ELECTROENCEPHA LOGRA M 



243 



Matthews [1934]). Many investigators have found that both auditory 

 and tactile stimuli usually block the a rhythm. An electroencephalo- 

 gram showing the effective blocking of the a rhythm by a luminus and 

 auditory stimulus is presented in Fig. VI-17. 



The pattern of the encephalogram changes systematically with the 

 onset of sleep (Davis, Davis, Loomis, Harvey, and Hobart [1938]). 

 In general, with the approach of drowsiness (" floating "), the a rhythm 

 (10 cycles), if present in the waking record, diminishes in voltage and the 

 time intervals between trains of a waves become longer. In the inter- 

 vals between these trains the a waves are reduced. Moderately deep 



Schema of potentials during a nights sleep 



Hours 

 Alpha 

 Wake- 

 fulness 



1 



Alpha & 



delta 



Light 



sleep 



2 



Delta 

 Deep sleep 



3 4 



Null 

 Light sleep 



5 

 Inter- 

 mittent 

 Alpha 



Wakefulness 



6 

 Alpha 



Fig. VI-18. Predominance of brain potentials through the night. The a waves 

 (black heavy line) in per cent presence; 14 per second waves (dashes) in per cent 

 presence and 5 waves (dots) in extent of predominance. Oral temperature (black 

 thin line). Below the graph the stages of sleep are indicated. Record begun at 

 time of retiring, arrow indicates beginning of sleep. Fluctuations represent changes 

 due to shift of state of sleep. (By courtesy of H. Blake, R. W. Gerard, and N. Kleit- 

 man [1939].) 



sleep shows well-developed 14-cycle " spindles " (Fig. VI-12, E) lasting 

 for about 1 second, superimposed on a background of random oscillatory 

 motion. As sleep becomes deeper, the random waves increase in volt- 

 age and wavelength; in very deep sleep, they may attain an amplitude 

 of several hundred microvolts and last for 2 seconds or more. 



An interesting graph summarizing the predominance of brain poten- 

 tials through a night's sleep is shown in Fig. VI-18 (Blake, Gerard, and 

 Kleitman [1939]). The heavy black line shows the percentage of a 

 waves present. The dashed line indicates the percentage of 14-cycle 

 waves. Delta waves are shown in extent of predominance by the dotted 

 line. Note how the oral temperature parallels the percentage presence 

 of a waves. This correlation is reasonable since a rhythm is closely 

 associated with tonus (Jasper and Andrews [1938]). 



The effect of drugs on the human encephalogram was summarized by 

 Gibbs, Gibbs, and Lennox [1939] as follows: Drugs that cause a sleep- 



