SIGNIFICANCE OF THE PHOTIC STIMULUS 

 ON THE EVOKED RESPONSES IN MAN 



E. Garci'a-Austt, J. BoGACz AND A. Vanzulli 



INTRODUCTION 



If the retina of a normal man placed in a dark and quiet room is stimu- 

 lated with flashes ot constant intensity and frequency, the subject under- 

 goes a series of psychological phenomena. Of these phenomena the most 

 constant are those pertaining to attention. The attention state follows a 

 defined time pattern. The first flashes, taking the subject by surprise, 

 arouse his attention. Subsequently, through monotonous repetition, the 

 flashes lose significance and he becomes uninterested. If under these condi- 

 tions the stimulus is discontinued at regular intervals for a constant 

 period of time, a new interest is added, darkness. The stimuli regain 

 significance on being preceded by a period of darkness and once more the 

 subject becomes attentive to them, but he again loses interest on becoming 

 habituated to the regular repetition of the association of darkness and 

 flicker. If on this same attention level a constant tone is added at the end of 

 the period of darkness, the flashes regain significance and the same pheno- 

 menon is repeated. 



Is there any method of objectifying the neurophysiological processes 

 associated with these modifications in attention; The study of the evoked 

 potentials contribute important elements t(^ the elucidation of this 

 problem. In this connection Hernandez-Peon and Donoso (1957), Joiivet 

 and Courjon (1958) worked on subcortical recordings in man. Obviously 

 this method does not allow for a systematic study. 



It has been shown that from scalp recordings it has been possible to 

 extract the evoked potentials by the use of different methods, taking into 

 account either the space factor (Grey Walter, 1954; Remond and Ripoche, 

 1958), or the time factor (Dawson, 1947; Calvet, Cathala, Hirsch and 

 Scherrer, 1956). 



In the present investigation, methods of temporal superposition have 

 been used to study in man the influence of the changes in the significance 

 of the photic stimulus on the evoked response. 



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