Where is the nucleus accumbens?
The nucleus accumbens is found in an area of the brain called the basal forebrain. There is a nucleus accumbens in each hemisphere; it is situated between the caudate and putamen. The nucleus accumbens is considered part of the basal ganglia and also is the main component of the ventral striatum. The nucleus accumbens itself is separated into two anatomical components: the shell and the core. These two contiguous areas have overlapping connections, but may make different contributions to the functions of the nucleus accumbens.
What is the nucleus accumbens and what does it do?
The most widely recognized function of the nucleus accumbens is its role in the "reward circuit" of the brain. When we do anything that is considered rewarding (e.g. eat food, have sex, take drugs), dopamine neurons (along with other types of neurons) in an area of the brain called the ventral tegmental area (VTA) are activated. These neurons project to the nucleus accumbens, and when they are activated it results in an increase in dopamine levels in the nucleus accumbens. The nucleus accumbens is an important component of a major dopaminergic pathway in the brain called the mesolimbic pathway, which is stimulated during rewarding experiences.
This association between rewarding experiences and dopamine levels in the nucleus accumbens initially caused many neuroscientists to believe the main role of the nucleus accumbens was in mediating reward. Thus, it is often implicated in addiction and the processes that lead to addiction. However, since the initial links were made between the nucleus accumbens and reward, it has been discovered that dopamine levels in the nucleus accumbens rise in response to both rewarding and aversive stimuli. This finding led to a re-evaluation of the functions of the nucleus accumbens, and indeed of the functions of dopamine as a neurotransmitter.
The most widely accepted perspective now is that dopamine levels don't rise only during rewarding experiences but instead rise anytime we experience something that can be deemed either positive or negative. Dopamine signaling may be involved with storing information about environmental stimuli associated with these different types of experiences. These memory stores can be called upon in the future to help us remember how to realize the pleasurable experiences again or how to avoid the aversive ones. In addictive processes, it may be that the connection between an initial pleasurable drug experience and the stimuli associated with it becomes too strong. This can foster compulsive drug-seeking after exposure to a related environmental cue (e.g. cigarette craving upon smelling cigarette smoke).
Neuroscientists are still trying to understand the exact role of the nucleus accumbens in these associative learning processes. At this point, however, it seems safe to say that the nucleus accumbens is an important brain area in forming memories involving salient environmental stimuli, both positive and negative.