intracranial electrodes are needed, and how such electrodes operate. It also discusses the basic principles of cortical stimulation mapping and different methods of using intracranial electrodes for stimulation purposes, and relevant concepts involved in the process such as charge density and electrode surface area. It reviews different electrodes used for mapping including subdural surface electrodes and depth electrodes.

Electrical Stimulation for Mapping of the Epileptogenic Zone

Electrical cortical stimulation mapping (ECM) is a neurophysiological technique used to help identify the epileptogenic zone (area of brain responsible for a person’s epilepsy). ECM accomplishes this in two ways: (1) direct stimulation of a specified cortical area resulting in the patient’s habitual seizures and (2) the understanding of specific cortical function based on ECM studies. This chapter begins by reviewing neurophysiological concepts underlying ECM. It then continues with an anatomical survey of cortical function as defined by ECM and an explanation of how the identified cortical functions relate to understanding seizure semiology. It proceeds to examine the techniques for electrically provoking seizures and interpretation of such provoked seizures. It concludes by exploring advanced electrophysiological concepts and how they may improve characterization of the epileptogenic zone. In short, this chapter seeks to conceptualize the use of ECM in accurately identifying the epileptogenic zone and informing safe surgical treatment when necessary.

Selective Interference with Syntactic Encoding during Sentence Production by Direct Electrocortical Stimulation of the Inferior Frontal Gyrus

Cortical stimulation mapping (CSM) has provided important insights into the neuroanatomy of language because of its high spatial and temporal resolution, and the causal relationships that can be inferred from transient disruption of specific functions. Almost all CSM studies to date have focused on word-level processes such as naming, comprehension, and repetition. In this study, we used CSM to identify sites where stimulation interfered selectively with syntactic encoding during sentence production. Fourteen patients undergoing left-hemisphere neurosurgery participated in the study. In 7 of the 14 patients, we identified nine sites where cortical stimulation interfered with syntactic encoding but did not interfere with single word processing. All nine sites were localized to the inferior frontal gyrus, mostly to the pars triangularis and opercularis. Interference with syntactic encoding took several different forms, including misassignment of arguments to grammatical roles, misassignment of nouns to verb slots, omission of function words and inflectional morphology, and various paragrammatic constructions. Our findings suggest that the left inferior frontal gyrus plays an important role in the encoding of syntactic structure during sentence production.