Question

What is an appropriate sampling frequency for measuring EEG in any sleep disorder? Justify your answer and How is the measurement setup build?

Answer 1

Solution-  The sampling frequency is usually expressed in Hz, for example, 240 Hz is 240 times per second. The minimum acceptable sampling frequency is 2.5 times greater than the highest frequency of interest but most digital EEG systems will sample at 240 Hz. The figure given below shows the Sampling Rate of 240Hz

Some recordings that involve recording activity directly from the brain surface may have high-frequency activity, for example 200 Hz. Therefore some digital EEG systems will have an optional sampling frequency of 480 Hz available.

EEG Measurement Setup- The EEG measurement set up is explained as follows-

(1) EEG Lead System - Silver/silver chloride electrodes are used as surface electrodes in this setup. At the skull, the distance between the two electrodes is given as 10% and 20% of the distance between the specified points.

Nasion and Inion are two reference points near human ear lobes(as shown in fig 2). Above the head, the distance between nasion and inion is divided into 5 points. Nasion - Inion distance is measured through the apex and three electrodes are placed as 1 in the frontal, 1 in the middle, and 1 in the parietal.

They are placed at distances of 10%, 20%, 20%, 20%, 20% and 10% of this length(fig 2). Now, similarly the nasion - inion distance is measured along the temporal lobes and five electrodes are placed as two in the frontal, 2 in the temporal and 1 in the occipital lobes, respectively. They are placed on both sides with lengths of 10%, 20%, 20%, 20%, 20% and 10%.

Finally, 2 in the frontal, 2 in the central, and 2 in the parietal are determined at the periphery of the remaining six electrodes. So, the setup consists entirely of 19 electrodes placed on the brain skull, and an electrode that serves as a reference is placed in the ear lobe. This is popularly known as the 10 - 20 EEG system. Shown in the diagram below-

     

(2) EEG Recording Setup - A 21-electrode system is used here. The electrodes are connected to an 8-channel selector. The output from the 8-channel connector goes to the differential amplifier bank. The differential amplifier is composed of preamplifiers used to reduce noise. 50 Hz noise is created due to A.C interference which can be reduced by using differential amplifiers. The output received from the differential amplifier is connected with the signal processing unit. After further processing, the display unit displays the data. This system helps to record the potentials generated from the sensory parts of the brain. As shown in the figure below-