5 : 4/ Electrical Potentials of the Brain 99 



occipital region where vision is projected on the surface of the cortex. 

 If the eye is suddenly focused on a bright image, the a-rhythm is abolished 

 leaving only higher frequency, low voltage waves in the eeg pattern. 

 With continued concentration, the a-rhythm returns. The a-rhythm 

 is also altered by blinking. With the eyes closed, it is slower than with 

 the eyes open. 



Similarly, the eeg pattern is altered by anesthesia and by sleep. With 

 anesthesia, the a-rhythm tends to build up and then later to disappear. 

 As one falls asleep the eeg pattern changes dramatically. During the 

 drifting-off stage, the a-rhythm tends to disappear. (In individuals 

 lacking an a-rhythm when awake, one appears during the drifting-off 

 stage.) As the a-rhythm disappears, 4-6 cps waves appear. In the 

 next stage, 14-16 cps spikes with the spindle shape of /3-waves appear; 

 this is the "dream" stage. With full sleep, very slow \- 3 cps waves 

 predominate. A sudden stimulus produces an 8-14 cps (a-wave) burst 

 superimposed on the slow waves. 



Eeg patterns not only are dependent on the state of awareness and 

 optical activity, but they also vary considerably during development. 

 On the scalp of a year-old baby, there appear the first orderly eeg 

 rhythms. These are occasional bursts of 4-8 cps, especially in the 

 occipital area. By four years of age, 7-8 cps appear. At nine years of 

 age, the frontal and parietal waves are slower than in adults, and 9-10 

 cps rhythms are more common in the occipital region. Even at 14 

 years of age, when people in many parts of the world are supporting 

 themselves and reproducing, childish forms are found in the eeg. By 

 19, however, all the records are adult in form. 



There can be no question that the eeg provides real clues as to the 

 mental state and activity. It varies with age, with sleep, and anesthesia, 

 and with shutting the eyelids. Eeg changes associated with certain 

 abnormal states are well known and used clinically. They are discussed 

 in Section 5. Likewise, eeg records are used routinely in behavioral 

 experiments to indicate alarm reactions, conditioning, and so forth. 

 None of these answer the fundamental question of the function and 

 origin of the eeg potentials. 



In spite of many experiments, the role of the eeg potentials is still 

 obscure. The simplest hypothesis would seem to be that they represent 

 some type of scanning by the brain of impulses coming in on the sensory 

 neurons and of information within the brain. This hypothesis is 

 simplest to one who has worked with large digital computers. The 

 experimental data either supporting or refuting this hypothesis are very 

 weak. 



Perhaps the place to start studying the role of the eeg potentials is 

 in discovering their origin. Here, the experimental data are conflicting. 



