242 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



centage of time the a frequency appears in a monopolar occipital record 

 at a level 2 cm above the inion (Davis and Davis [1936]). This record 

 is to be taken at least 10 seconds after the eyes have been closed. 



The /3 activity is present all over the living brain. It represents physi- 

 ological activity of the brain which is modified by physiological changes 



Fig. VI-16. A human electroencephalogram. The a rhythm is suppressed by 

 visual stimulation. Subject in darkness with eyes open. Light turned on at first 

 arrow and off at second. Note survival of /3 waves. Electrodes 5 mm apart and 

 5 cm lateral to midline over periparastriate area. Original record. (By courtesy of 

 D. B. Lindsley [1938].) 



such as sleep. One may identify the /3 activity of the brain as the resid- 

 ual occipital activity when the eyes are opened in a bright light. This 

 type of encephalogram is illustrated in Fig. VI-16, which shows that the 

 a activity ceases during illumination but resumes after light is cut off, 



;■ i , . . ■«« J - ".- ■• \»/*r ••„ •„/■ «/v vV 



Fig. VI-17. A. This shows how the a rhythm of the right (top record) and left 

 occipital areas differ in amplitude and blocking when a luminous stimulus is " turned 

 on " at first arrow and " turned off " at second. 



B. Blocking of a rhythm by an auditory stimulus (buzzer). On and off at arrows. 

 (Original electroencephalograms by D. B. Lindsley [1938].) 



but that the /3 waves are present at all times. Note that the composite 

 wave at the right-hand edge resembles the composite (a + j3) graph of 

 Fig. VI-13. 



It has been found that the /5 waves usually are diminished by appre- 

 hension or emotional attention, and reduced by drowsiness (Adrian and 



