EEG

By sreeremya.sAsst professor,sree narayana guru

college,cbe

• Electroencephalograms (EEGs) are recordings of the minute (generally less that 300

• μ• V) electrical• potentials produced by the brain. Since 1924, when Hans

Berger reported the recording of rhythmic• electrical activity from the human scalp, analysis of EEG

activity has been conducted primarily in clinical• settings to detect gross pathologies and epilepsies and in

research facilities to quantify the central effects• of new pharmacologic agents.

• In 1875, Richard Caton published the first account documenting the recording of spontaneous brain

• electrical activity from the cerebral cortex of an experimental animal. The amplitude of these electrical

• oscillations was so low (i.e., in the microvolt range) that Caton’s discovery is all the more amazing because

• it was made 50 years before suitable electronic amplifiers became available. In 1924, Hans Berger, of the

• University of Jena in Austria, carried out the first human EEG recordings using metal strips pasted to

• the scalps of his subjects as electrodes and a sensitive galvanometer as the recording instrument. Berger

• was able to measure the irregular, relatively small electrical potentials (i.e., 50 to 100• μ• V) coming from• the brain.

• EEG Recording Techniques• Scalp recordings of spontaneous neuronal activity in the brain, identified as the EEG, allow

measurement• of potential changes over time between a signal electrode and a reference electrode

[Kondraski, 1986].• Compared with other biopotentials, such as the electrocardiogram, the EEG is extremely

difficult for an• untrained observer to interpret, partially as a result of the spatial mapping of functions

onto different regions• of the brain and electrode placement. Recognizing that some standardization was

necessary, the International• Federation in Electroencephalography and Clinical Neurophysiology adopted the 10–20

electrode• placement system. In addition to the standard 10–20 scalp array, electrodes to monitor eye

movement, ECG,• and muscle activity are essential for discrimination of different vigilance or behavioral states

• For long-term recordings, as in seizure monitoring, electrodes present major problems. Needle electrodes,

• which must be inserted into the tissue between the surface of the scalp and the skull, are sometimes

• useful. However, the danger of infection increases significantly. Electrodes with self-contained miniature

• amplifiers are somewhat more tolerant because they provide a low-impedance source to interconnecting

• leads, but they are expensive. Despite numerous attempts to simplify the electrode application process

• and to guarantee long-term stability, no single method has been widely accepted.

• Use of Amplitude Histographs to Quantify the EEG• In general, the EEG contains information regarding changes in the electrical potential

of the brain• obtained from a given set of recording electrodes. These data include the

characteristic waveforms with• accompanying variations in amplitude, frequency, phase, etc., as well as the brief

occurrence of electrical• patterns, such as spindles.• Any analysis procedure cannot simultaneously provide information regarding• all these variables.• Consequently, the selection of any analytical technique will emphasize changes in

one• particular variable at the expense of the others. This observation is extremely

important if one is to• properly interpret the results obtained using a given technique.

• Standard Amplitude• The variance of the EEG amplitude distribution is directly related to the total power of the EEG. For• example, a flat EEG will provide low variance values, while a widely oscillating EEG will yield high• variance values. To avoid confusion and use units that are more familiar to electroencephalographers,• the term• standard amplitude• is often used.• Skewness• The degree of deviation from the symmetry of a normal or gaussian distribution is measured by• skewness.• This third central moment of the amplitude histogram has a value of zero when the distribution is• completely symmetrical and assumes some nonzero value when the EEG waveforms are asymmetrical• with respect to the baseline (as is the case in some characteristic sleep patterns, murhythms, morphine• spindles, barbiturate spiking, etc.). In general, a nonzero value of the skewness index reflects the

presence• of monophasic events in the waveform. The following methods can be used to obtain the measure of• skewness.