l84 BRAIN MECHANISMS AND LEARNING 



very interested to know what might be the physiological meanijig of this inhibition. 

 According to our supposition, the hippocampus inhibits the orientation reflex, 

 which is a necessary step in the development of the conditioned reflex. 



Olds. Wc found that if we were on the surtace of the hippocampus itself, so that 

 we were stimulating the hippocampal cells proper, we very rarely got grand mal 

 seizure perceptible in the animal's behaviour. When we were in dentate we almost 

 invariably did get grand mal at such low thresholds that it was impossible to do 

 any research at all. Now as to whether our results in the hippocampus proper might 

 be from hippocampal seizure which could be recorded although not observed I 

 certainly would not be at all surprised if that were true. I have often conceived of 

 these experiments in the light of trving to tind the place where localized seizures 

 would have the same effect as a generalized electro-convulsive shock. We are trying 

 to look for a place where we can, so to speak, jam the network by excessive activity. 



Adey. I was very interested in Dr Olds's observations of the role of the anterior 

 hippocampus in this particular type of activity. I wonder whether he has also 

 examined the ventral parts of the hippocampus arch and whether he has come to 

 anv conclusions about the relative significance of the more ventral zones ot the 

 hippocampus as opposed to the dorsal. My reason for asking this is primarily that 

 we have studied quite extensively the course of the normal wave discharges in the 

 hippocampus in the course of various learning procedures. I would mention that 

 there does not appear to be a homogeneity of the whole aspect of the hippocampus 

 in this regard, and that, briefly, the dorsal hippocampus and the entorhinal cortex 

 are regions which appear to be particularly concerned in the type of behaviour in 

 which we are interested, to the point where one might summarize by saying that 

 the region between the hippocampal pyramidal cells and the dentate gyrus, where 

 Dr Olds mentions this extreme sensibility to seizure, this region and the adjacent 

 entorhinal cortex appear to be concerned in what we might term the execution of 

 planned behaviour, basing that opinion on certain very regular slow-wave dis- 

 charges. If I might raise one further point, that is the question of these curious 

 polyphasic discharges which Dr Olds saw when he was recording from the vicinity 

 of the dentate gyrus. I wonder whether he may not be seeing in fact a muscle 

 discharge if his microelectrode recording set-up is the typical one with the mono- 

 polar method of recording. The absence of the normal muscular paralysing agents 

 will very often produce volume conducted muscular effects that may be extremely 

 confusing in the interpretation of the micro-electrode recording. 



Olds. In all cases that we have reported, in which are found this special pheno- 

 mena of peculiar effects immediately after the stimulus we have very careful 

 control showing that we do not get these effects except when we apply the stimulus 

 immediately after the unit response. In all other respects, I assure you this looks like 

 artefact. And it is only because it has this physiological significance, namely that the 

 proximal unit has to fire before we apply our stimulus in order for us to see this 

 response, it is only for that reason that I even mention it. 



Let me go back to following the hippocampus around to its posterior arch. To 

 me it looks like the hippocampus all the way around has the dentate gyrus facing it. 

 I did show three sagittal sections in my large map and if you had looked carefully 

 you would find that in all cases if the electrodes w^re placed just above the arch of 

 the hippocampus proper, we got the rather total inhibitory effect without self- 

 stimulation. Finally, as to your notion that the dentate might be involved particu- 



