M. R. COVIAN, C. TIMO-IARIA AND R. F. MARSEILLAN 44I 



COMMENTS 



The activity in sensory systems is subject to modification by central 

 regulatory mechanisms and background states, sufficiently pronounced to 

 be capable of influencing perception and so to be of interest for psychology. 

 Upon the arrival at the cortex, afferent discharge may show variation 

 related to spontaneous cortical activity (Bishop, 1944), to cortical thalamic 

 cxcitabiUty cycles (Chang, 1950, 195 1) or to influence of other afferent 

 stimuli (Gellhorn et al, 1954). The possibility that not only spontaneous 

 but also some electrical and chemically evoked cortical potentials can be 

 abolished during electrocortical arousal has been pointed out by a number 

 of investigators. Disappearance of recruiting potentials has been observed 

 by Moruzzi and Magoun (1949) and later confirmed by other investi- 

 gators (Jasper, Naquet and King, 1955) and it has been reported that local 

 strychnine waves can be blocked during sensory arousal. It has also been 

 demonstrated more rccendy that intermediary stages of afferent trans- 

 mission are also subject to influence by regulatory mechanisms. Reticular 

 inhibition of the medullary relays in sensory paths was shown by 

 Hernandez-Peon and Hagbarth (1955) and by Hernandez-Peon and 

 Scherrcr (1955) in experiments on the trigeminal nucleus and by 

 Hernandez-Peon, Scherrer and Velazco on nucleus gracilis. 



While some authors reported no interaction between the mechanisms 

 that give origin to the primary cortical response and those which give rise 

 to spontaneous periodic activity (Morison and Dempsey, 194- ; Moruzzi 

 and Magoun, 1949; Jasper, 1949) other investigators observed some kind 

 of interaction (Jasper and Ajmonc-Marsan, 1952; Bremer, 1954). 



The present experiments support the findings that show that reticular 

 formation is capable of exerting a clear influence upon cortical evoked 

 potentials. The pathway used in this case by the stimulus ot the reticular 

 formation to reach the cortex seems to be the slower one as described by 

 Adey, Scgundo and Livingston (1957), which shows rapid fatigue, fails to 

 follow repetition rates faster than 2-3 stimuli per second and which 

 conducts exclusively or predominandy in an upward direction. The 

 possibility of spreading current to the medial lemniscus is rejected by the 

 results of the control experiments. The positive-negative evoked potential 

 in the cortical forepaw projection areas is unequally affected by the 

 previous reticular stimulation. There is a different susceptibility of the 

 positive and negative components of the primary response, the negative 

 being the more susceptible. Parma and Zanchetti (1956) observed a 

 reticular blocking of cortical potentials evoked by thalamic stimulation. 



