The limbic system is part of the brain that includes structures such as the amygdala, hypothalamus, and hippocampus that are implicated in reactive aggression. Reactive aggression is a response to a perceived threat, as opposed to proactive aggression, which is a response in anticipation of a reward. The limbic system also connects to the cingulate gyrus, which is responsible for focusing attention on emotionally significant events. In addition, the limbic system has connections to the prefrontal cortex which is involved in forward planning and anticipation of reward. The limbic system plays a key role in how an organism responds to environmental threats and challenges and thus is believed to be the key factor in whether we respond aggressively or not to an external stimulus.