100 Electrical Potentials of the Brain /5 : 5 



Some people have found that a small part of the cortex, when isolated 

 from the remainder of the brain, will continue to produce eeg rhythms. 

 Others have claimed, on the basis of their experiments, that large areas 

 or all of the cortex must remain intact to produce normal eeg rhythms. 

 Still others believe, again on the basis of experiments, that the eeg 

 patterns involve closed neuron circuits which include both the cerebral 

 cortex and the thalamus. 



The scanning hypothesis cannot explain how some normal persons can 

 lack the a-rhythm which is so predominant in most others. Further- 

 more, there is no simple explanation of the variation of the spatial 

 distribution from one head to another. Again, the speed or frequency 

 of the a-rhythm does not relate to any known sensory, motor, or thought 

 process of the majority of normal persons. (See, however, the dis- 

 cussion of epilepsy in the next section.) The theories that include the 

 thalamus as part of the feedback loop are difficult to reconcile with the 

 absence of changes of the eeg on the scalp of persons with thalamic 

 tumors. The lack of any definite cellular knowledge regarding the 

 origin of the eeg makes it extremely hard to interpret. 



5. Abnormal Electroencephalographic Patterns 



Clinically, abnormal eeg patterns are used to localize brain tumors and 

 to study epilepsy. Although various investigators have reported a 

 relationship between psychological disturbances and eeg patterns, these 

 seem so uncertain that they will not be discussed further here. Both 

 the tumor and epilepsy patterns have been intensively studied, and the 

 results not only are clinically useful but they serve to emphasize our 

 inability to directly relate brain activity and eeg patterns. 



The most reliable method of detecting brain tumors is the so-called 

 "pneumoencephalograph." In this method, the fluid spaces of the 

 brain and spinal cord are drained, the fluid being replaced with air. 

 X-ray photographs are then taken. The contrast in X-ray opacity 

 between the brain and air is large (although between brain and fluid it 

 is negligible) . A tumor is discovered from a distortion of the ventricles. 

 This method of diagnosis has a definite mortality rate, it is extremely 

 painful, and it fails to reveal small tumors. 



By contrast, the eeg can show a brain tumor two years before the 

 pneumoencephalograph does, is not painful, and has a zero mortality 

 rate. Its use is limited by its complexity and the volumes of records 

 which must be analyzed, and by its failure to show tumors below the 

 surface of the cerebral cortex. Using 24 electrodes, as shown in Figure 5, 

 there are 276 possible pairs. If eight pairs are recorded at one time, for 



