1. EEG An Introduction Aamir Saeed Malik Neuro-Signal Processing Group Universiti Teknologi Petronas Malaysia

2. Hans Berger  The first recording of the electric field of the human brain was made by the German psychiatrist Hans Berger in 1924 in Jena  He gave this recording the name electroencephalogram (EEG  From 1929 to 1938, he published 20 scientific papers on the EEG

3. EEG Activity  Spontaneous activity: is measured on the scalp or on the brain and is called the electroencephalogram  The amplitude of the EEG is about 100 µV when measured on the scalp  Its about 1-2 mV when measured on the surface of the brain  The bandwidth of this signal is from under 1 Hz to about 50 Hz (see figure)  As the phrase "spontaneous activity" implies, this activity goes on continuously in the living individual

5. EEG Activity  Evoked potentials: are those components of the EEG that arise in response to a stimulus (which may be electric, auditory, visual, etc)  Such signals are usually below the noise level and thus not readily distinguished  One must use a train of stimuli and signal averaging to improve the signal-to-noise ratio

6. EEG Activity  Single-neuron: behavior can be examined through the use of microelectrodes which impale the cells of interest  Through studies of the single cell, one hopes to build models of cell networks that will reflect actual tissue properties

7. Nerve Cells  The number of nerve cells in the brain has been estimated to be on the order of 10 11  Cortical neurons are strongly interconnected  Here the surface of a single neuron may be covered with 1,000-100,000 synapses (Nunez, 1981)  The resting voltage is around -70 mV, and the peak of the action potential is positive  The amplitude of the nerve impulse is about 100 mV; it lasts about 1 ms  The complexity of brain structure and its electrophysiological behavior have thus far precluded the evaluation of voltage source  In ECG or EMG, it is possible to evaluate the source function  The quantitative EEG is based on a statistical treatment, whereas the clinical EEG is largely empirical

8. EEG Topography  It refers to building visual maps of the brain using EEG  An important application of multichannel EEG is to try to find the location of a epileptic focus or of a tumor, even when they are not visible in a x-ray or CT scan of the head  Epileptic focus is a small spot in the brain where the abnormal activity originates and then spreads to other parts of the brain

9. EEG Topography  Each horizontal tracing corresponds to an electrode pair placed on a particular area of the patient's scalp  By noting the set of channels where abnormal waves occur (such as those marked in red), the neurologist is able to infer the parts of the brain where the abnormality is located  This is very difficult to scan and to interpret, and subject to many errors

10. EEG Topography  With the possibility of recording simultaneously a great number of digitized channels of EEG, a new technique was born: EEG brain topography, at the end of the 80s  Special techniques are used to plots the activity on a color screen or printer, by coding the amount of activity in several tones of color  For example, black and blue might depict low EEG amplitude, while yellow and red might depict larger amplitudes  The spatial points lying between electrodes are calculated by mathematical techniques of interpolation (calculating intermediary values on the basis on the value of its neighbors)  Thus a smooth gradation of colors is achieved

11. EEG Topography  This approach gives a representative view of the location of alterations of rhythm, amplitude, etc.  Pinpointing the exact location of EEG alterations was also made much more easier  In addition, the use of the cinè mode (animations using several sequential pictures taken from the brain maps) made possible the dynamic study of brain function in action cinè mode

12. EEG Topography  Its used to determine the presence of tumors and focal disease of the brain (including epilepsy, arteriovenous mal-formations and stroke)  It is also appropriate when disturbances in consciousness and vigilance are present, such as narcolepsy (the abrupt onset of sleep), coma, etc  In addition, EEG brain topography is being increasingly used to monitor the effects of withdrawal of psychoactive drugs, and in infectious diseases of the brain, such as meningites,  It is used also to follow up patients who where subjected to brain operations  In psychiatry, EEG brain topography has been of value in identifying disorders of biological origin, such as schizophrenia, dementias, hyperactivity and depression, brain atrophy and attention deficit disorders in children

13. EEG Topography  Spectral analysis is done to show the frequency components of a wave  It can tell us how much of each of the pure waves (alpha, beta, theta, delta, etc.) are present and mixed in a single channel recording  What you see in figure is a tridimensional diagram showing the time axis from left to right, the frequency component orthogonal to it, and the intensity in microvolts on the vertical axis. A color scale is used to differentiate amplitudes.

14. EEG Topography  Another recent development is the use of powerful graphical processing software to render three-dimensional reconstructions of the head and of the brain  The electrical activity parameters recorded in the EEG brain topography are depicted as 3D color maps  Dynamic video animations are produced, showing the alterations of electrical activity as a function of time