238 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



The discharges in the optic nerve are of the same type as those 

 observed in other sensory nerves. This is a very fundamental general- 

 ized conclusion. 



Electroencephalogram 



In 1929 Berger discovered that rapid changes in electrical potential 

 could be detected between various points on the crown of the head. For 

 instance, normal human subjects possessed a more or less regular 

 rhythm of 10 pulses per second. The impulses had an average differ- 

 ence of potential of about 50 microvolts. 



These electrical potential fluctuations, popularly called " brain 

 waves," may be detected with the aid of two small pad-electrodes, 

 moistened with electrocardiogram paste, which were placed on the scalp 

 in circuit with an amplifier and a mechanical recording oscillograph. A 

 sensitive instrument will resolve the electrical rhythm patterns so that at 

 least four distinct, and more or less intense, frequencies can be identi- 

 fied. These wave patterns are classified as a (Berger), 0, y, and 8 

 rhythmic electrical potentials. 



The a rhythms appear as trains of waves lasting from 1 to 30 seconds. 

 Their frequency for each individual is quite constant. They usually 

 have a frequency of about 10, with extremes of 8.5 and 12.0 per second 

 according to Pauline A. Davis [1941], and attain a maximum peak volt- 

 age of 200 microvolts, though typical voltages run from 25 to 75 micro- 

 volts. 



The (3 rhythms have a frequency of over 20 per second and a peak 

 voltage of 50 microvolts. They appear in short sequences or irregular 

 groups. They are often difficult to distinguish from muscle potentials. 



Fast y rhythms have been described, but they are supposed to be 

 difficult to distinguish from muscle potentials. Their frequency is over 

 35 per second. Their peak voltage is about 10 microvolts. 



The 5 rhythms have a frequency of 4 per second or less with a 

 maximum peak voltage of 500 microvolts though typical voltages vary 

 from 100 to 300 microvolts. 



A freehand copy of a typical normal human electroencephalogram is 

 shown in Fig. VI-12. Tuned electrical filters permit the passage of 

 fairly narrow frequency bands, so that the recorded electroencephalo- 

 grams take the forms shown in B, C, and D. Note especially how the 

 amplitude of the a waves is modulated at a period of about 1 second. 

 Such modulations are often quite irregular in some subjects. The a 

 waves are nearly sinusoidal in form, though often very sharp and 

 unsymmetrical. 



