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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



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TIME IN SECONDS 



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FIG. lo. Alpha frequency changes in the same subject as in figures 7 and 8 under the influence of 

 pipradrol (Meratran). The steady frequency has risen by 0.4 cycles per sec. and the rate of change 

 during attention is abrupt and discontinuous, but again the frequency returns to precisely its orig- 

 inal value. [From Walter (61).] 



Effects of Activation and Stimulation 



The effects of generalized or nonspecific activation 

 on the spontaneous alpha rhythms are sHght but 

 characteristic; of equal interest are the relations be- 

 tween responses to specific physiological stimuli and 

 the distribution and time relations of the intrinsic 

 rhythms. The most dramatic and effective changes 

 are of course those produced by visual stimulation. In 

 one of the early reports on the effect of flicker, Adrian 

 & Matthews (i) described these as 'driving the alpha 

 rhythm', l:)ut more detailed study (42, 53) has shown 

 that, except in certain circumstances, responses to 

 photic stimulation are distinct from alpha waves. 

 Those circumstances in which alpha rhythms are in- 

 volved in evoked responses are of particular interest 

 in the study of spontaneous activity since the interac- 

 tion between rhythmic external stimuli and intrinsic 

 brain rhythms indicates a possible function of the 

 latter. Bishop (10, 11) reported cyclic changes in the 

 excitability of the visual system of the rabbit and the 

 relation of such changes to intrinsic alpha rhythms 

 has Ijeen studied by several other experimenters, 

 Bartley & Bishop (6), Gastaut et al. (23), Lindsley 

 (40) and Lansing (37) In some animals there seems 

 to be good evidence for periodic fluctuations in the 

 excitability of the visual system, but in human sub- 

 jects the observations are inconclusive because of the 



wide variation between individuals. The properties 

 of the retina interfere .seriously with attempts to meas- 

 ure the time relations between visual stimulus times 

 and evoked responses; in effect, the retina acts as an 

 integrator for brief flashes of light so that the volley 

 of impulses in the visual pathways is a very ragged 

 one, spread over a period of o\er 50 m.sec. even when 

 the stimulus is a flash of light lasting only a few micro- 

 seconds. This means that even if the central visual 

 structures were totally ' blind' for half of every alpha 

 wave, they could still receive signals that fell in the 

 blind phase because the afferent volley would outlast 

 the critical period. 



Synchronization of Alpha Rhythms 



There is a further complication to be considered; 

 the alpha rhythms can only be 'driven' over a narrow 

 range of frequencies, Ijut this range is enough to 

 allow them to be synchronized or locked in phase by 

 afferent visual signals. This is demonstrated clearly in 

 toposcope records; this device can be arranged to 

 deliver brief flashes of light in a time-sequence of 

 doublets or triplets at any chosen repetition rate. In this 

 way the pattern of true evoked responses can be distin- 

 guished from the superficially similar but functionally 

 distinct pattern of synchronized alpha rhythms. When 



