420 BRAIN MECHANISMS AND LEARNING 



potential produced in this way (Litschitz, Palestini and Arniengol, 1959) 

 with the possibihty of recuperation to the previous ampUtude by some 

 known dishabituating agents (extrastiniuhition, rest, changes in the 

 characteristics of the stimuh, etc.). 



EEG recordings of primary and secondary^ visual responses were made 

 before and after midpontine section. Each animal was its own control. 

 The preliminary recording was made on unanacsthetized dark-aciapted 

 cats with permanently nnplanted bipolar electrodes in striate cortex and 

 gyrus lateralis anterior. The pupil was dilated by homatropine and the 

 head was maintained in the same position by means of a metal cone. 



Midpontine section produced several changes in primary and secondary 

 evoked photic potentials. In the first place, an increase in the amplitude of 

 these potentials was recorded. This phenomenon will be discussed later. 

 Secondly, a remarkable decrease in the speed of habituation of primary 

 potentials was noticed. Tt was clear that it could be obtained only after a 

 great number of stimuli and, in some cases, was not present during the 

 whole test period (Fig. 3). Thirdly, the secondary potential had a time- 

 course of habituation similar to that shown by the primary one (Fig. 4). 

 This is quite different from that to be seen in normal animals. In a previous 

 communication (Lifschitz, Palestini and Armengol, 1959) a description 

 has been given of the rapid habituation of the secondary potential registered 

 in the most anterior part of the gyr.us lateralis, while habituation of the 

 evoked potential in the striate area needed the application of hundreds and 

 even thousands of stimuli to develop. There is a very striking diftercnce 

 between both potentials, for the secondary response practically ciisappears 

 after the first twenty to thirty stimuli and even after only three to five 

 stimuli (EGG recordings). Other authors have also described this prompt 

 habituation of secondary responses (Jasper, Ricci and Doane, 1958) and 

 have stressed their peculiar plasticity which has specially drawn our 

 attention. The difference in habituation rates between primary and secon- 

 dary potentials is altered in the MP? since, as we have already noted, 

 both responses have a similar temporal course of habituation in this 

 preparation. 



So far, the mechanisms of attention and habituation have generally been 

 attributed to inhibitory tonic inffuences originating in central regions of 

 the brain stem. This hypothesis is based on experiments showing the 

 blocking of sensory transmission at the level of gracilis nucleus (Hernandez- 

 Peon, Scherrer and Velasco, 1956), trigeminal nucleus (Hernandez-Peon, 



^ Wc call photic secondary responses those recorded in the gyrus lateralis anterior and 

 suprasylvian gyrus (Buser and Borenstein, 1959). 



