286 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



PROPERTIES OF ALPHA ACTIVITY AS TYPICAL 

 OF INTRINSIC RHYTHMS 



Early Rijmrls 



The early studies of alpha rhythms in human beings 

 were made with straiehtforward recording devices — 

 mirror oscillographs, cathode ray oscillographs and, 

 later, ink writing recorders. These provide simple 

 graphs of voltage changes with respect to time; it is 

 unfortunate that the human eye is severely limited in 

 its capacity to analyze a curve of this sort, being at- 

 tracted to the most prominent features and tending to 

 ignore or misrepresent the minor ones. Berger's early 

 studies, original and detailed though they were, pro- 

 vided only limited information about the distribu- 

 tion and complexity of the alpha rhythms because he 

 employed only one or two separate recording chan- 



nels; his impression that the alpha rhythms were de- 

 veloped by the whole brain was due to this limitation. 

 During the last 20 years however, the technical trend 

 has been toward multiplication of channels and 

 elaijoration of analyzing de\ices. There is no doubt 

 that in consequence the picture of alpha activity has 

 become progressively more and more involved and 

 controversial. Adrian & Matthews (i), and Adrian 

 & Yamagiwa (2) were the first to prove that the 

 alpha rhythms usually arose in the posterior regions of 

 the l)rain, and they were able to demonstrate in some 

 subjects an abrupt change in the sign of the potential 

 gradient of the alpha waves over the scalp, an effect 

 which has since become known as a ' phase-re\ersal 

 focus'. In the simplest case, a potential distribution 

 of this sort could be produced Ijy an equivalent gen- 

 erator within the head oriented radially with its 

 axis projecting toward the surface at the 'focus', and 



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FIG. 5. Primary records, frequency analysis and period display from a subject of the alpha M 

 type. Upper portion contains foiu' primary records and a frequency analysis which is continued in 

 the middle portion; the lowermost record is the period display. Immediately on eye closure there 

 is a brief burst of alpha activity at i o to 1 1 cycles per sec, but even this is sometimes lacking. The 

 spectrum of brain rhythms is almost 'white,' although the absence of regular rhythms makes the 

 faster activity seem more obvious. 



