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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



alpha activity during visual attention and mental vigi- 

 lance is almost invariable. When means are available 

 for instrumental analysis, a convenient form for the 

 measure of alpha activity is 'aisundance'. This in- 

 cludes dimensions both of amplitude and of persist- 

 ence. It is comparable with a measure of energy, but 

 this term is undesirable because of its precise connota- 



FiG. 6. An a.ssembly of cut-outs showing Huctuations in 

 abundance of alpha components over long periods during the 

 performance of various tasks: (/I) in a subject of the alpha- 

 responsive type showing an immediate and sustained rise in all 

 alpha components on eye closing and a lack of response to 

 mental activity with the eyes shut; (/?) with greatly increased 

 analyser gain the alpha components are barely perceptible in 

 an alpha-M type subject; (C) in a responsive versatile subject 

 the various components fluctuate over a wide range during the 

 performance of a psychological task. 



tion in physical systems. When analyzed in terms of 

 the abundance of its various components, the changes 

 of alpha activity during spontaneous or induced varia- 

 tions of behavior can be plotted as abundance with 

 respect to time (fig. 6). Necessarily, each component 

 is arbitrarily identified by its frequency, usually as a 

 whole number of cycles per second, but a composite 

 plot of these arbitrary components reveals the nature 

 and extent of the complexity previously referred to. 

 Often the various components exhibit some degree 

 of independence, particularly during the performance 

 of an exacting task, and the individuality of the spon- 

 taneous rhythms is considerably emphasized by this 

 procedure. The classical responsive types of alpha 

 rhythm rise and fall inversely according to the in- 

 tensity of concentration of the subject, but during a 

 period of tranquillity the pattern of frequency and 

 distribution may remain almost constant so that even 

 short samples of record are similar to one another. 



.At the other extreme, subjects showing little alpha 

 activitN display an extremely varied pattern, even 

 during rest. Those alpha components which are 

 present in such records fluctuate in abundance from 

 moment to moment, so that extremely long samples 

 of record must be taken if the samples are to resemble 

 one another. 



This observation has .suggested that a useful measure 

 of alpha character would be the length of sample 

 necessary for all of a set of such samples to fall within 

 a specified range of variation. This computation is 

 performed automatically with a wave analyzer fitted 

 with an electronic averaging device. This measure 

 pro\ides an estimate of the repertory of a person's 

 alpha activity and has been found to be related to 

 the .scope and variety of interests in a population of 

 normal young adults. The aspect of cerebral men- 

 tality defined in this way has been termed versa- 

 tility (jo). Detailed analysis of alpha responses has 

 suggested that, in some people at least, relative 

 abundance of the slower alpha components may be 

 associated with rest or inactivity of the mechanisms 

 of internal imagination, while abundance of the 

 faster components is more closely linked with the lack 

 of significant afferent signals from the receptors. 

 Thus with the eyes closed and the subject relaxed, the 

 most abundant rhythm may be at 9 cycles per sec; 

 but when the subject is given a mental task to per- 

 form with the eyes shut, the frequency of the dominant 

 rhythm may seem to rise to, say, i i c\cles per sec. In 

 some cases, however, the apparent acceleration is due 

 to attenuation of the lower frequency components 

 during concentration, leaving the higher frequency 



