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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



Adrian & Matthews (6) and more recently Adrian 

 (5) also adhere to the view that attention to a stimulus 

 or even attempts to see or hear a stimulus not actually 

 present is responsible for the disappearance of the 

 alpha waves and their replacement with low-ampli- 

 tude fast and irregular waves. This type of reaction 

 we now call an 'activation' or 'arousal' response and 

 attribute it to increased activity in the reticular sys- 

 tem, including the ARAS and DTPS. However, it 

 iii.n also be a function of selective attention since the 

 waves block with sensory stimulation, mental arith- 

 metic, or attempting to visualize a scene or recall a 

 melody. 



We need measures of activation and attention, and 

 the EEG offers promise in this regard. Behaviorally a 

 person may feign sleep and deceive an observer but 

 not a trained EEG observer, for his record would 

 show alpha waves or some degree of activation or 

 arousal responses. He could not feign sleep charac- 

 teristics. If a person is supposed to be awake and alert 

 and fixating an area in order to make observations, 

 one would expect the EEG record to show either a 

 condition of activation with low-amplitude fast waves 

 or alpha waves if he has become somewhat habituated 

 to the condition. If either picture shifts to slow and 

 sporadic alpha waves or to even slower delta waves, 

 or mixtures of the two, one can predict quite reliably 

 that the subject is not alert and attentive, but on the 

 contrary is drowsy. Usually the subject will admit 

 dosing or drowsing if questioned at that time; or if a 

 stimulus for reaction-time measurement is given at 

 such a time, it will either be missed entirely or the 

 reaction will be slow. 



Although the alpha rhythm averages about 10 

 waves per see. in older children and adults, it has a 

 much lower frequency in very young children. During 

 the waking slate in newborn infants there is no organ- 

 ized or persistent alpha rhythm over the sensor) areas 

 ill the brain. By ;j or 4 months the sensory zones de- 

 velop a persistent alpha rhythm of about 3 or 1 waves 

 per sec I hese increase to about 5 or 6 per sec. at 1 

 year of age and by to or 1 2 years oi age have reached 

 the adult frequency of to per sec. Figure c| shows the 

 longitudinal development of the occipital alpha 

 rhythm during the waking st.ite in the same child 

 Imiii infant v tn adulthood. Prior to the development 

 ■.I .111 alpha rhythm it is difficult to attract and main- 

 tain the attention of a child of 1 or 2 months of aye. 

 After the child develops an occipital alpha rhythm, the 

 waves can be blinked bv a visual stimulus, indicating 

 that activation via ARAS is possible Before the in- 

 fant develops an alpha rhythm in the occipital area, 





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fig. 9. Longitudinal development of occipital alpha rhythm 

 in the same subject from early infancy to -M years of age. 

 Alpha rhythm typically appears in a persistent pattern by the 

 third or fourth month, with a frequency of 3 to 4 waves per 

 sec. By 1 year the frequency is r , to 6 per sec, and by 10 years 

 it may attain the adult average frequency of about 10 per sec. 

 Note that as frequency increases, voltage diminishes. 



repetitive photic stimulation will induce a rhythmic 

 activitv in synchrony with the stimulus, but only in 

 the immediate range of the natural alpha rhythm 

 which would appear at ■] or 4 months of age. There is 

 some reason to believe that the alpha rhythm is main- 

 tained or synchronized bv unspecific nuclei of the 

 DTPS but is ea|)able of activation bv either ARAS or 

 DTPS. Thus the establishment of a rhythm in a voting 

 infant must mean that the sensory /one involved has 

 taken on a new functional role. The convergence of 

 specific and unspecific influences upon a cortical 

 zone in which rhythmicity and activation properties 

 have been attained indicates that perceptual capacity 

 is also now available. This extension of sensory and 

 perceptual capacities to the cortex implies that a new 

 level of consciousness has been attained as well. At- 

 tention, perceptual discrimination and consciousness 

 are matters of degree, level, temporal relations and 

 perhaps other factors. One wonders about the young 



infant with slow alpha waves ,\\k\ slow development 

 11I an evoked potential following stimulation. I o w hat 



