Memories are funny, aren't they? You can be going along, minding your business, and then, all of a sudden, a memory pops into your head. Sometimes memories are triggered （触发） by a song that reminds you of the setting where you first heard it. Or it may be a smell that makes you think of a delicious meal with your friends or family. Perhaps hearing kids arguing makes you remember an unhappy situation with childhood friends. These spontaneous（自发的） memories can he "sweet," or they might be "sour." Did you ever wonder what's going on in your brain that makes your memories produce positive or negative emotions?
Researchers at the Salk Institute for Biologica Studies in California have made a discovery that could help to uncover this secret. The explanation is a bit technical （for those of us who are not brain scientists）, but basically it all focuses on a single molecule（分子）called neurotensin（神经降压素）. Our surrounding environment is constantly presenting our brain with stimuli（刺激）. The molecule neurotensin acts as a key indicator of whether an experience is labeled as positive or negative, depending on the nature of the stimuli. Our brain subsequently associates similar stimuli with the pleasant or painful memory that we experienced.
The study by the Salk Institute was published in the journal Nature. The focus of their experiments was the brain circuitry of laboratory mice. After a sound was played, the mice received either a pleasant swallow of sugary water or, an unpleasant electrical shock or puff of air. The researchers discovered that neurotensin was the signal that associated the sound with reward or with punishment When the gene（基因） that regulates neurotensin was removed from the mice's brains, they had more difficulty connecting the sound with the sugary water. However, the negative responses were enhanced.
Understanding how neurons in the brain respond to neurotensin could have influences on one's mental health. Addictions may be caused by overly high levels of neurotensin （connected with positive rewards）. On the other hand, depression and anxiety can result when decreased levels of neurotensin allow negative. responses to dominate. Thus, regulating neurotensin could. help to maintain mental stability. One other thing: The research study showed that the brain may tend to be a pessimist, focusing on fearful memories that cause negative emotional responses.
Perhaps insights into neurotensin will help us have fewer sour memories and more sweet ones.