5 : 4/ Electrical Potentials of the Brain 



97 



Because the eeg patterns vary from one area of the brain to another, 

 many studies use eight to 12 electrodes on each half, or as many as 24 

 on the entire cortical area. Figure 5 shows a typical electrode arrange- 

 ment. The form of the eeg from the human scalp is not very different 

 from that on the surface of the cortex ; accordingly, most human studies 



Temporal j 



Left J 

 EarQ 

 21 



Occipital 



Occipital 

 Protuberance 



Figure 5. Electrode arrangement on scalp is indicated by dots. 

 The differences between pairs of these electrodes are recorded 

 to obtain graphic records such as shown in Figures 6 and 7. 



use the potentials on the outside of the scalp. These are smaller by a 

 factor of 100, that is, up to 10 mv on the cortex but 100 /av or less on the 

 scalp. This large difference is caused by the high electrical resistance 

 of the bone. The eeg potentials outside the meninges are the same as 

 those on the cortex itself. 



The eeg patterns are normally observed with oscilloscopes or recorded 

 with pen galvanometers on moving paper. The exact form observed 

 depends on the equipment used. Most eeg equipment responds to 

 signals from 1-100 cps, although there are potentials outside of this 

 frequency range. One of the most striking features of a graphic eeg 

 record is that there appear to be pronounced frequencies present, as 

 shown in Figure 6. 



However, if the eeg voltages are fed into frequency analyzers of con- 

 ventional types, no sharply delineated frequencies are found. Instead, 

 there is a continuous spread of energy from zero to 100 cps per second, 

 albeit with peaks in certain frequency regions. This continuous spec- 

 trum is in sharp contrast to the visually recognized discrete frequencies. 



