Which of the following instruments is used for recording the electrical activity of the brain?

Electroencephalograph (EEG) is an instrument for recording the electrical activity of the brain, by suitably placing surface electrodes on the scalp. The nerve cells of the brain generate electrical impulses that fluctuate rhythmically in distant patterns. In 1929, German scientist Hans Berger published the results of the first study to employ an electroencephalograph and record these brain-wave patterns. The recording produced by such an instrument is called an electroencephalogram, commonly abbreviated EEG.

To record the electrical activity of the brain, 8 to 16 pairs of electrodes are attached to the scalp. Each pair of electrodes transmits a signal to one of several recording channels of the electroencephalogram. This signal consists of the difference in the voltage between the pair. The rhythmic fluctuation of this potential difference is shown as peaks and troughs on a line graph by the recording channel.

The EEG of a normal adult in a fully conscious but relaxed state is made up of regularly recurring oscillating waves known as alpha waves. When a person is excited or startled, the alpha waves are replaced by low-voltage rapid irregular waves. During sleep, the brain waves become extremely slow. Such is also the case when a person is in a deep coma. Other abnormal conditions are associated with particular EEG patterns. For example, irregular slow waves known as delta waves arise from the vicinity of a localised area of brain damage.