THE HIPPOCAMPUS 



1387 



In cats electrolytic lesions in the hippocampus in- 

 duce seizures which show most of the characteristics 

 of psychomotor epilepsy (47). Seizures may also occur 

 as the result of secondary injury to the hippocampus 

 when the region of primary destruction is elsewhere, 

 particularly in the amygdala (47; Naquet, R., et al., 

 personal communication). Lesions involving the 

 medial part of the amygdala in the cat frequently 

 interrupt the anterior choroidal artery. When this 

 happens, there is a destruction of many of the cells 

 within the hippocampus, JDoth \entral and dorsal. 

 Animals with this type of lesion show motor fits, 

 usuallv of a clonic type, with a march from the face 

 through the neck, forelimbs, trunk, to the hind limbs, 

 accompanied by salivation and often preceded by 

 prodromal 'arrest reactions' or by more complex 

 signs, for example pouncing as if to catch a small 

 animal, striking or patting at the air, or signs sug- 

 gestive of fear. \'ery often these seizures may be 

 triggered by peripheral stimulation, by touching the 

 animal, by repetitive click stimulation or by strobo- 

 scopic stimulation. The same forms of stimuli also at 

 times induced what appeared to be sensory fits. The 

 animals may show apparent fear, retreating from the 

 investigator, crouching at the back of the cage, snarl- 

 ing and hissing, and showing dilated pupils and 

 piloerection. Sometimes this does not appear in- 

 stantaneously but only after a more or less prolonged 

 period of afferent stimulation. Sometimes when the 

 animal is stroked, regions of hyperesthesia seem to 

 appear and the animal will strike out at the investi- 

 gator when certain parts of the body are reached, 

 especially in the region of the pelvis and axilla. The 

 onset of such a "seizure" is most commonly initiated 

 by an arrest reaction in which the animal suddenly 

 ceases whatever it is doing, remains motionless for a 

 few seconds and possibly looks around in a worried 

 fashion. At times such behavior is not accompanied by 



any further signs, although after a period of peripheral 

 stimulation, as previously described, the animal may 

 show what seem to be unreasoning fears, for example 

 fear of kittens or of other small animals. Studies on 

 conditioning by MacLean (75) and by Lissak (per- 

 sonal communication) seem to indicate that hippo- 

 campal stimulation will cause the arrest of a con- 

 ditioned response. 



General Conclusions on Functional 

 Role of the Hippocampus 



Because of the su.sceptibility of the hippocampus to 

 seizure discharges, and because these seizure dis- 

 charges are accompanied by a condition resembling 

 psychomotor epilepsy notably a.ssociated with prodro- 

 mal signs and hallucinations, it is evident that at the 

 present time great caution must be applied in inter- 

 preting its functional role. This caution is especially 

 emphasized by the absence of clear-cut effects re- 

 sulting from destruction of the main efferent pathways 

 in the fornix. The rhinencephalon occupies a phylo- 

 genetic position midway between those of the reticular 

 activating system, the midbrain tegmentum and 

 thalamus, and that of the neocortex. Within the 

 rhinencephalon the hippocampus and piriform cortex 

 may be considered to represent the ancient cerebral 

 hemisphere, or ancient cortex, of primitive verte- 

 brates. It is reasonable to suppose that, of all rhinen- 

 cephalic structures, the hippocampus represents 

 perhaps the highest level of integration. It is clearly 

 established that it is in important relationship with the 

 reticular activating system and it seems reasonable to 

 suppose that it modifies the latter's activity, but at the 

 present our knowledge has done little more than con- 

 firm the predictions of Herrick in 1933 (53). How- 

 ever, evidence is accumulating to suggest the partici- 

 pation of Amnion's horn in 'emotional' responses. 



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