Neuroscientific 연구 및 실시간 기능성 두피 매핑을위한 인적 Electrocorticographic (ECoG) 신호를 녹음하기

Summary

우리는 침략적 간질 모니터링을 진행하고 인간에서 연구 목적으로 electrocorticographic 신호를 수집하기위한 방법을 제시한다. 우리는 데이터 수집, 신호 처리 및 자극 프레 젠 테이션을 위해 BCI2000 소프트웨어 플랫폼을 사용하는 방법을 보여줍니다. 특히, 우리는 SIGFRIED, 실시간 기능적 뇌 맵핑을위한 BCI2000 기반 도구를 보여줍니다.

Abstract

Neuroimaging studies of human cognitive, sensory, and motor processes are usually based on noninvasive techniques such as electroencephalography (EEG), magnetoencephalography or functional magnetic-resonance imaging. These techniques have either inherently low temporal or low spatial resolution, and suffer from low signal-to-noise ratio and/or poor high-frequency sensitivity. Thus, they are suboptimal for exploring the short-lived spatio-temporal dynamics of many of the underlying brain processes. In contrast, the invasive technique of electrocorticography (ECoG) provides brain signals that have an exceptionally high signal-to-noise ratio, less susceptibility to artifacts than EEG, and a high spatial and temporal resolution (i.e., <1 cm/<1 millisecond, respectively). ECoG involves measurement of electrical brain signals using electrodes that are implanted subdurally on the surface of the brain. Recent studies have shown that ECoG amplitudes in certain frequency bands carry substantial information about task-related activity, such as motor execution and planning¹, auditory processing² and visual-spatial attention³. Most of this information is captured in the high gamma range (around 70-110 Hz). Thus, gamma activity has been proposed as a robust and general indicator of local cortical function^1-5. ECoG can also reveal functional connectivity and resolve finer task-related spatial-temporal dynamics, thereby advancing our understanding of large-scale cortical processes. It has especially proven useful for advancing brain-computer interfacing (BCI) technology for decoding a user’s intentions to enhance or improve communication⁶ and control⁷. Nevertheless, human ECoG data are often hard to obtain because of the risks and limitations of the invasive procedures involved, and the need to record within the constraints of clinical settings. Still, clinical monitoring to localize epileptic foci offers a unique and valuable opportunity to collect human ECoG data. We describe our methods for collecting recording ECoG, and demonstrate how to use these signals for important real-time applications such as clinical mapping and brain-computer interfacing. Our example uses the BCI2000 software platform^8,9 and the SIGFRIED¹⁰ method, an application for real-time mapping of brain functions. This procedure yields information that clinicians can subsequently use to guide the complex and laborious process of functional mapping by electrical stimulation.

Prerequisites and Planning:

Patients with drug-resistant partial epilepsy may be candidates for resective surgery of an epileptic focus to minimize the frequency of seizures. Prior to resection, the patients undergo monitoring using subdural electrodes for two purposes: first, to localize the epileptic focus, and second, to identify nearby critical brain areas (i.e., eloquent cortex) where resection could result in long-term functional deficits. To implant electrodes, a craniotomy is performed to open the skull. Then, electrode grids and/or strips are placed on the cortex, usually beneath the dura. A typical grid has a set of 8 x 8 platinum-iridium electrodes of 4 mm diameter (2.3 mm exposed surface) embedded in silicon with an inter-electrode distance of 1cm. A strip typically contains 4 or 6 such electrodes in a single line. The locations for these grids/strips are planned by a team of neurologists and neurosurgeons, and are based on previous EEG monitoring, on a structural MRI of the patient’s brain, and on relevant factors of the patient’s history. Continuous recording over a period of 5-12 days serves to localize epileptic foci, and electrical stimulation via the implanted electrodes allows clinicians to map eloquent cortex. At the end of the monitoring period, explantation of the electrodes and therapeutic resection are performed together in one procedure.

In addition to its primary clinical purpose, invasive monitoring also provides a unique opportunity to acquire human ECoG data for neuroscientific research. The decision to include a prospective patient in the research is based on the planned location of their electrodes, on the patient’s performance scores on neuropsychological assessments, and on their informed consent, which is predicated on their understanding that participation in research is optional and is not related to their treatment. As with all research involving human subjects, the research protocol must be approved by the hospital’s institutional review board. The decision to perform individual experimental tasks is made day-by-day, and is contingent on the patient’s endurance and willingness to participate. Some or all of the experiments may be prevented by problems with the clinical state of the patient, such as post-operative facial swelling, temporary aphasia, frequent seizures, post-ictal fatigue and confusion, and more general pain or discomfort.

At the Epilepsy Monitoring Unit at Albany Medical Center in Albany, New York, clinical monitoring is implemented around the clock using a 192-channel Nihon-Kohden Neurofax monitoring system. Research recordings are made in collaboration with the Wadsworth Center of the New York State Department of Health in Albany. Signals from the ECoG electrodes are fed simultaneously to the research and the clinical systems via splitter connectors. To ensure that the clinical and research systems do not interfere with each other, the two systems typically use separate grounds. In fact, an epidural strip of electrodes is sometimes implanted to provide a ground for the clinical system. Whether research or clinical recording system, the grounding electrode is chosen to be distant from the predicted epileptic focus and from cortical areas of interest for the research. Our research system consists of eight synchronized 16-channel g.USBamp amplifier/digitizer units (g.tec, Graz, Austria). These were chosen because they are safety-rated and FDA-approved for invasive recordings, they have a very low noise-floor in the high-frequency range in which the signals of interest are found, and they come with an SDK that allows them to be integrated with custom-written research software. In order to capture the high-gamma signal accurately, we acquire signals at 1200Hz sampling rate-considerably higher than that of the typical EEG experiment or that of many clinical monitoring systems. A built-in low-pass filter automatically prevents aliasing of signals higher than the digitizer can capture. The patient’s eye gaze is tracked using a monitor with a built-in Tobii T-60 eye-tracking system (Tobii Tech., Stockholm, Sweden). Additional accessories such as joystick, bluetooth Wiimote (Nintendo Co.), data-glove (5^th Dimension Technologies), keyboard, microphone, headphones, or video camera are connected depending on the requirements of the particular experiment.

Data collection, stimulus presentation, synchronization with the different input/output accessories, and real-time analysis and visualization are accomplished using our BCI2000 software^8,9. BCI2000 is a freely available general-purpose software system for real-time biosignal data acquisition, processing and feedback. It includes an array of pre-built modules that can be flexibly configured for many different purposes, and that can be extended by researchers’ own code in C++, MATLAB or Python. BCI2000 consists of four modules that communicate with each other via a network-capable protocol: a Source module that handles the acquisition of brain signals from one of 19 different hardware systems from different manufacturers; a Signal Processing module that extracts relevant ECoG features and translates them into output signals; an Application module that delivers stimuli and feedback to the subject; and the Operator module that provides a graphical interface to the investigator.

A number of different experiments may be conducted with any given patient. The priority of experiments will be determined by the location of the particular patient’s electrodes. However, we usually begin our experimentation using the SIGFRIED (SIGnal modeling For Realtime Identification and Event Detection) mapping method, which detects and displays significant task-related activity in real time. The resulting functional map allows us to further tailor subsequent experimental protocols and may also prove as a useful starting point for traditional mapping by electrocortical stimulation (ECS).

Although ECS mapping remains the gold standard for predicting the clinical outcome of resection, the process of ECS mapping is time consuming and also has other problems, such as after-discharges or seizures. Thus, a passive functional mapping technique may prove valuable in providing an initial estimate of the locus of eloquent cortex, which may then be confirmed and refined by ECS. The results from our passive SIGFRIED mapping technique have been shown to exhibit substantial concurrence with the results derived using ECS mapping¹⁰.

The protocol described in this paper establishes a general methodology for gathering human ECoG data, before proceeding to illustrate how experiments can be initiated using the BCI2000 software platform. Finally, as a specific example, we describe how to perform passive functional mapping using the BCI2000-based SIGFRIED system.

Protocol

1. 전극 현지화 슬라이스 당 256 X 256 픽셀,보기의 전체 분야, 아니 보간, 1 밀리미터 슬라이스 폭 선호 화살 크로스 – 섹션 : 환자의 머리 사전 수술 T1-가중 구조적 MRI로 (1.5T 또는 3T)를 수집합니다. 격자 및 스트립의 외과 이식을 관찰. 이식 격자 및 스트립의 위치에 디지털 현장에서의 전극의 사진, 그리고 신경 외과 의사의 노트를 수집합니다. 고해상도의 수술 두개골 X 선 이?…

Discussion

연구 ECoG 데이터를 수집하는 것은 매우 멀티 – 징계 팀 임상 신경학, 신경 외과, 기초 신경 과학, 컴퓨터 과학 및 전기 공학 문제 해결과 함께, 임상의와 연구자 간의 긴밀한 협력이 필요합니다. 보상 ECoG 신호와 높은 감마 주파수 범위 (70-110Hz)에서 특정 amplitudes에서 매우 가치가있다는 것입니다. 그들은 감각,인지의 신경 상호로 과학적인 통찰력을 제공하고 모터는 높은 공간과 시간적 해상도 모두?…

Materials

1. 8 x 16-channel g.USBamp amplifiers ( http://gtec.at )
2. 2 x 64-channel break-out box (splitter head-box)
3. 2 x Connection cable from splitter to clinical system
4. 2 x Connection cable from splitter to four g.USBamps
5. 2 x Four-way power adapter for four g.USBamps
6. 2 x Four-way sync adapter to synchronize four g.USBamps
7. 1 x Sync cable to synchronize two sets of four g.USBamps
8. 1 x Potential-equalization clamp + cable for g.USBamp
9. 18 x Touchproof jumper cables
10. 2 x Four-way USB 2.0 hubs
11. Power strip
12. Laptop or desktop computer (see section 2.1)
13. Secure, moveable cart for all of the above
14. Eyetracker (or ordinary LCD monitor) for patient
15. Moveable tray table for the patient monitor
16. Other peripherals (joysticks etc) for patient behavioral responses
17. BCI2000 software
18. CURRY software
19. MATLAB software