EEG and MEG  




What is Electro EnchephaloGraphy?

       In the late 1920s a German psychiatrist named Hans Berger was measuring the brain waves of his daughter when she was doing mental arithmetic.  He found that the activity increased when she was trying to multiply difficult numbers. From this evidence, he deduced that the frequency of the wave pattern from the recording reflected the amount of wave activity in his daughter's brain.  

      Since then, EEG has been perfected as a cheap and accurate way of measuring brainwave activity in the outer layer of the brain.  Sensitive electrodes are attached to the head, and the signals are amplified to give a graph of electrical potential versus time.  It can be measured and compared at different spots on the head simultaneously, to give a 2-dimentional activity map of the cerebral cortex.  EEG is often used to diagnose sezure disorders, tumors, head injuries, degenerative diseases and brain death.  It is also used in research on brain activity.   

      Sometimes, the brain waves are recorded for a special stimulus, and the experiment is repeated several times.  The graphs are then averaged, and the resulting data are called event-related potentials.  These were important in the 60s and 70s when researchers were trying to find out if certain regions of the cerebral cortex were specialized to perform some tasks.  ERPs have also been useful for finding out how long the brain uses to process different kinds of information, and monitor levels of attention and stress for various experiments. 

      The major advantage of EEG over visual methods is a very low price.  In addition, the equipment is portable; all you need is some electrodes, a reliable signal amplifier and a computer which records or shows the data.  The signals can even be transmitted via a radio link before they are recorded.  This way, it is possible to record the brain activity of people while they are performing a task or activity.  It is also the only way of measuring brain activity in real time, since the time aspect of the graph is measured in milliseconds. 

      The major drawback of EEG is that it cannot measure activity or structure beyond the cerebral cortex.  Hence, EEG is often used in combination with MRI.  This will allow brain activity to be determined in relation to subcortical (beneath the cortex) structures. 

      If you are going to have your brain activity monitored with EEG, there are very few preparations to make.  The method is completely noninvasive, and there are no dieting rules.  Depending on the extent of the experiment, you may have to shave little spots of your head for the electrodes to work.  Also be sure to be well rested and relaxed, as stress or lack of sleep will change your brain wave patterns. 

      There is a similar method to EEG called magnetoencephalography, which measures the magnetic fields created by the electrical activity of the brain, rather than the activity itself.  Some scientists claim that these signals are less distorted by the skull and skin tissue, but no experimental data exists to support this.  Hence, the accuracies of EEG and MEG seem to be comparable.