[MUSIC] Hello, my name is Kirill Nourski. I'm an associate professor of neurosurgery at the University of Iowa. And in this part of the methods overview, we will talk about intracranial electrophysiology. Intracranial electroencephalography, or iEEG, is perhaps the crown jewel among the methods used in modern neurolinguistics. The term refers to electrophysiological recordings made directly from the human brain. Such recordings are possible in neurosurgical patients, and are only possible based on clinical indications. Let's start with a general classification of methods. The principle approach of intracranial electrophysiology can be termed electrocorticography, or ECoG, or stereo-electroencephalography, or sEEG, depending on how recording electrodes are placed. The former, ECoG, is used when electrodes are placed on the brain surface. Whereas, the latter, sEEG, is when electrodes are implanted deep into the brain tissue. More on that later. There are two additional approaches that are sometimes used in conjunction with intracranial recordings that are worth mentioning. One is electrical stimulation, where the same electrodes can be used to stimulate brain tissue. The other is local cooling of the cortical surface. Intracranial recordings are possible in the following clinical contexts. It can be placement of electrodes in the brain for accurate localization and resection of a epilepsy seizure focus in neurosurgical patients. Or intracranial recordings are sometimes also used in patients who undergo resection of a brain tumor. Regarding duration, the two broad classes of experiments that are based on intracranial electrophysiology are acute, or intraoperative, and chronic. Those are carried out over 1 to 2 weeks, typically. The majority of iEEG studies are carried out in epilepsy patients. For a patient to be a candidate for neurosurgical treatment, three criteria have to be met. First, these patients have to fail antiepileptic drug treatment, so that the surgical option is essentially the last resort for them. Also, the disorder, the epilepsy disorder, has to have a focal nature. That is to say, it has to be a single resectable focus. Also, these patients are expected to expect a projected considerable improvement in their quality of life to make the surgery worthwhile. Patients who volunteer to be research subjects participate in both clinical and additional research protocols. The pre-surgical clinical evaluation includes video EEG monitoring, structural MRI scanning, as well as neuropsychological testing. Additional research protocols include functional MRI, audiological testing and ear mold casting, language casting, and additional psychological and cognitive testing. The method of intracranial EEG offers several advantages over other non-invasive approaches to studying neurolinguistics. First of all, intracranial EEG is a direct measure of activity of neuronal populations, unlike, for example, functional MRI or PET. This method has a very high resolution, both in time and space. On the order of milliseconds in time and on the order of millimeters in space, which makes it a lot better than both fMRI or PET, and EEG or MEG. This method offers an exceptionally high signal-to-noise ratio, allowing examination of high-frequency cortical activity. And also, it is characterized by a low susceptibility to artifacts, such as those generated by eye movements or other muscle artifacts. When compared to other methods, iEEG has a remarkably high resolution, both in time and space, spanning from milliseconds to days, and from single neuron level to coverage of entire brain lobes. It should be born in mind, however, that placement of electrodes for iEEG studies is dictated only by the clinical necessity, and thus the electrode coverage is always discontinuous or patchy. Approximately half of the neurosurgical epilepsy patients have their diagnosis formulated based on non-invasive methods, and thus proceed to surgery without the need to implant electrodes chronically. This offers an opportunity for research to be performed in the operating room. These experiments are called acute or intraoperative studies. The clinical background, thus, is to perform intraoperative recording to gather additional information about seizure focus localization. As well as to perform intraoperative electrical stimulation to test for and avoid resection of critically important cortex in the patient. The acute experimental approach has several advantages. First of all, it offers a relatively flexible choice of recording stimulation or cooling devices to use. These devices can be placed directly on the cortical surface, and they can be repositioned at the researcher's will without damaging the underlying tissue. At the same time, this method has several limitations. First and the foremost, time that is available for research is quite limited, typically to about half an hour or so. Also, as you can imagine, the range of research tasks that the patient, even an awake patient during an awake craniotomy, can perform in the operating room is quite limited. Finally, the environment of the operating room is plagued by a great deal of ambient noise. And that includes the acoustic noise, as well as electromagnetic interference.