ELECTROENCEPHALOGRAPHY — WALTER 



245 



of between 8 and 13 cycles per second. Even with the eyes shut, some 

 alpha rhythms are usually diminished when the subject thinks hard, 

 and some are blocked when a vivid visual image is called up. There 

 is wide variation between individuals, but the general picture in a 

 given subject is very constant and is almost like a signature (figs. 1 and 

 2). It is important to note the paradoxical inverse relation between 

 the prominence of the alpha rhythms and the level of functional 

 activity and attention. In babies, there is little alpha activity : there 

 are only much slower swings of electric potential in all areas (fig. 3). 

 In older children, rhythms at about 6 cycles per second are prominent 

 up to the age of 7 or 8 (fig. 4), but the alpha rhythms are beginning to 

 appear, and the adult pattern is usually established by the age of 12. 

 The 6-cycles-per-second activity is sometimes called theta rhythm; it 



NORMAL — 



REST 



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EYES open:' 



Figure 1. — Electroencephalogram from a normal i)€rson showing the effect of 

 closing the eyes. Note the increase in alpha activity at 10 cycles per second 

 in both occipital regions and the associated peak at 10 in the frequency 

 analysis ( shown in bottom line of graph ) . 



is often larger when the child is unhappy or angry. It comes from the 

 temporal lobes of the brain and deeper regions, and is found in many 

 adults whose behavior is childish in the sense that they are short- 

 tempered, aggressive, and hard to get on with. 



In sleep, the alpha rhythms disappear first, and the E. E. G. becomes 

 less individual. In deep sleep it resembles a baby's record, but in 

 lighter sleep there is usually intermittent activity at 14 cycles per 

 second, and this is sometimes prominent just before a subject wakes up 

 from a dream. Hypnosis produces little change unless the subject is 

 of the type who goes into a very deep trance, when the alpha rhythms 

 are said by some to respond to the suggested, rather than the real, 

 conditions. 



When the brain is injured, or invaded by a tumor, the affected area 

 tends to develop slow discharges called delta activity (fig. 5). Al- 



