400 BRAIN MECHANISMS AND LEARNING 



original magnitude by sodium pentobarbital, but remained remarkably 

 stable in this condition (Fig. 5). 



Tactile Iwbitiiation at the spinal cord in decorticated cats. After establishing 

 that afferent neuronal habituation also occurs at spinal sensory neurones in 

 intact cats, the possible participation of the cerebral cortex in this phen- 

 omenon was explored in cats whose neocortex had been totally removed 

 2 or 3 months before. In all of them, the spinal evoked potentials also 

 dechned, sometimes at a faster rate, with repetition of the tactile 

 stimuli. 



Tactile habituation at the spinal cord in decerebrated cats. It being evident 

 that the neocortex is not necessary for the establishment of afferent neu- 

 ronal habituation at the spinal cord, it was deemed convenient to test 

 whether structures in the forebrain, temporal lobe or diencephalon might 

 play a role in the demonstrated habituatory spinal sensory suppression. 

 With this aim in view, the brain stem was severed at the mid-collicular 

 level leaving intact all the mesencephalic tegmentum in the remaining 

 brain stem. In these mesencephahc animals, habituation of tactile evoked 

 responses at the spinal cord was readily observed (Fig. 6), and not 

 infrequently the number of stimuli required for extinguishing those 

 responses was again less than in intact animals. Dishabituation was also 

 observed following a transient increase of the stimulus intensity. 



From the results just presented it is obvious that tactile habituation at the 

 spinal cord can take place in cats with only the sub-diencephahc portion 

 of their central nervous system. If, according to the view proposed by one 

 of us (Hernandez-Peon, 1955, 1957, 1959), afferent neuronal habituation 

 at second-order sensory neurones results from centrifugal inhibitory 

 influences proceeding from the reticular system of the lower brain stem, 

 the spinal responses extinguished by habituation in the mesencephalic 

 preparation should be enhanced by interrupting the postulated descending 

 inhibitory path to the spinal sensory neurones. This hypothesis was tested 

 by performing a high transection of the spinal cord in those decerebrated 

 cats which had already been habituated to repeated tactile stimuli. 



Tactile habituation at the spinal cord in spinal cats. Shortly after severance 

 of the spinal cord at C2, the tactile evoked potentials were remarkably 

 enhanced. Very often, as illustrated in Fig. 6, not only did the evoked 

 potentials recover the magnitude they showed before habituation, but the 

 enhancement surpassed the original size. These results clearly indicate that 

 a descending influence which acts upon spinal sensory neurones originates 

 in the lower brain stem itself. Another conclusion to be drawn from the 

 same results is that the electrophysiological correlates of afferent neuronal 



