INTRINSIC RHYTHMS OF THE BRAIN 



289 



ones unmasked. In this condition the siilyect is attend- 

 ing to imaginary or endogenous signals and is ignor- 

 ing external ones. 



The intricate and elusive relations between the 

 various brain rhythms and mental functions have 

 been explored also by Mundy-Castle (43) who has 

 identified three types of theta thythm and two of beta 

 rhythm, as well as various categories of activity in the 

 alpha range of frequencies. The statistical relations 

 between types of brain rhythm and psychological 

 character have been analyzed in detail by Werre (66) 

 who concludes that, although no unique associations 

 can be established between any single EEG variable 

 and any specific psychological parameter, none the 

 less certain electrical patterns are contingent on psy- 

 chological grouping. For example, alpha frequency is 

 related to the performance of psychotechnical tests, 

 since subjects with low frequencies perform slowly but 

 steadily, those with high frequencies fast and regu- 

 larly and those with complex alpha rhythms er- 

 ratically. 



Although the alpha-blocking effect is seen most 

 clearly in response to visual stimuli, it can also be pro- 

 duced in some subjects by nonvisual stimuli which are 

 novel and startling. The effect of nonvisual stimuli 

 usually wears off quite rapidly, but if an ineffective 

 stimulus is then accompanied or followed by an 

 effective visual one the neutral stimulus may become 

 'conditioned'. Conditioning of alpha blocking was 

 first studied intensively by Jasper & Shagass (31) 

 and has recently been extensively employed in the 

 experimental analysis of learning by Gastaut ti al. 

 (24). They attribute generalized desynchronization 

 of intrinsic rhythms by a novel stimulus to activation 

 of the brain stem reticular system, and local desyn- 

 chronization by a specific stimulus to activation of the 

 thalamic reticular system. 



Degree of Constancy and Range of I'aiuilnm 

 in Alpha Frequemy 



Although the frequency and distribution of alpha 

 rhythms in any particular subject are characteristic 

 and individual, the rate of an alpha rhythm can some- 

 times be shifted slightly. The range of normal varia- 

 tion is limited to a fraction of a cycle per second, 

 however, and such changes cannot be identified and 

 measured ea.sily in conventional records. The effect is 

 easily demonstrated with a toposcope display system 

 (56, 57, 61, 64) designed to emphasize and correlate 

 rhythmic activity in many regions. An example taken 

 from a normal subject is shown in figure 7. The fre- 



quency of the major alpha rhythms is here 8.80 cycles 

 per sec. over a wide area at rest. During mental ac- 

 tivity the rhythm disappears in all but two derivations 

 in the right centroparietal region where the frequency 

 rises to 9.45 cycles per sec. and then gradually subsides 

 to its original frequency in about 90 sec. (fig. 8). This 

 example illustrates two very important features of 

 alpha activity; first, the extreme constancy in fre- 

 quency in tranquil conditions; second, the degree of 

 independence of the two hemispheres and even of 

 adjacent regions during activity. As can be seen in 

 figure g, one minute after the start of the experiment 

 when the right centroparietal region is showing alpha 

 rhythm at 9.1 cycles per sec, the left temporoparietal 

 derivation has resumed alpha activity but at 9.35 

 cycles per sec. Nevertheless, when the period of at- 

 tention is over, all regions return precisely to their 

 original rate of 8.80 cycles per .sec. 



This degree of constancy is by no means unusual 

 and has been reported afso by Brazier & Casby 

 (12) and Barlow & Brazier (5) using an entirely 

 different method of analysis and correlation. The pat- 

 tern of frequency fluctuation, however, is an individual 

 character and is related to the complexity of the rest- 

 ing alpha activity; if there are .several rhythms, the 

 apparent changes in frequency are usually abrupt 

 and extensive and may be in the direction of accelera- 

 tion or deceleration. Changes of this type are at- 

 tributable to the substitution of one process for 

 another rather than to changes of rate in the same 

 process. In general, a particular alpha rhythm seems 

 to be capable of aijout ±0.5 cycles per sec. variation 

 within the physiological range; a greater change can 

 be induced by the administration of drugs but such 

 alterations are associated with signs of intoxication. 

 For example, ingestion of 1 00 ml of alcohol in i }'2 

 hr. in one subject reduced the alpha frequency from 

 10 to 9 cycles per .sec, but the subject was seriously 

 inebriated and relapsed into a prolonged stupor 40 

 min. later. Conversely, activating drugs such as 

 amphetamine or pipradrol compounds in sufficient 

 doses may raise the frecjuency of an alpha rhythm 

 by as much as i cycle per sec, but this is associated 

 with marked mental stimulation and agitation (fig. 

 10). The effects of hallucinogenic drugs such as LSD 

 25 are also related to mental change; doses sufficient 

 to raise the apparent alpha frequency, from e. g. 10 to 

 12 cycles per sec, induce characteristic transforma- 

 tions in mood and character. 



