98 



Electrical Potentials of the Brain /5 : 4 



Electronic studies, using autocorrelators, have revealed that emphasized 

 frequencies do occur. These frequencies are not maintained for many 

 periods, so that they do not give sharp peaks on a frequency analyzer. 



The characteristic eeg rhythms of humans are classified by size, shape, 

 and frequency. The most outstanding normal pattern is the so-called 

 "a-rhythm." These waves are between 8 and 13 cps. They are more 

 sinusoidal and regular than the other types of waves. On the surface 

 of the scalp, the a-waves are about 20 /xv or less in amplitude. They 

 are usually present in adult humans, especially in the occipital and 

 parietal areas (see Figure 5 for the location of these areas) . However, 



Classical Normal Recording in Normal Control Subject 

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Figure 6. Normal eeg patterns illustrating abolition of the 

 a-rhythm with eyes open. If eyes remained open for longer 

 period of time, the a-rhythm would build up again. Original 

 figure of R. G. Bickford, M.D., Mayo Clinic. 



many normal adults completely lack a-waves both on the scalp and on 

 the cortex. 



Besides the a-rhythm, there are so-called "/?- waves" in the frequency 

 range of 14-50 cps. They are always present in normal adults. The 

 /3-waves are smaller in amplitude than the a-waves and are usually 

 spindle shaped. In some adults, there are large voltage (50-100 /xv), 

 slow ( |-4 cps) S-waves, as well as slightly faster #-waves in the frequency 

 range 5-7 cps. Both the 8- and the 0-waves are often associated with 

 abnormalities. 



Although the eeg patterns are not the same for all normal adults, 

 they do vary with the activity of the central nervous system. The most 

 studied example is the a-rhythm, which occurs predominantly in the 



