604 BRAIN MECHANISMS AND LEARNING 



VISUAL EVOKED RESPONSES 



Dawson (1947) was the first author to record through the scalp the 

 evoked responses to electrical stimulation of the peripheral nerves in man. 

 He used the superposition of fifty or more successive sweeps in a single 

 record, leading as is customary the potentials on the Y axis and the sweep 

 synchronized with the stimulus on the X axis. 'In such a record any 

 waves which follow the stimulus by a regular interval will all occur at the 

 same place on the trace; they may thus be detected although they are no 

 larger than the deflection produced by the spontaneous activity of the 

 brain and scalp muscles. As deflections due to these causes are not regularly 

 related to the stimulus, they tend to occur at a different place on each 

 successive sweep and therefore appear oidy as a thickening of the baseline' 

 (Dawson, 1947). Ciganek (1958 a, b) used this technique tor recording the 

 evoked potentials by photic stimuli in man. 



Calvet, Cathala, Hirsch and Scherrcr (1956), and Calvet, Cathala, 

 Contamin, Hirsch and Scherrer (1956) used another technique of super- 

 position for recording in man the evoked visual, somato-sensorial and 

 auditory responses. They led the cortical potentials on the Z axis thus 

 causing variations in the luminous intensity of the cathode ray. On the 

 X axis the sweep was synchronized with the stimulus, and on the Y axis 

 a high frequency was fed to obtain a wide luminous strip. On a single 

 frame twenty to fifty successive sweeps were superimposed. By this means 

 a strip was obtained whereon the asynchronic waves of the EEG were 

 superposed, giving a grey tone, and the evoked potentials were added 

 together, giving a clear contrast between light and shadow. The strip was 

 analysed after magnification by a photoelectric cell, and a linear record 

 thus obtained. 



In this investigation these two techniques were employed to record the 

 evoked visual response in forty-three experiments carried out on normal 

 adults. Dawson's technique (1947) of superposition is simple, reliable, and 

 permits an adequate study of the sequence of the potentials. However, as 

 it is purely a procedure of superposition without adding the potentials, its 

 amplitude was small. When working with the stroboscope at a distance 

 of over 3 m. from the retina the response was not recorded. The technique 

 of superposition and addition of Calvet ct al. is more complicated, requires 

 a longer procedure and is less safe. A straight baseline cannot always be 

 obtained owing to differences in the luminous intensity of the cathode 

 rays. Potentials are distorted by contrast, by the non-linearity of the 

 photographic process and by the need to use the photo-electric cell wnth 



