262 BRAIN MECHANISMS AND LEARNING 



histor\- ot the stimulus when we apply it to the animal and I think this should be 

 extended also to his comment about immediate learning, rapid one-trial learning, 

 in the higher species. It is a different matter in the lower species — in the higher 

 species it is not done with unfamiliar material. You are dealing with something 

 that has a long history, as tar as the elements of the response are concerned. One 

 weakness of manv investigations of learning is their disregard of the past naturalistic 

 history of the animal. The study of learning without regard to the past history of 

 the animal is totally unrealistic. 



Palestini. I want to add some observations regarding the pretrigeminal pre- 

 paration, that is to say, a cat with a midpontine transsection. These cats remain 

 permanently awake. There is a desynchronized activity of the cortex and they 

 present ocular vertical movements of the eyes. We believe that these movements 

 are purposeful, because when a mouse is presented to the cat, the cat follows it with 

 its eyes. These movements are maintained during the whole period of stimulation 

 by the passing object. This contrasts with the normal intact cats. We think that in 

 the lower pons and medulla there exist inhibitory structures, which exert an 

 ascending inhibition towards higher levels. We believe that lower structures also 

 must be taken into account in explaining all the inhibitory phenomena. 



Gerard. I wanted to talk on the same subject as Dr Hebb. Dare one draw the 

 conclusion that, because a single event has led to a permanent trace, this excludes 

 the need of some continued activity to produce morphological, chemical and 

 other changes in the neurone pattern? A single impulse may continue to cause 

 activity, by reverberation or other mechanisms, for very long times. This has been 

 demonstrated by sending brief: stimuli to several centres, which then continue 

 discharging tor months. Further, there is a continued dynamic state, with responses, 

 memories, etc., which is still quite different from the permanent structured en- 

 gramme which is established and endures long after the dynamic memory has 

 gone. The fact that one can get a permanent change after one stimulus does not 

 exclude the reverberation type of mechanism in establishing fixation. 



Chow. We have prepared cats with lesions in the n. centrum medianum bilater- 

 ally and these cats have been presented with learning problems before lesion. After 

 lesion they can still remember these problems. 



Segundo. It appears to me that Dr Grastyan made a distinction between 'orienta- 

 tion' and 'startle' responses. Does he believe the difference to be qualitative or 

 simply quantitative in the sense that they constitute different intensities of response 

 to novel, or significant stimuli. 



Hernandez-Peon. Have you done any lesion experiments on the hippocampus 

 and observed the effect on the extinction of orientation reflex? 



Grastyan. I think that Dr Morrell's observations are partly in agreement with 

 my own — namely the second stage in which the big increase ot unit discharge is 

 observed in the hippocampus and disappearance of unit discharge in the reticular 

 formation. There seems to be a discrepancy concerning the first stage when both 

 structures show an increase in firing. Unfortunately we don't know the relation- 

 ship of this firing to the gross electrical recording and the behaviour manifestations. 

 Perhaps this is a transitory stage in which both structures were activated. In the 

 third phase — and this would be an answer to Dr Hernandez-Peon as well, the unit 

 activity in the hippocampus disappeared completely — which might correspond 

 to the picture we got during the final phase of extinction, when the stimulus 



