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The ventral striatum is a conglomeration of several brain areas. It consists of the main part of the olfactory tubercle, a multi-sensory processing centreYamaguchi, M. (2017). Functional sub-circuits of the olfactory system viewed from the olfactory bulb and the olfactory tubercle. Frontiers in Neuroanatomy, 11, 33., and the nucleus accumbens, which is innervated with dopamine when we experience reward and reinforcementUbeda-Bañon, I., Novejarque, A., Mohedano-Moriano, A., Pro-Sistiaga, P., de la Rosa-Prieto, C., Insausti, R., ... & Martinez-Marcos, A. (2007). Projections from the posterolateral olfactory amygdala to the ventral striatum: neural basis for reinforcing properties of chemical stimuli. BMC neuroscience, 8(1), 1-10.. It also encompasses the continuity between the caudate nucleus and putamen, ventral to the rostral internal capsuleHaber, S. N., & McFARLAND, N. R. (1999). The concept of the ventral striatum in nonhuman primates. Annals of the New York Academy of Sciences, 877(1), 33-48.. The internal capsule, a white matter tract, is overlaid with strands of grey matter between the caudate and putamen, creating a striped appearance; hence the brain region receiving the Latin name 'striatum', meaning 'striped'.

The ventral striatum receives afferent projections from various brain regions, including;

• Topographic cortico-striatal projections from the cerebral cortex, primarily from the orbital prefrontal cortexHaber, S. N. (2011). 11 neuroanatomy of reward: A view from the ventral striatum. Neurobiology of sensation and reward, 235.
• The brain stem, particularly midbrain dopaminergic cells from the ventral tegmental areaHaber, S. N. (2011). 11 neuroanatomy of reward: A view from the ventral striatum. Neurobiology of sensation and reward, 235.
• The thalamusGiménez‐Amaya, J. M., McFarland, N. R., De Las Heras, S., & Haber, S. N. (1995). Organization of thalamic projections to the ventral striatum in the primate. Journal of Comparative Neurology, 354(1), 127-149.
• The basolateral amygdala, which encodes emotional meaning of environmental stimuli and helps provide contextual information to inform motivational engagementFudge, J. L., Kunishio, K., Walsh, P., Richard, C., & Haber, S. N. (2002). Amygdaloid projections to ventromedial striatal subterritories in the primate. Neuroscience, 110(2), 257-275.
• The hippocampal formation, which consults relevant prior memories of events and stimuli, which helps inform both decision-making and regulates responses to rewardsFriedman, D. P., Aggleton, J. P., & Saunders, R. C. (2002). Comparison of hippocampal, amygdala, and perirhinal projections to the nucleus accumbens: combined anterograde and retrograde tracing study in the Macaque brain. Journal of Comparative Neurology, 450(4), 345-365.

Efferent projections from the ventral striatum project to the lateral hypothalamus, ventral pallidum, globus pallidus and the substantia nigraHaber, S. N., & McFARLAND, N. R. (1999). The concept of the ventral striatum in nonhuman primates. Annals of the New York Academy of Sciences, 877(1), 33-48.

The ventral striatum contains smaller and more densely packed dopaminergic neurons compared to its dorsal counterpartHaber, S. N. (2017). Anatomy and connectivity of the reward circuit. In Decision neuroscience (pp. 3-19). Academic Press..

Dopamine neurons play a key role in the reward circuit, also known as the mesolimbic pathwayWise, R. A. (2002). Brain reward circuitry: insights from unsensed incentives. Neuron, 36(2), 229-240.. The mesolimbic pathway is a dopaminergic projection from the ventral tegmental area to the nucleus accumbens of the ventral striatumTelzer, E. H. (2016). Dopaminergic reward sensitivity can promote adolescent health: A new perspective on the mechanism of ventral striatum activation. Developmental cognitive neuroscience, 17, 57-67.. The release of dopamine to the nucleus accumbens regulates motivational cognition, incentive salience, and reinforcement learningHauser, T. U., Eldar, E., & Dolan, R. J. (2017). Separate mesocortical and mesolimbic pathways encode effort and reward learning signals. Proceedings of the National Academy of Sciences, 114(35), E7395-E7404..

File:Mesocorticolimbic Circuit.png

The nucleus accumbens has been found to respond to the anticipation of positive gains, which is additionally correlated to self-reported predictions of financial or object gainsKnutson, B., Fong, G. W., Bennett, S. M., Adams, C. M., & Hommer, D. (2003). A region of mesial prefrontal cortex tracks monetarily rewarding outcomes: characterization with rapid event-related fMRI. Neuroimage, 18(2), 263-272.. Anticipation of loss has been found to be processed elsewhere, in the insulaPaulus, M. P., & Stein, M. B. (2006). An insular view of anxiety. Biological psychiatry, 60(4), 383-387., suggesting that specifically the anticipation of positive gains is processed within the nucleus accumbens. This neural activity can also act as a predictor of an individual’s purchasing decisions; willingness to pay for a product has been found to correlate with activation in the nucleus accumbensKnutson, B., Rick, S., Wimmer, G. E., Prelec, D., & Loewenstein, G. (2007). Neural predictors of purchases. Neuron, 53(1), 147-156..

Functional magnetic resonance imaging evidence suggests observed differences in neural activity in the ventral striatum between participants can be used to predict an individual’s donation behaviours in charity-giving gamesHarbaugh, W. T., Mayr, U., & Burghart, D. R. (2007). Neural responses to taxation and voluntary giving reveal motives for charitable donations. Science, 316(5831), 1622-1625.. Participants with larger ventral striatum activation responses when giving money to charity were more likely to give larger amounts of money to charity, even at their own financial expense. Altruistic behaviour was directly correlated to high neural responses in the ventral striatum.

In the Prisoner’s Dilemma paradigm, individuals demonstrate increased neural activation in both the orbitofrontal cortex and the anteroventral striatum when reciprocating co-operative behaviour or experiencing mutual co-operationRilling, J. K., Gutman, D. A., Zeh, T. R., Pagnoni, G., Berns, G. S., & Kilts, C. D. (2002). A neural basis for social cooperation. Neuron, 35(2), 395-405.. Anteroventral striatum activity was specifically activated for conspecific interactions and did not occur in trials vs computer opponents. Neural activity was also compared to a control condition where participants received a free reward, and found the act of co-operation significantly enhanced these reward processing regions. The study suggests we feel more rewarded when obtaining rewards via mutually co-operative social interactions.

More ventral striatum activity was correlated with an increased likelihood of continued co-operation, and deactivation of the anteroventral striatum occurred in both players when one of the players stopped co-operating.

The ability to determine who will reciprocate co-operation is vital in social interaction, and informs our decisions of whether to interact or avoid someoneCosmides, L., & Tooby, J. (2000). 87 The Cognitive Neuroscience of Social Reasoning.. A study investigating the neural activity of this evaluation process found that activity in the amygdala, extending anteriorly into the ventral striatum (the right putamen and nucleus accumbens), is significantly increased when participants view the faces of those who had previously co-operated in social dilemma gamesSinger, T., Kiebel, S. J., Winston, J. S., Dolan, R. J., & Frith, C. D. (2004). Brain responses to the acquired moral status of faces. Neuron, 41(4), 653-662.

Neural alterations to ventral striatal circuitry form the basis of addictive disordersVolkow, N. D., & Morales, M. (2015). The brain on drugs: from reward to addiction. Cell, 162(4), 712-725.. Over-expression of DeltaFosB (a splice variant of protein FosB) can be found in the D1-receptor neurons of the ventral striatumRobison, A. J., & Nestler, E. J. (2011). Transcriptional and epigenetic mechanisms of addiction. Nature reviews neuroscience, 12(11), 623-637..

DeltaFosB is the most significant bio-molecular mechanism in addiction, as it causes alterations in gene expression in mesolimbic and mesocortical pathways. As a transcription factor, DeltaFosB transcribes genetic information in a cell from DNA into RNA, which ribosomes then translate into proteins. Over-expression of DeltaFosB in the nucleus accumbens causes over-production of GluR2 within the dopamine neuron. This temporarily stimulates the neuron and increases the sensitivity to the rewarding effects of addictive stimuli, particularly drug stimulants. GluR2 binds to AMPA receptors on the neuron, which are responsible for excitatory synaptic transmission. When AMPA receptors are blocked, it reduces calcium ion permeability and reduces the overall excitability in the D1-receptor neuronZhang, Y., Crofton, E. J., Li, D., Lobo, M. K., Fan, X., Nestler, E. J., & Green, T. A. (2014). Overexpression of DeltaFosB in nucleus accumbens mimics the protective addiction phenotype, but not the protective depression phenotype of environmental enrichment. Frontiers in behavioral neuroscience, 8, 297.. This results in the dopamine neuron eliciting reduced neuronal firing in the absence of the addictive stimuli.

Over-expression of DeltaFosB in the nucleus accumbens is responsible for the behavioural effects seen in addictive behaviours (such as administering drugs or seeking addictive stimuli)Robison, A. J., & Nestler, E. J. (2011). Transcriptional and epigenetic mechanisms of addiction. Nature reviews neuroscience, 12(11), 623-637..

DeltaFosB is increasingly expressed in the nucleus accumbens when individuals repeatedly overdose on addictive drugs, or over-expose themselves to addictive stimuli.

Studies investigating the pre-frontal striatal pathways in nicotine addiction have demonstrated that as nicotine cravings decrease, activity in the dorso-lateral prefrontal cortex increases (which reflects control over habits and behaviour, via the deployment of cognitive strategies to regulate cravings), which in turn exerts a decrease in activity on the ventral striatumKober, H., Mende-Siedlecki, P., Kross, E. F., Weber, J., Mischel, W., Hart, C. L., & Ochsner, K. N. (2010). Prefrontal–striatal pathway underlies cognitive regulation of craving. Proceedings of the National Academy of Sciences, 107(33), 14811-14816..

In rhesus macaques, lesions of the ventral striatum impaired learning of how to obtain rewards through selecting imagesRothenhoefer, K. M., Costa, V. D., Bartolo, R., Vicario-Feliciano, R., Murray, E. A., & Averbeck, B. B. (2017). Effects of ventral striatum lesions on stimulus-based versus action-based reinforcement learning. Journal of Neuroscience, 37(29), 6902-6914.. However, the ability to select rewarding actions remained intact, suggesting the ventral striatum is primarily involved specifically in reinforcement learning.

Individuals with focal lesions affecting the ventral striatum demonstrated significant impairment in recognising signals of anger and aggression in others, compared to individuals with more dorsal basal ganglia lesionsCalder, A. J., Keane, J., Lawrence, A. D., & Manes, F. (2004). Impaired recognition of anger following damage to the ventral striatum. Brain, 127(9), 1958-1969.. It is theorised that this failure to identify aggression reflects a deficiency in understanding behaviours that aim to procure, or protect, valued and/or contested resources.

Neuroimaging of individuals with obsessive-compulsive disorder found that increased connectivity of the ventral striatum, amygdala and ventromedial prefrontal cortex was correlated with aggressive symptoms, whilst increased connectivity in the ventral striatum and insula was indicative of intrusive sexual thoughtsNakao, T., Okada, K., & Kanba, S. (2014). Neurobiological model of obsessive–compulsive disorder: Evidence from recent neuropsychological and neuroimaging findings. Psychiatry and clinical neurosciences, 68(8), 587-605..