Electroenephalography

Student Handout Electroencephalography (EEG) Introduction In this laboratory, you will explore the electrical activity of the brain. You will record and analyze electroencephalograms (EEGs) from a volunteer; look at interfering signals, and examine the effect on alpha and beta waves by opening and shutting the eyes, auditory and mental cues. Background The cerebral cortex contains huge numbers of neurons. Activity of these neurons is to some extent synchronized in regular firing rhythms ('brain waves'). Electrodes placed in pairs on the scalp can pick up variations in electrical potential that derive from this underlying cortical activity. EEG signals are affected by the state of arousal of the cerebral cortex, and show characteristic changes in different stages of sleep. EEG signals are also affected by stimulation from the external environment, and brainwaves can become entrained to external stimuli. Electroencephalography is used, among other things, in the diagnosis of epilepsies and the diagnosis of brain death. Recording the EEG EEG recording is technically difficult, mainly because of the small size of the voltage signals (typically 50 µV peak-to-peak). The signals are small because the recording electrodes are separated from the brain's surface by the scalp, the skull and a layer of cerebrospinal fluid. A specially designed amplifier, such as the Bio Amplifier built into the PowerLab, is essential. It is also important to use electrodes made of the right material, and to connect them properly. Even with these precautions, recordings may be spoiled by a range of unwanted interfering influences, known as 'artifacts'. In this laboratory you will record EEG activity with two electrodes: a frontal electrode on the forehead, and an occipital electrode on the scalp at the back of the head (Figure 1). A third (ground or earth) electrode is also attached, to reduce electrical interference. In clinical EEG, it is usual to record many channels of activity from multiple recording electrodes placed in an array over the head.

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Student Handout Electroencephalography (EEG)

Figure 1. Equipment setup (with PowerLab 15T). Origins of the EEG signals The EEG results from slow changes in the membrane potentials of cortical neurons, especially the excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs). Very little contribution normally comes from action potentials propagated along nerve axons. As with the ECG, the EEG reflects the algebraic sum of the electrical potential changes occurring from large populations of cells. Therefore, large amplitude waves require the synchronous activity of a large number of neurons. The rhythmic events that these waves reflect often arise in the thalamus whose activity is in turn affected by a variety of inputs including structures in the brainstem reticular formation. Components of the EEG waveform The EEG waveform contains component waves of different frequencies. These can be extracted and provide information about different brain activities. The LabTutor software is set up so that the raw EEG signal is displayed in channel 1. Digital filtering allows this to be analyzed into the component frequencies of interest that are displayed in other channels. Each these waves (or rhythms) provides information about different brain states. These waves are:

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Student Handout Electroencephalography (EEG) 1. Alpha (8 to 13 Hz; average amplitudes 30 to 50 µV) Alpha rhythm is seen when the eyes are closed and the subject relaxed. It is abolished by eye opening and by mental effort such as doing calculations or concentrating on an idea. It is thus thought to indicate the degree of cortical activation, the greater the activation, the lower the alpha activity. Alpha waves are strongest over the occipital (back of the head) cortex and also over frontal cortex. 2. Beta (13 to 30 Hz; <20 µV) In awake, alert individuals with their eyes open, the dominant rhythm is beta. It may be absent or reduced in areas of cortical damage and can be accentuated by sedative-hypnotic drugs such as benzodiazepines and barbiturates. 3. Theta (4 and 8 Hz; <30 µV) Theta rhythm is said not to be seen in awake adults but is perfectly normal in awake children up to adolescence. It is normal during sleep at all ages. (Note however, that some researchers separate this frequency band into two components, low theta (4 - 5.45 Hz) activity that they correlate with decreased arousal and increased drowsiness, and high theta (6 - 7.45 Hz) activity that it is claimed is enhanced during tasks involving working memory.) 4. Delta (between 0.5 and 4 Hz; up to 100 - 200 µV) Delta rhythm is the dominant rhythm in sleep stages 3 and 4 but is not seen in the conscious adult. It tends to have the highest amplitude of any of the component EEG waves. Note that EEG artifacts caused by movements of jaw and neck muscles can produce waves in the same frequency band. 4. Gamma (between 30 and 50 Hz) Some people also recognize gamma waves but their existence and importance is more controversial. It may be associated with higher mental activity, including perception and consciousness and it disappears under general anesthesia. One suggestion is that the gamma rhythm reflects the mental activity involved in integrating various aspects of an object (color, shape, movement, etc) to form a coherent picture. Interestingly, recent research has shown that gamma waves are enhanced in Buddhist monks during meditation and are absent in schizophrenics.

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Student Handout Electroencephalography (EEG) It is not presently possible to relate the EEG waves to specific underlying neuronal activities. In general, the more active the brain the higher the frequency and the lower the amplitude of the EEG. Conversely, the more inactive the brain the lower the frequency and the higher the amplitude of the signal. The EEG during sleep It is established that the EEG pattern provides an indicator of the sleep state. Sleep consists of two very different alternating stages, non-REM and REM (rapid eye movements) sleep. Non-REM sleep is often described in four stages that are characterized by a progressive increase in sensory thresholds, an increase in EEG wave amplitude, and a decrease in EEG wave frequency. Stage 1 is marked by drowsiness and drifting in and out of consciousness, This is followed by stages 2 and 3 and then 4. Sleepers then move back through the stages except that rather than stage 1, REM sleep occurs. The whole cycle lasts approximately 90 minutes so that, over the course of an 8 hour 'sleep', the cycle is repeated 4 to 6 times. In the later cycles, the REM component is longer and stages 3 and 4 become shorter.

Figure 2. Sleep cycles. These stages can be correlated with EEG activity. Stage 1 is associated with decreasing beta activity, alpha activity that becomes less obvious and the emergence of theta activity. Stage 2 has irregular theta activity, short bursts of waves of 12 - 14 Hz called sleep spindles, and sudden increases in wave amplitude (K complexes).

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Student Handout Electroencephalography (EEG)

Figure 3. Sleep spindles. Stages 1 and 2 are relatively "light" stages of sleep. In stages 3 and 4, delta activity predominates with the distinction between the two being that in Stage 3 sleep there is delta activity for less that 50% of the time. In stages 3 and 4 we are in deep sleep. In REM sleep, which can last from 20 to 60 minutes or more, the EEG is similar to that in Stage 1. REM sleep is the stage most associated with dreaming. Although the EEG shows significant activity during REM sleep, motor activity is inhibited. Levels of brain serotonin and nor-epinephrine alter during these sleep stages. In non-REM sleep stages 1 to 4, serotonin levels are increased whereas during REM sleep, nor-epinephrine, corticosteroids and, in males, testosterone is secreted. Non-REM sleep is characterized by decreases in blood pressure, and heart and respiratory rates. In REM sleep, there is marked variation in heart rate and blood pressure and irregular breathing. In sleep studies, EOGs and EMGs are often recorded in addition to the EEG. Non-REM sleep is characterized by rolling, uncoordinated and slow eye movements and passively decreased muscle tone, whereas REM sleep has rapid, coordinated eye movements (hence the name) and a little EMG activity reflecting the active inhibition of muscle in this state.

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Student Handout Electroencephalography (EEG)

Figure 4. Sleep stages. The EEG and changes in intracranial metabolism Changes in the EEG can be detected in response to changes in the chemical environment of the neurons. One easy way to demonstrate this in a student laboratory is to observe the effects of hyperventilation. Hyperventilation lowers blood PCO2. Since CO2 , being lipid soluble, readily crosses the blood-brain barrier and cell membranes, this in turn results in decreased P CO2 (hypocapnia) in the brain interstitial fluid and within the neurons and glial cells. Thus extracellular and cellular pH is elevated - acute respiratory alkalosis. In addition, blood vessels in the brain constrict with reduction in brain blood flow. The consequences are a change in neuronal activity with slower rhythms and higher amplitudes (increased delta and theta activities) as well as some decrease in alpha activity. There is still debate about whether these EEG changes are a consequence of the metabolic changes or of hemodynamic factors. One possibility is that they arise from depressant effects of the hypocapnia on the brainstem reticular formation and are analogous to the EEG changes seen in the transition from wakefulness to sleep. The EEG and the functions of the cerebral hemispheres Efforts have also been made to use EEG recordings to dissect out the contributions of the two hemispheres to brain function. It has been argued that the left hemisphere is the 'logical' half of the brain concerned with reasoning, problem solving and language while the right hemisphere is the more intuitive, creative side concerned with images and spatial processing rather than with language. Careful reading of the literature reveals this to be a major

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Student Handout Electroencephalography (EEG) oversimplification of cortical organization. In reality, there is little published EEG evidence to lend credence to this hypothesis. The EEG and personality Attempts have also been made to relate personality to EEG patterns, perhaps the most famous example being Eysenck's Cortical Arousal Model of Introversion and Extraversion. Eysenck argued that there is some 'optimal' level of electrical activity in the cortex. If we fall below this we tend to be bored and fall asleep; above this we are unable to deal with the activity and feel overwhelmed. In this construct, extraverts need additional mental stimulation (people around them, loud music, etc) to reach this optimal cortical activity whereas introverts avoid such additional stimulation as their cortical activity is already in the optimal region. There has been considerable debate about the extent to which EEG findings support this hypothesis. Further Reading 

Kraemer et al., Nature, Vol. 434, Page 158 (2005).

What you will do in the laboratory There are five exercises that you will complete during this Lab. 1.

EEG artifacts. In this exercise, you will learn to recognize common artifacts seen while recording an EEG.

2.

Alpha & Beta Rhythm. Here you will learn how best to elicit alpha waves in an EEG recording.

3.

Effects of mental activity. In this part of the laboratory, you will do some simple arithmetic and observe the effects on the EEG activity.

4.

Effects of auditory stimulation. Here you will examine the effects on the EEG of the volume and the type of music.

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