The neurobiology that underpins these processes has received much research interest. One particularly influential series of studies in non-human primates indicated that midbrain dopamine cells initially fired in response to a novel reward. After repeated exposure, the neurons stopped firing during predictable reward delivery and instead fired when they were exposed to stimuli that were predictive of the reward.57 A new reward—and subsequently the cues associated with the reward—triggered phasic dopamine cell firing and activation of dopamine D1 receptors, which are necessary for conditioning to occur (table 2, circuits 1–4).58 This process allows previously neutral stimuli to become endowed with incentive salience and strengthens the learned association with repeated exposure to the cues, which creates strong motivation to seek a reward.
