Our experiences can cause brain circuits to change and form new memories. For example, when we hear an unpleasant soun the amygdala heightens our perception of the sound. This heightened perception is deemed distressing and memories are formed associating the sound with unpleasantness. If the noise startles us, we have an automatic flight or fight response. This response involves the activation of the sympathetic division of the peripheral nervous system. Activation of the nerves of the sympathetic division results in accelerated heart rate, dilated pupils, increase in metabolic rate, and increase in blood flow to the muscles.