General explaining about EEG Mehran Ahmadlou 86133001 Why

General explaining about EEG Mehran Ahmadlou 86133001

Why neocortex ? • First, the activity of one neuron is too small to be recorded. To be visible on the scalp, electrical activity must involve thousands of neurons acting synchronously. The neocortex has a high density of neurons, which facilitates their synchronous activation. • Second, it is not enough to act synchronously. Electrical activity is visible only if the activity of the underlying neurons adds up. To add up the activity must be generated by parallel neurons. The neocortex is composed of pyramidal cells aligned in parallel. • Third, the electric and magnetic field decrease with the distance from the source. The neocortex is closer to recording sensors.

the 10 -20 international system • It is based on anatomical location and on percentage of distance among these points giving the 10 or 20% in the system name. The original 10 -20 system has only nineteen electrodes but has been extended to accommodate more than 200 electrodes. 128 channels • In this system the electrodes' locations are related to specific brain areas. For example, electrodes C 3 and C 4 are above the motor cortex

F (frontal), C (central), T (temporal), P (posterior), and O (occipital).

• Each EEG signal can therefore be correlated to an underlying brain area. • A better correlation can be obtained by registering the electrode's locations with MRI of the subject's head. • Electrical Recording with Differential Amplifiers : Reference and “ Active ” Electrodes Bipolar and “ Monopolar ” Montages of the Average Reference Bias

Characterization of EEG Activity General EEG frequencies : eta: 13 -30 Hz (desynchronized) alpha: 8 -12 Hz (synchronized) theta: 3. 5 -7. 5 Hz (synchronized) delta: < 3. 5 Hz (synchronized) Lower frequencies are usually higher amplitude (“synchronized”) activity

• The amplitude of the normal EEG can vary between approximately -100 and +100 µV, and its frequency ranges up to about 40 Hz.

Figure 7. 5 (a) Four types of EEG waves. Figure 7. 5 (b) When the eyes are opened, alpha waves disappear.

Figure 7. 5 (c) Different types of epilepsy yield abnormal waveforms. (From Guyton, A. C. Structure and Function of the Nervous System, 2 nd. Ed. Copyright 1972 by W. B. Saunders. Reprinted by permission of W. B. Saunders. )