INTRINSIC RHYTHMS OF THE BRAIN 



-'95 



the region of the neuron concerned is electronegative 

 with respect to the surrounding tissue. Li et al. (38) 

 have suggested that the microlocation of spontaneous 

 rhythms in the layers II to V of cat cortex may be 

 similar to that of the recruiting responses evoked by 

 thalamic stimulation. Stimulation of this type is 

 effective only when its frequency is close to that of the 

 spontaneous cortical rhythms, that is at 5 to 8 cycles 

 per sec, and this relation is reflected in a correspond- 

 ence between the phase of the recruiting response, 

 the spontaneous rhythms and the unit discharges. It 

 is not clear, however, whether the spontaneous ac- 

 tivity in these preparations is functionally homologous 

 with the alpha rhythms in human subjects. 



In some cases, therefore, the spontaneous rhythms 

 can act as electrotonic escapements, but there are 

 many occasions when the spontaneous rhythms and 

 unit discharges are quite unrelated. This \ariability is 

 manifest even in conditions of normal adaptation; 

 Ricci et al. (46) have described the complexities of 

 the relations between unit firing and surface rhythms 

 in the occipital cortex of monkey during the estab- 

 lishment of conditioned responses to sounds associated 

 with light flashes at 7 per sec. They conclude that 

 such a response "is a complex pattern of interwoven 

 inhibitory and excitatory processes", in which the 

 electric fields of relatively slow wave-like spontaneous 

 rhythms are interlaced with the rapid all-or-none 

 discharges of individual cells. A similar image was 

 employed by \Valter C58) to describe the topologic 

 details of evoked and spontaneous activity in human 

 subjects engaged in learning: "an interweaving of re- 

 ciprocal electric filaments to generate an intricate and 

 duraijle texture of significant association." 



Such observations suggest that an important factor 

 in cerebral mechanisms must be the geometry of the 

 electric fields in the region of neurons and their 

 processes. It is often forgotten that these fields have 

 vectorial as well as scalar aspects — they have direc- 

 tion as well as magnitude. As already mentioned, the 

 geometric and time relations of alpha rhythms as 

 seen on the scalp might be due to the adventitious 

 effect of remoteness from the source in a volume con- 

 ductor. Walter & Dovey (63) reported observations 

 of alpha rhythms in the depths of the occipital lobe in 



patients investigated for the delimitation of cerebral 

 tumors, but they could not study the details of this 

 activity. Recently Cooper et al. (18) have been able 

 to obtain toposcopic records of alpha rhythms derived 

 from electrodes implanted in the brains of patients 

 with no organic brain disease. In these experiments 

 the subjects were provided with up to 70 fine wire 

 electrodes in various regions of the brain as described 

 by Dodge et al. (20) and Sem-Jacobsen et al. (48). In 

 one subject it was possible to record from intracerebral 

 electrodes connected to the amplifiers in the network 

 pattern customary for toposcope studies of scalp po- 

 tentials. These records (fig. 1 1) of alpha activity from 

 the depths of the brain show phase — and space — re- 

 lations quite similar to those found in the superficial 

 fields. The efTects of synchronization by photic stimu- 

 lation and of blocking by attention were also similar. 

 As reported by Sem-Jacobsen et al. C49), the greatest 

 amplitude of the alpha rhythms was found about 2 

 cm below the surface of the brain, but the rhythm 

 existed also between pairs of electrodes 4 to 5 cm 

 deeper. This extension of the alpha activity could not 

 be due to purely electric conduction, since the phase 

 of the waves was not alwaxs identical in the various 

 regions and bursts of activity sometimes occurred in 

 one region and not in others. 



The conclusion from these studies is that some 

 alpha rhythms involve deep structures as well as cor- 

 te.x and the time relations of the alpha waves indicate 

 .some sort of spread from front to back and depth to 

 surface. The tendency of the transverse components 

 of the rhythm to be phase-shifted by 90° with respect 

 to the longitudinal ones suggests that there may be two 

 interlocked processes, one generated by a corticobasal 

 mechanism, the other, essentially corticocortical with 

 peaks corresponding in phase to the moment of most 

 rapid potential change — that is, zero potential — of 

 the corticobasal process. 



Relation Between .Alfiha Rhythms and Effector Fiuution 



The possibility that the alpha cycle may act as a 

 gating mechanism for afferent signals has suggested 

 that a similar relationship might be found for efferent 

 ones. Kibbler et al. (32), Kibbler & Richter (33) and 



16 and 19 are synchronized at twice the group rate (8.4 cycles 

 per sec.) and channels 21 and 22 respond at 4 X 42 = 16.8 

 cycles per sec. E. Flicker stimulation with triplets at 5.6 groups 

 per sec. evokes true replica in channel 1 7, rhythms at 1 1 .2 cycles 

 per sec. in channels 16, 18, 19 and 20, and at 3 X 5.6 = 16.8 

 in channels 21 and 22. All these patterns were associated con- 



sistently with specific frequencies and modes of stimulation 

 and were similar to those deri\'ed from scalp records. The 

 arithmetic and geometric relations of the various features in 

 such records suggest the presence of a number of mechanisms, 

 each with its own domain, intrinsic rhythmicity and responsive- 

 ness to stimulation. 



