CHANGES IN CORTICAL EVOKED POTENTIALS BY 

 PREVIOUS RETICULAR STIMULATION 



Miguel R. Covian, Cesar Timo-Iaria and Ricardo F. Marseillan 



It is a well-established fact that the central nervous system possesses 

 mechanisms by means of which the afferent inflow can be influenced, and 

 that an important role in these mechanisms is played by the reticular 

 formation of the brain stem. Reticular stimulation has been shown to 

 affect the sensory discharge at various levels in the central nervous system. 

 On the other hand on a background of continuous electrical activity there 

 arises in the cortex on peripheral stimulation a primary response — only 

 in the correspondent receptor areas — and a number of waves which 

 follow it and spread to a somewhat wider area. The primary response is 

 the one which has been studied in most detail ; it occurs regularly on any 

 kind of peripheral stinuilation and is the most closely connected with the 

 arousing of sensation. 



The literature is contradictory regarding the interaction between those 

 nerve elements which determine development of the primary responses 

 and the central core of the brain stem which, receiving impulses coming 

 from all sensory channels, is an adequate region for the integration of these 

 different impulses. 



In the present paper the influence of previous stimulation of the reticular 

 formation upon the cortical evoked potential obtained by a tactile stimulus 

 is studied in an attempt to understand the interaction between the specific 

 and non-specific, diffuse projection systems. 



METHODS 



Experiments of this type were performed upon twenty-four cats. The 

 animals were anaesthetized with initial (60 mg. kg.) and maintenance doses 

 of chloralose given intravenously. The somatic sensory area of the left 

 cortex was exposed by craniectomy and reflection of the dura and covered 

 by mineral oil. A unipolar electrode connected to a Dumont double beam 

 oscilloscope through a Grass AC pre-amplifier registered the electrical 

 activity of the cortex with negativity upwards in all records. Photographic 

 records of the evoked potentials were taken either in individual sweeps or 



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