Since the middle of the 20th century, neuroscientists have investigated the mechanisms of positive emotion in the brain and body. Before that time, positive emotions were regarded as too subjective for rigorous scientific study. But a better understanding of the brain chemicals known as neurotransmitters and increased ability to use technology to create images of the living brain opened new opportunities for study.
In the 1950s, psychologists identified a "pleasure center" in an area of the brain known as the nucleus accumbens. They found that laboratory animals would press a lever to deliver an electrical stimulus to their own brain's "pleasure center" repeatedly until they were exhausted—undeterred by hunger, thirst, or pain. When researchers stimulate the nucleus accumbens of people, they smile, laugh, and report feeling pleasure, happiness, or euphoria. Later, by mapping connected areas, the researchers identified a reward circuit in the brain that involves the prefrontal cortex (the thinking part of the brain) and several underlying areas, including the nucleus accumbens and the amygdala (which triggers emotions).
The brain responds to a pleasure stimulus by activating a reward system. When the brain receives a positive sensory stimulus (something that feels good), it sends a signal to the ventral tegmental area (VTA) in the midbrain. The VTA releases dopamine into the nucleus accumbens (the pleasure center) and into the septum, the amygdala (part of our emotional response system), and the prefrontal cortex (which is involved in thinking).