Around the central core of the brain is the limbic system, a set of structures that are closely interconnected with the hypothalamus and appear to impose additional controls over some of the instinctive behaviours regulated by the hypothalamus and the brain stem. One part of the limbic system, the hippocampus, plays a special role in memory. The effects of surgical removal of the hippocampus have demonstrated that it plays a critical role in storing new events as lasting memories, yet is not necessary for the retrieval of older memories. The limbic system is also involved in emotional behaviour. Monkeys in which some regions of the limbic system are damaged react with rage at the slightest provocation, suggesting that the destroyed area normally has an inhibiting influence. Monkeys with damage to other areas of the limbic system no longer express aggressive behaviour and show no hostility, even when attacked. They simply ignore the attacker and act as if nothing had happened.