THE HIPPOCAMPUS 



I381 



has a well-developed dentate gyrus despite the fact 

 that the cetaceans are completely anosmatic and lack 

 olfactory nerves. The hippocampus is often well de- 

 veloped in human monsters born without olfactory 

 bulbs and is generallv believed to reach its highest 

 state of development in the microsmatic animal, man. 

 Tlie completely subordinate role of olfactory impulses 

 in the function of the hippocampus was finally demon- 

 strated by the experiments of .Swann and of Allen. 

 Swann (108, log) showed that olfactor\' discrimina- 

 tion was not affected by ablation of the hippocampus. 

 Allen (10, II) demonstrated that olfactory conditioned 

 refle.xes still persisted after the hippocampus had been 

 removed and also that they could be established after 

 its ablation. Thus, the hippocampus is of minor signifi- 

 cance even in highly coordinated olfactory reflex 

 mechanisms (22, 65, 74). 



Herrick's view that the hippocampus correlates the 

 diencephalic and cortical structures is implicit not 

 only in his own monumental studies (53, 54), but also 

 in the earlier work of Ramon y Cajal (90, 91). 

 Certainly no one can dispute this point of view, but 

 the specific function is another matter and up to this 

 time no truly satisfactory theory concerning the role 

 of the hippocampus has been advanced. Ablation of 

 the hippocampus, without serious damage to large 

 areas of the brain, has not yet proved feasible. Elec- 

 trolvtic or surgical injuries, locally placed within the 

 hippocampus without its total destruction, produce a 

 variety of changes; but it is striking to note that these 

 changes are remarkably similar to the changes seen 

 following local stimulation in the conscious animal, 

 and the question arises whether they are, in fact, the 

 results of destruction or the results of irritation. Elec- 

 trolytic or severe experimental infarction may give 

 rise to a condition resembling psychomotor epilepsy 

 (47, 48), and it is quite likely that some of the be- 

 havioral changes observed — abnormal fears, pupillary 

 dilatation, anisocoria and hyperesthesia — may repre- 

 sent a part of the seizure process. This is particularly- 

 likely since these behavioral changes seem to be 

 triggered by peripheral stimulation of various kinds. 

 It is interesting to compare the seizures and behavioral 

 changes observed following electrolytic lesions with 

 the similar changes described after stimulation (14, 

 51, 61-63, 66-70, 76). Secondary damage to the 

 hippocatnpus, induced by interruption of its blood 

 supply (47, 48; Naquet, R., et a!., personal communi- 

 cation), seems to induce effects identical with those of 

 electrolytic lesions placed exclusively in the hippo- 

 campus. The behavioral changes seen following 

 lesions tend to occur particularly in the first 3 weeks 



postoperatively. This is usually followed by an interval 

 which seems to be virtually symptom-free; but the 

 effects may then recur after an interval of some 

 months, suggesting the gradual formation of an ir- 

 ritative scar within the central nervous svstem. Apart 

 from seizure discharges and their concomitant effects, 

 stimulation of the hippocampus has yielded disap- 

 pointing results. Kaada found that "it did not yield 

 any significant effects on any of the somato-motor or 

 autonomic activities recorded in this study (except for 

 .some facilitation of cortically-induced movements 

 when the three central motor areas were activated)." 

 Carlson el al. (25) believed that the autonomic effects 

 seen following stimulation of the fornix and mammil- 

 lary body could be attributed to the spread of current 

 to adjacent hypothalamic areas; Penfield & Erickson 

 (84) also obtained negative results. Kaada (61), how- 

 ever, saw pupillary dilatation in man, which can be 

 confirmed for the cat, while Kaada c& Jasper (63) ob- 

 served changes in respiration, and MacLean & 

 Delgado (76) saw changes in emotional behavior. 

 Damage to the fornix and septum lucidum produces 

 changes in rage threshold, according to Spiegel el al. 

 (106) and Rothfield & Harmon (99). Green & 

 Arduini (46) noted that lesions of the septum lucidum 

 in rabbits, involving the precommissural fornix h\ii 

 not the postconimissural fornix, resulted in curious be- 

 havior in which the animals seemed to be hyper- 

 reactive and at times would apparently attack the 

 observer in fear. They noted that this type of lesion 

 blocks the theta rhythm; this will be discussed below. 

 Brady & Nauta (18, 19) made somewhat similar ob- 

 servations on the rat but noted that after a period of 

 about 40 days the response disappeared. Therefore, 

 it is not clear whether the response is an irritative one 

 or whether the recovery is due to some local process of 

 adaptation. Section of the fornix, incidental to section 

 of the corpus callosum, did not produce behavioral 

 changes in man in the observations of Akelaitis et al. 

 (6-8), while Wheatley (114) observed no changes 

 following complete electrolytic destruction of the 

 fornices (postcommissural fornix and partial damage 

 to septum). Dott (31) noted no symptoms which 

 could be attributed to section of the fornices and 

 incision of the lower part of the septum lucidum in 

 two patients. It should be pointed out, however, that 

 in these patients there were also a number of signs of 

 hypothalamic damage. Garcia-Bengochea et al. (36) 

 also found no detectable changes in monkeys in a 

 variety of performance tests. Possible roles of the hip- 

 pocampus in conditioning and learning will be con- 

 sidered by Galambos & Morgan in Chapter LXI of 



