SpecialIssue Normal Electroencephalogram D (original) (raw)
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Physiologic Basis of the EEG Signal
A Practical Handbook, 2011
The EEG is the recording of the spontaneous electrical activity generated by cerebral neurons. Like all other cells in the body, neurons have high concentrations of potassium (K + ) and chloride (Cl − ) ions inside, while high concentrations of sodium (Na + ) and calcium (Ca 2+ ) ions are kept outside. This leads to a voltage difference of about −60 to −70 mV with respect to the outside of the cell membrane. Such a voltage difference is modified by the flux of ions depending on the opening and closing of ion channels induced by electrical or chemical stimuli. A reduction of charge separation across the membrane, due to an influx of positive charged ions into the cell, results in a less negative membrane potential and is termed depolarisation, whereas an increase in charge separation leading to a more negative membrane potential is called hyperpolarisation.
Sanei/EEG Signal Processing, 2013
Introduction to EEG The neural activity of the human brain starts between the 17th and 23rd week of prenatal development. It is believed that from this early stage and throughout life electrical signals generated by the brain represent not only the brain function but also the status of the whole body. This assumption provides the motivation to apply advanced digital signal processing methods to the electroencephalogram (EEG) signals measured from the brain of a human subject, and thereby underpins the later chapters of the book. Although nowhere in this book do the authors attempt to comment on the physiological aspects of brain activities there are several issues related to the nature of the original sources, their actual patterns, and the characteristics of the medium, that have to be addressed. The medium defines the path from the neurons, as so-called signal sources, to the electrodes, which are the sensors where some form of mixtures of the sources are measured. Understanding of neuronal functions and neurophysiological properties of the brain together with the mechanisms underlying the generation of signals and their recordings is, however, vital for those who deal with these signals for detection, diagnosis, and treatment of brain disorders and the related diseases. A brief history of EEG measurements is first provided. 1.1 History Carlo Matteucci (1811-1868) and Emil Du Bois-Reymond (1818-1896) were the first people to register the electrical signals emitted from muscle nerves using a galvanometer and established the concept of neurophysiology [1,2]. However, the concept of action current introduced by Hermann Von Helmholz [3] clarified and confirmed the negative variations that occur during muscle contraction. Richard Caton (1842-1926), a scientist from Liverpool, England, used a galvanometer and placed two electrodes over the scalp of a human subject and thereby first recorded brain activity in the form of electrical signals in 1875. Since then, the concepts of electro-(referring to registration of brain electrical activities) encephalo-(referring to emitting the signals from the head), and gram (or graphy), which means drawing or writing, were combined so that the term EEG was henceforth used to denote electrical neural activity of the brain.
An Insight to the Human Brain and EEG
SpringerBriefs in Applied Sciences and Technology, 2018
The human brain is a major part of the central nervous system (CNS). The CNS and the peripheral nervous system (PNS) are two major sections of the human nervous system. The CNS comprises of the brain and the spinal cord, whereas, the PNS connects the CNS to primary sensory organs of the body such as the eye, ear, nose, etc., and other organs of the body. It comprises of the spinal nerves, 12 cranial nerves, and the autonomic nerves which regulate the cardiac muscles, blood vessel wall, and gland muscles. The CNS receives information from the sensory organs and resends this information to the PNS. This chapter gives details on the human brain and its constituent features. It also discusses electroencephalography (EEG) recording, the physics behind it, EEG electrodes, their composition, and other necessary details. The last section is about the conventional EEG placement methodologies. 2.1 Neurons Neurons can be called the basic units of the nervous system. Any cell in the nervous system has neuron as its primary component. Neurons can be of the following three types:
Spatial spectra of scalp EEG and EMG from awake humans
Clinical Neurophysiology, 2003
Objectives: Evaluate spectral scaling properties of scalp electroencephalogram (EEG) and electromyogram (EMG), optimal spacing of electrodes, and strategies for mitigating EMG. Methods: EEG was recorded referentially from 9 subjects with a 64 channel linear array (electrodes 3 mm apart) placed parasagittally or transversely on forehead or occiput, at rest with eyes open or closed, or with deliberate EMG. Temporal (PSD t) and spatial (PSD x) power spectral densities were calculated with 1-dimensional fast Fourier transform (FFT) for comparison with earlier analyses of intracranial EEG. Results: Scaling of PSD t from scalp resembled that from pia: near-linear decrease in log power with increasing log frequency (1/f a). Scalp PSD x decreased nonlinearly and more rapidly than PSD x from pia. Peaks in PSD t (especially 4-12 Hz) and PSD x (especially 0.1-0.4 cycles/cm) revealed departures from 1/f a. EMG power in PSD t was more 'white' than 1/f a. Conclusions: Smearing by dura-skull-scalp distorts PSD x more than PSD t of scalp EEG from 1/f a scaling at the pia. Spatial spectral peaks suggest that optimal scalp electrode spacing might be ~1 cm to capture nonlocal EEG components having the texture of gyri. Mitigation of EMG by filtering is unsatisfactory. A criterion for measuring EMG may support biofeedback for training subjects to reduce their EMG. Significance: High-density recording and log-log spectral display of EEG provide a foundation for holist studies of global human brain function, as an alternative to network approaches that decompose EEG into localized, modular signals for correlation and coherence.
ELECTROPHYSIOLOGICAL AND NEUROIMAGING TECHNIQUES IN NEUROPSYCHOLOGY
Technological advances have come about in all areas of medicine, and the techniques utilized for diagnosis of neuropsychological problems are no exception. These advances have moved neuropsychology from a practice emphasizing assessment to determine focal and diffuse lesions to one of developing interventions to compensate for brain damage or neuro-developmental differences. Historically, neuro-psychology has concentrated on the ability to diagnose cerebral lesions on the basis of behavioral data. This emphasis was necessary because technology was unable to provide the evidence for such diagnoses. With the advent of magnetic resonance imaging (MRI), lesions, brain tumors, and brain conditions that previously could be seen only with surgery or at autopsycan now be observed in the living patient. Because the neuropsychologist will consult on cases that utilize neuroradiological and electrophysi-ological techniques, it is important to understand what these basic techniques involve and what they can tell the clinician. This chapter will provide information about common neuroradiological and electrophysi-ological techniques. ELECTROPHYSIOLOGICAL TECHNIQUES Procedures utilizing an electrophysiological technique provide an assessment of electrical activity associated with incoming sensory information. Electrodes are attached to the scalp and electrical brain activity is recorded through the use of a computer and an amplifier for the signals. Electrical brain activity is very weak and requires a differential amplifier to record these signals through a recorder attached to a personal computer. Each electrode is placed on the scalp according to various conventions, the most common of which is the 10—20 universal system (Jasper, 1958). Figure 3.1 provides an overview for electrode placement. Each electrode provides a signal from a particular region, and each signal is referred to a common reference electrode. The function of the reference electrode is to provide a reference to be used to subtract the signals from the individual electrodes. Because each electrode provides some natural interference to the signal, the reference electrode serves as a baseline for this interference, and the interference is thus subtracted from each electrode. In this way, each electrode provides information about the unique degree of electrical activity from that selected region of the scalp. Because muscle contractions, muscle movement , and eye movement can interfere with the signal , the patient is observed carefully and brain waves that show such movement are removed. These techniques include electroencephalography and event and evoked potentials. Each of these techniques will be discussed in this chapter as well as research using such techniques for the study of neurodevelopmental disorders such as learning disabilities and attention deficit disorders. Electroencephalography Electroencephalographs (EEGs) are recorded in patients who are considered at risk for seizure disorders and abnormal brain activity resulting from brain tumors. They also have been found helpful for use with 51