The previous studies were comparisons between experience-enriched animals and animals maintained in isolation. Animals which were housed in colonies, but given no special treatment, showed intermediate effects in those situations studied. The Berkeley group emphasized that the finding of changes in the brain subsequent to experience does not prove that the changes have anything to do with memory storage, but do establish the fact that the brain can respond to environmental pressure. However, the results are compatible with the hypothesis that long-term memory storage involves the formation of new somatic connections among neurones. Calculations of the amount of additional material required to permit this to exist are compatible with the increases observed. A number of investigators have studied the effects of antimetabolites and drugs on the learning process. Since their specific metabolic effects are known in other tissues, the rationale is that if these materials do interfere with memory, then specific types of metabolic activities may be implicated in the deposition of the engram. One of the initial studies of this type was conducted by Dingman and Sporn ([ref.122]), presently at the National Institute of Mental Health. They showed that 8-azaguanine, a purine antagonist, injected intra-cisternally was incorporated into the RNA of the brains of rats. Associated with this incorporation was an impairment of the maze-learning ability of the animals. These findings have been confirmed. Flexner and his associates injected puromycin, an inhibitor of protein synthesis, into the brains of mice, which were then trained to perform in a maze. Losses of short-term or long-term memory were obtained, depending upon the site of the injection. The results indicate that the hippocampal region is the site of recent memory. The hippocampal region is of interest in connection with memory processes for a number of other reasons. Adey et al. ([ref.123]) and his group observed a transient fall in electrical impedance in this region when cats learned to perform in a T-maze in response to a visual cue. It was supposed that the electrodes were situated within glial cells of the dendritic zone of the hippocampal pyramidal cell layer. Extinction of the learned habit abolished the briefly evoked impedance changes, which subsequently reappeared with retraining.