5 : 2/ Electrical Potentials of the Brain 89 



eeg potentials can be observed only with electronic amplifiers which 

 discriminate both against other potentials of physiological origin and 

 against electrical noise. 



The characteristic form of the eeg pattern has been used clinically 

 and experimentally. Various types of epilepsy have typical eeg patterns 

 which are useful for diagnosis and occasionally in treatment. Brain 

 tumors likewise may be located from an eeg if the tumor is sufficiently 

 close to the brain's surface. Many brain injuries can be diagnosed from 

 alterations in the patterns of the potentials near the injury. Behavioral 

 experiments use eeg patterns to indicate alarm reactions, sensory res- 

 ponses, and so forth. 



From the viewpoint of this text, the more significant application of 

 these so-called "brain waves" is that they may indicate the operation of 

 the central nervous system. Many theories have been proposed, based 

 on the form of these brain potentials. To date, none of these theories 

 has been altogether successful. The eeg potentials are a building block 

 which may eventually lead to an undejstanding of the function of the 

 brain. 



The potentials associated with brain activity may be as large as 100 

 microvolts on the human scalp; these can be observed electronically. 

 In laboratory animals, it is more difficult, if not impossible, to measure 

 eeg potentials outside the skull. Small electrodes inserted through the 

 skull onto the surface of the brain indicate potentials similar to those 

 found on the human scalp. In other studies, electrodes are inserted 

 into the interior of the brain. Potentials measured within the brain, 

 with electrodes so large (diameter 0.01 mm or greater) that they respond 

 to some type of average of the activity of many cells, are also referred 

 to as eeg potentials. The instrument used to record the potentials is 

 called an electroencephalograph and the record an electroencephalogram. 



2. The Central Nervous System 



The eeg potentials result from the action of the central nervous system. 

 To aid in discussing these brain potentials, an outline of the anatomy 

 of the central nervous system is given in this section. In Section 3 of 

 this chapter, some of the actions of the central nervous system are inter- 

 preted by analogy with electronic feedback networks. 



The central nervous system, as is the case with all other nervous 

 tissue, is made up of neurons. Some carry information into the central 

 nervous system; these are sensory or afferent neurons. Others carry 

 spike potentials out of the central nervous system and are called motor 



