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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



hundred laboratories. There is the further advantage 

 that such correlation with function as may be estab- 

 lished can be followed more closely in human sub- 

 jects whose mentality and behavior are more familiar 

 and comprehensible than are those of other animals. 

 The rhythmic wave-like potential changes desig- 

 nated as ' alpha rhythms' bv Berger are the most 

 prominent and peculiar feature of human brain ac- 

 tivity, and these rhythms will be taken as representa- 

 tive of rhythmic activity in normal conditions. An 

 example appears in figure 2. Since Berger's original 

 discoveries, brain rhythms have been subclassified, 

 not only on the basis of their frequency and ampli- 

 tude but also on their provenance and functional 

 correlation. Employing these criteria, three other main 

 classes of rhythm have been identified in human sub- 

 jects:' theta rhythms' with a frequency of 4 to 7 cycles 

 per .sec, occupying typically the parietal and tem- 



poral regions of the brain and associated with child- 

 hood, and emotional stress in some adults (fig. 3); 

 'delta rhythms' with frequencies from less than i up 

 to sM cycles per sec, associated with deep sleep in 

 normal adults, with infancy and with organic brain 

 disease (fig. 4); and 'beta rhythms' with frequencies 

 higher than 14 cycles per sec, generally associated 

 with activation and tension. In considering the nature 

 and correlations of the alpha rhythms, it should always 

 be recalled that these other rhythmic phenomena 

 exist and that their mechanisms may be as different 

 from those of alpha rhythms as are their functional 

 associations. Moreover, in the realm of brain pathol- 

 ogy, relatively enormous rhythmic discharges are as- 

 sociated with certain types of epileptic seizures, and 

 these again may originate in a manner quite different 

 from that of the normal alpha rhythms. 





1001 1" 'i'jiV^ll,'^' 



FIG. 2. An example of the classical effect of eye closure on alpha rhythms in a normal subject. 

 Upper five traces were recorded from electrode sites shown in the diagram in the upper left corner. 

 These five primary records show the typical burst of alpha activity as the eyes are closed, followed by 

 marked amplitude modulation, the alpha rhythms being most prominent in the posterior occipital 

 derivations. The sixth trace representing the frequency analysis, indicates the presence of two com- 

 ponents at 9 and 10 cycles per sec. The seventh trace, that showing the period analysis, indicates 

 the presence of wave intervals varying from 90 to 1 1 o msec. The three methods of display are es- 

 sentially complementary since each system emphasizes certain characters at the expense of others; 

 all the information is present in the primary records but is not easily extracted from them visually. 



