M. PALESTINI AND W. LIFSCHITZ 423 



These facts, and especially the last mentioned, make us think of possible 

 differences in the mechanisms responsible for these two plastic activities, 

 for it is unlikely that the same mechanisms would account for such dis- 

 similar characteristics. Attention combines the functional traits of a 

 transitory influence which has at the same time an activating effect upon 

 the EEC Habituation, on the contrary, is the result of repeated stimula- 

 tion, it appears slowly and is maintained for a longer period. 



It has already been noted that state of attention can be provoked in the 

 MPP when a heterosensory stimulus is applied. In this respect, the animal 

 behaves normally, while habituation in pretrigeminal cats shows the 

 above-mentioned alterations. In other words, this particular kind of brain 

 stem transsection seems capable of dissociating both phenomena. This 

 suggests that attention is a function of the mesencephalic reticular forma- 

 tion or of even more anterior regions. On the same basis, habituation of 

 cortical potentials might be considered a function of caudal reticular 

 mechanisms. Anyhow, we should point out that this is probably not the 

 only habituating influence, since we have sometimes observed, in the MPP, 

 a delayed decrease in the evoked potentials. It can be said that this part of 

 the reticular formation acts as a habituation-facilitating mechanism. 



When a more rostral pontine section is made, a synchronized EEG can 

 be observed as a rule. This fact differentiates the rostropontine from the 

 midpontine animal, but both preparations show the same slow type of 

 habituation (Fig. 5). This points to the possible independence of syn- 

 chronizing and habituating mechanisms. 



Sharpless and Jasper (1956) remark that there is no strict correlation 

 between the magnitude of the auditory evoked potentials in the cerebral 

 cortex and habituation of the arousal reaction. Although this last is seen 

 after twenty or thirty repetitions of the arousal stimulus, the evoked 

 potentials remain of the same size or increase. These authors believe it 

 necessary to distinguish between habituation of the arousal reaction and 

 decrease of the evoked potentials as a consequence of the former. On the 

 other hand, a detailed study of the zones of mesencephalic reticular forma- 

 tion that evoke the arousal reaction and those that inhibit responses in the 

 dorsal cochlear nucleus (Jouvct and Desmedt, 1956) has induced these 

 authors to assume that each of these functions has a different reticular 

 organization. 



Moreover, it is well known that stimulation of the reticular formation 

 produces dcsynchronization of the EEG, while different authors have 

 observed the facilitation or diminution of cortical evoked responses during 

 stimulation of this structure (Rossi and Zanchetti, 1957). All these facts lead 



