EEG QUESTIONS AND ANSWERS 2023

EEG QUESTIONS AND ANSWERS 2023 What is an EEG? Measurement of voltage fluctuations at the surface of the scalp generated by the electrical activity of the brain. These voltage fluctuations are called _______ and are recordings of ___________. Brain waves Voltage difference versus time How many electrodes are plugged into each amplifier on the EEG machine? Two What does the EEG recorder detect? The voltage differences between the recording and reference electrode for each channel of detection. The amplifier compares the two electrode signals coming into it and cancels out signals that are the same. Therefore the EEG signal is the difference in electrical activity detected by two electrodes. One electrode pair is referred to as a _____. Channel Modern EEg recordings have from 8 to 40 channels to produce a ____________ as shown here. Multichannel recording. The closer the electrodes are to each other, the ____ differences in their brain waves. Less What happens if the electrodes are too close? The EEG channel will display a straight line. Therefore, when recording EEGs, a technician will measure the patient's head to ensure symmetrical and equidistant placement of electrodes. The voltage emitted from the scalp is on the order of ____ Microvolts (10^-6) The EEG amplifiers increase this signal by a factor of.... Several hundred thousands (10^5) for detection and recording. What is the digitized data analyzed by? Fourier transforms (FT) and filtering. What is FT? A mathematical way to convert the wave data from being a function where voltage varies with time to a description of how much of the EEG signal (called power) on any given channel occurs at a given frequency. What are the three aspects of EEGs? 1. Brain waves occurring at different frequencies 2. A number of specific types of wave patterns 3. Artifacts What are artifacts? Disturbances caused by technical defects, usually transitory, and include: electrode movement, with loss of contact; muscle activity obscuring the EEg, movements of the head, scratching the scalp; and sweating. Figure 2 illustrates eye movement artifacts in the multichannel EEG recording. What are brain waves? Listed from highest to lowest frequency (alpha, beta, theta, delta). The various brain waves are associated with global aspects of awareness and subjectivity as listed below. Frequency of gamma waves 40 Hz Description of gamma waves. Widely spread out brain areas that are active at the same time display gamma oscillations. Gamma frequencies are thought to reflect what is called BINDING. This is the property of the brain that unifies our awareness into a single gestalt. Not visible in the EEG tracing, must be extracted by fourier analysis. Frequency of beta waves 13-20 Hz Description of beta waves. Usually seen on both sides in symmetrical distribution and is most evident frontally. Low amplitude, high frequency of waves. May be absent or reduced in areas of cortical damage. It is the dominant rhythm in patients who are alert or anxious or who have their eyes open. Frequency of alpha waves? 8-13 Hz Description of alpha waves. Alpha is usually best seen in the posterior regions of the head on each side, being higher in amplitude on the dominant side. It is brought out by closing the eyes and by relaxation, and abolished by eye opening or altering by any mechanism (thinking, calculating). It is the major rhythm seen in normal relaxed adults-it is present during most of life especially beyond the thirteenth year when it dominates the resting tracing. Frequency of theta waves. 4-8 Hz Description of theta waves. Abnormal in awake adults but is normal in children up to 13 years and in sleep in children and adults. Seen normally in hippocampus (which cannot be detected using surface EEG recording). In the hippocampus, theta is thought to represent activation of the hippocampus. Seen as a focal disturbance in focal subcortical lesions. Seen in generalized distribution in diffuse disorder or metabolic encephalopathy. Frequency of delta wave. 4 Hz Description of delta wave. Distribution is generally broad or diffused, may be bilateral, widespread. high amplitude waves Not present in the normal adult EEG, if it is, then its a sign of disease Associated with NREM stages 3&4. Physiological correlates: not moving, low-levels of arousal. Present in newborn infants. How are electrodes configured onto the head? Geometrical array of even-spaced points How does software code the amount of activity? As colors, somewhat like a weather map (i.e. black and blue depict low EEG amplitude, yellow and red depict larger amplitudes). Points between electrodes are filled in by interpolation resulting in a smooth gradation of colors to visualize whole brain activity. What is the main indication of EEG brain topography? To determine the presence of tumors and focal disease of the brain such as epilepsy, AVM, and stroke. Also appropriate when disturbances in consciousness and vigilance are present, such are narcolepsy (the abrupt onset of sleep) or coma. Increasingly used to monitor the effects of withdrawal of psychoactive drugs, infectious disease of the brain (i.e. meningitis), as well as to follow up patients who were subjected to brain operations. Are brain waves causal? NO! They do not cause brain events, but instead passively reflect brain events. They are epiphenomena of the brain's activity at the cellular and circuit levels. Populations of cells generate brain waves when they _____. Depolarize in synchrony. The EEG recordings detect activity mainly in the _________ of the cerebral cortex. upper layers The potential difference causing EEG signals probably represents the ____________ generated by extracellular currents associated with radially arranged ______ of pyramidal ells in layers II and II, and layers V and VI of the cerebral cortex. Summed voltages Dendrites EPSPs and IPSPS, NOT _______, contribute most to EEG signals. Action potentials What is the massive synchrony of millions of neurons that underlies the EEG signal due to? 1. Feedback circuits between the thalamus and cerebral cortex. Thalamocortical neurons project to layer 4 of the cerebral cortex, and layer 6 corticothalamic neurons project back to the thalamus. 2. Within these circuits, voltage and modulated dependent intrinsic conductances give rise to rhythmic activities over wide earths (i.e. mm to cm scales) of cortical surfaces. What are evoked potentials? Also called event related potentials (ERPs), are EEG signals that result when the brain is exposed to a specific activity. The activity can be sensory, cognitive, or motor. What are sensory evoked potentials? The subject is exposed to a stimuli on a repetitive basis (i.e. every 1 sec for 500 exposures), and the EEG recorded. Resulting EEGs are all averaged. This cancels out the EEG activity not related to the stimulus and reveals the ERP form.