ELECTROENCEPHALOGRAPHY WALTER 251 



watching its spontaneous activity. An analogy for this is the de- 

 velopment of radar, in which instead of trying to pick up the sound 

 or radiation from an aeroplane, a radio pulse is transmitted toward 

 it and the echoes produced are observed in relation to the original 

 signal. In the study of the brain the transmitted signals are called 

 stimuli and the echoes responses, but the resemblance in both method 

 and results is very close. The sound from an aeroplane travels too 

 slowly to indicate the position of the source, and any radio signals it 

 emits may be deliberately or inadvertently confusing to the observer, 

 but an aircraft cannot avoid reflecting the radar pulse. In the brain, 

 the spontaneous actions or thoughts which it initiates may be delayed, 

 by a variable time, after the first electrical sign of their occurrence, 

 and the spontaneous activity is usually too varied and uncontrollable 

 to correlate with other physiological variables, but the central nervous 

 system cannot escape the influence of external stimuli, since to re- 

 spond to them is one of its prime functions. Moreover the stimuli 

 can be given at moments controllable by the observer and at known 

 frequencies. 



By using techniques precisely similar to those developed for radar, 

 the response to a regular stimulus can be displayed on a cathode-ray 

 oscilloscope, so that scores of successive responses are precisely super- 

 imposed — whereas the random or spontaneous activity is blurred and 

 unobtrusive. This method has been turned to great account by Daw- 

 son [1] - in the study of the electrical responses to stimulation of sen- 

 sory nerves in the limbs. Results of considerable general interest have 

 been obtained by using rhythmic flashes of light for investigating the 

 response characteristics of the visual cortex [2], [3], [4]. When 

 rhytlmiic stimulation of this sort is used, the frequency analyzer al- 

 ready mentioned is of great value, since the regular responses at the 

 stimulus frequency can be traced through the brain, even when they 

 are smaller than the irregular background oscillations, because their 

 steady recurrence is seen by the analyzer as a prominent peak of ac- 

 tivity at a single frequency. 



This method has proved powerful in clinical applications, particu- 

 larly when the resting records present no diagnostic features. In 

 some epileptics, for example, it is possible to find a flash frequency 

 which sets up in the brain electrical activity which combines with the 

 oscillations already present to produce a diagnostically abnormal 

 pattern, and a clinical seizure. This observation, in combination with 

 the known features of the spectrum of epileptic records, has sug- 

 gested the hypothesis that the condition known as idiopathic epilepsy 

 is due to the occasional synchronization by sensory or motor impulses 

 of otherwise unrelated electrical rhythms. In cases with organic 

 disease of the brain the response to rhythmic flashes of light is often 



2 Numbers in brackets refer to authorities cited at end of article. 



