6o6 BRAIN MECHANISMS AND LEARNING 



the intensity of the stimukts, the amphtude in all the regions was increased. 



The evoked response had the same characteristics when it was recorded 

 in the subject after an interval of several days. 



Between the evoked responses from the occipital region and those from 

 other regions differences of latency, amplitude, phase and duration were 

 observed. The presence of a single generator located in the depth ot the 

 occipital region could explain the differences of amplitude and phase 

 owing to the different distance and orientation of the electrodes. How- 

 ever, the difi^erences in latency and duration of the response cannot be 

 satisfactorily accounted for by the physical propagation of the occipital 

 response, because for the frequencies recorded the reactance of the brain 

 coverings is negligible (Grey Walter, 1950). Furthermore during the 

 continuous or discontinuous flicker stimulation, as will be shown later on, 

 the response of the different regions may vary independently. 



It is necessary, therefore, to recognize that other regions of the brain are 

 activated. The magnitude of the delay (15-30 msec.) existing between the 

 occipital potential and the potentials of the other regions, leads to the belief 

 that the latter must course through pathways with more numerous 

 synapses. The small distance between the electrodes eliminates the possi- 

 bility that this delay may be due solely to axonic conduction. 



To sum up, the evoked visual response studied in these experiments is a 

 rhythmic, complex potential, of long' latency, of diffuse distribution over 

 the scalp, recorded during wakefulness with the eyes open or shut. 



Evoked visual responses have been the subject of numerous studies in 

 animals and in man since Bartlcy and Bishop (1933) described them for the 

 first time in the cat, by stimulation of the optic nerve. Shape and topo- 

 graphic distribution vary considerably according to the state of attentiveness. 



Two components of the evoked response are distinguished regardless of 

 whether the recording is made in the animal in the area of primary pro- 

 jection (Gerard, Marshall and Saul, 1936; Marshall, Talbot and Ades, 

 1943; Chang, 1950) or in man through the scalp (Ciganek, 1958b): the 

 initial component and the after-discharge, hi both cases the after-discharge 

 is similar. It is not known what relation may exist between the initial 

 component recorded in the area of specific projection, primary evoked 

 potential, and the initial component of the response recorded through the 

 scalp. Ciganek (r95Sb) maintains that some of the waves which constitute 

 the initial component correspond to the primary evoked response. 



The distribution of the evoked visual response is not limited to the 

 striate area, but in experimental conditions, in the anaesthetized animal 

 (Gerard, Marshall and Saul, 1936; Marshall, Talbot and Ades, 1943; Clare 



