Brain Reward System

The reward systems of the brain are a group of structures that are activated whenever we experience something rewarding, such as eating a nice-tasting food, having sex, or using an addictive drug.

Structures that are considered to be a part of this reward system are found along the primary dopamine pathways of the brain.

When exposed to a stimulus that is rewarding, the brain responds by releasing an increased amount of dopamine, the main neurotransmitter associated with rewards and pleasure.

Neurobiology of the Reward Pathway: From Perception to Behavioral Reinforcement

It’s worth noting that while the reward system reinforces beneficial behaviors (like eating when hungry), it can also lead to maladaptive behaviors when hijacked by drugs of abuse or in the context of disorders like addiction.

1. Perception of a Rewarding Stimulus:

The process initiates when the brain encounters a rewarding stimulus, whether it’s an external object or an internal thought.

This perception gets processed by various regions of the brain, including the hippocampus (which provides context based on past experiences) and the amygdala (which adds emotional weight to the stimulus).

2. Activation of the Ventral Tegmental Area (VTA):

Upon the recognition of the rewarding stimulus, the VTA is stimulated to produce and release dopamine. The exact mechanism can vary: for natural rewards, the VTA responds to signals from other parts of the brain.

For drugs of abuse, many directly increase dopamine production or release in the VTA.

3. Dopamine Transmission to the Nucleus Accumbens (NAc):

Once dopamine is released from the VTA, it travels through the mesolimbic pathway to the NAc.

In the NAc, dopamine binds to receptors on the surfaces of neurons, changing their activity. This change in neuronal activity is the underlying neurobiological event that translates to feelings of pleasure and reward.

4. Reinforcement Learning and Behavioral Response:

After the rewarding experience, the prefrontal cortex (which plays a role in decision-making and planning) assesses the entire event. It connects the pleasure from the NAc with the original stimulus and the action taken.

The stronger the pleasurable response in the NAc, the stronger the reinforcement signal sent to the prefrontal cortex and other areas responsible for memory and behavior.

As a result, the individual becomes more inclined to seek out or engage in that specific behavior or context in anticipation of the reward. Over time, through repeated exposures, this leads to learned behaviors or habits.

5. The Role of Feedback:

Continuous feedback loops exist between the NAc, VTA, and prefrontal cortex. These loops help refine and adjust behaviors to optimize reward-seeking actions over time.

For instance, if an anticipated reward doesn’t result in the expected pleasure, the feedback loop may reduce the motivation to pursue that specific reward in the future.

Reward pathways in the brain

Dopamine is mostly produced in an area of the brain called the ventral tegmental area (VTA), located within the midbrain.

Once produced in the VTA, dopamine is transported to other areas of the brain, through different pathways, the two main ones being the mesolimbic and the mesocorticol dopamine pathways.

The most important reward pathway in the brain is the mesolimbic dopamine pathway. When experiencing something rewarding, dopamine is activated in the VTA.

This neurotransmitter is then projected to an area called the nucleus accumbens via the mesolimbic pathway. The nucleus accumbens is an area found in the ventral striatum that is strongly associated with motivation and reward and is part of complex circuits involving the amygdala and the hippocampus.

The activation of the nucleus accumbens causes dopamine levels in this region to rise. Essentially, activation of the mesolimbic dopamine pathway tells us to repeat what just happened in order to feel the rewarding sensation.

Since the nucleus accumbens have circuits with the amygdala, a region of the limbic system associated with emotions, this attributes feelings towards the experienced reward.

For instance, after eating nice food, the amygdala contributes to our feelings of enjoyment and happiness as a result. Connections of the nucleus accumbens to the hippocampus, a region involved in memory, would help attribute memory and learning to the reward.

So, after eating nice food, our hippocampus would help with remembering where the food was purchased from and will remind us of our enjoyment of this food.

Therefore, all these brain areas work together to encourage the repetition of rewarding behaviors. The mesocortical dopamine pathway is another reward pathway but is less discussed.

This is also a pathway that stems from the VTA. During rewarding experiences, dopamine is activated in the VTA, which then travels from this region directly to the cerebral cortex, specifically the frontal lobes.

The frontal lobes are responsible for high cognitive function, thinking, planning, and motivation. Thus, the activation of this pathway brings about the conscious experience of the pleasure and reward being experienced.

Classic studies of desire and reward

In 1954, Olds and Milner completed experiments with rats to investigate which brain regions may be involved in rewards. They implanted electrodes at various points in the brains of the rats, which were then placed into a ‘ Skinner box.’

This contraption comprises a small chamber used to conduct conditioning research on animals, with a lever inside. When the rats would press the lever, they would receive a mild burst of electrical stimulation to their brains.

Their results indicated that there were various areas in the brain where the electrical stimulation is rewarding, so the rats will press the lever frequently to receive this rewarding sensation.

One of the rats in this experiment pressed the lever 7500 times in 12 hours to receive this electrical stimulation. The reward area which was most apparent when electrodes were placed there, was in the septal region, an area in the lower medial surface of the frontal lobe, with connections from the hippocampus, amygdala, and thalamus, among other areas.

Eventually, these rats would sometimes choose to receive the electrical stimulation than eat food.

In another experiment with more lenient conditions, the rats would eat enough food to thrive but would then spend the majority of the rest of their time excessively pressing the level for stimulation.

Other scientists were able to replicate similar findings to these in their experiments on primates and humans (Heath, 1972; Sem-Jacobsen, 1976).

Role of Dopamine

Dopamine is a neurotransmitter that is both excitatory and inhibitory in function, as well as a neuromodulator involved in reward, motivation, and addiction.

Dopamine binds to five subtypes of dopamine receptors: D1, D2, D3, D4, and D5. Dopamine binding to these receptors initiates a flow of signaling responsible for activating functions in the associated brain regions where each receptor type is most dominant.

In the reward pathways, specifically the mesolimbic pathway, dopamine is released during pleasurable experiences and binds to dopaminergic receptors located in the nucleus accumbens.

It was found in the previously mentioned experiments on rats that dopamine was activated during rewarding brain stimulation. With an influx of dopamine reaching the brain regions involved in reward and pleasurable feelings, the rats were likelier to continue seeking this stimulation.

The perspective on dopamine’s role has shifted slightly. Once believed to be the neurotransmitter responsible for directly causing pleasurable experiences, it is now thought to be involved with aspects of reward rather than the experience of enjoyment.

For instance, it has been suggested that dopamine is involved in encoding the memories associated with a reward, such as understanding how to achieve the experience again.

The importance of dopamine in these experiments could be determined as the scientists were able to measure the increased release of dopamine in the reward pathways after the rat received the reward.

They also discovered that they could prevent the rats from seeking brain stimulation by blocking dopamine release. This was done through either using dopamine-antagonizing drugs, which blocked the effects of dopamine, or through destroying the reward pathways.

After this, the rats no longer sought out brain stimulation. Through this experiment, they could identify the specific brain areas, as well as determine that dopamine was involved in the reward pathways.

Addiction

As indicated, dopamine is secreted by the brain during many activities that bring about pleasurable feelings and activates the dopamine reward pathways.

Drugs such as stimulants, opioids, ethanol, and nicotine trigger the release of more dopamine in the brain. Opioids and ethanol also increased the rate of cell firing.

These drugs will typically bind to brain receptors in regions associated with rewards, e.g., the VTA and nucleus accumbens. However, with drugs, they will force the brain to release a large amount of dopamine than would be released by healthy activities.

They will also prevent the brain from reabsorbing dopamine, making the pleasurable experience last unnaturally longer.

Overstimulating the reward systems in the brain can eventually result in addiction.

Addiction is the result of reinforcing or rewarding behaviors being carried out compulsively, despite any negative consequences, a main feature being that there is a loss of control over the amount of the addictive substance.

In a further study of rats, they were dispensed cocaine if they pushed a lever. The rats quickly learned to keep pushing the lever to get more cocaine and would also engage in drug-seeking behaviors and would increase their dosage if given the opportunity to.

Over time, with repeated use of addictive substances, the body becomes reliant on this to maintain rewarding feelings. A negative consequence is that although a lot of dopamine is stimulated around the brain and there are strong feelings of euphoria, this also causes serotonin levels to decrease.

Serotonin is an essential neurotransmitter associated with feelings of happiness. Due to addictive substances like drugs affecting serotonin levels, this can get to a point where everyday activities which an individual would have found pleasurable before do not bring them happiness anymore and could result in feeling very low in mood over the continued use of drugs.

This can also be because these activities do not produce the same amount of dopamine that they are now accustomed to.

Studies of desire and reward

As previously discussed, different reward regions and pathways are activated when experiencing a pleasurable stimulus. Researchers have since expanded on this research to look into whether the stimulus needs to actually be experienced for these reward centers to be activated or whether the anticipation of a reward triggers these areas.

Spreckelmeyer et al. (2009) used functional magnetic resonance imaging (fMRI) to investigate whether the anticipation of a monetary reward significantly affects the brain.

They found that through the fMRI data, there was activation of the neural structures relating to the reward system when anticipating a reward. It has also been found that the bigger the potential reward (the more money), the greater the brain activity in the reward areas.

These findings provide a better understanding of how reward systems work in the brain and that the brain areas can stimulate a reward response without experiencing a reward yet.

Sherman, Hernandez, Greenfield, and Dapretto (2018) investigated the neural structures that are activated in terms of social media rewards. The ‘Like’ option, which is prevalent on many social media platforms, is believed to give social rewards to those who receive them.

The researchers found this to be the case when the participants completed a task in an MRI scanner. This task was designed to mimic the social photo-sharing app Instagram.

When examining the neural correlates, there was activation in the brain’s reward circuits, especially in the VTA, when participants received Likes.

Similarly, they found that when the participants provided positive feedback (gave them Likes) through this task, this also activated the brain’s reward circuits.

References

Ekhtiari, H., & Paulus, M. (2016). Neuroscience for Addiction Medicine: From Prevention to Rehabilitation-Methods and Interventions. Elsevier.

Heath, R. G. (1972). Pleasure and brain activity in man: Deep and surface electroencephalograms during orgasm. Journal of Nervous and Mental Disease, 154, 3–18.

Olds, J., & Milner, P. (1954). Positive reinforcement produced by electrical stimulation of septal area and other regions of rat brain. Journal of Comparative and Physiological Psychology, 47 (6), 419–427

Sem- Jacobsen, C. W. (1976). Electrical stimulation and self- stimulation with chronic implanted electrodes: Interpretation and pitfalls of results. In A. Wauquier & E. T. Rolls (Eds.), Brain- stimulation reward (pp. 505–520). Amsterdam: Elsevier- North Holland.

Sherman, L. E., Hernandez, L. M., Greenfield, P. M., & Dapretto, M. (2018). What the brain ‘Likes’: neural correlates of providing feedback on social media. Social cognitive and affective neuroscience, 13(7), 699-707.

Spreckelmeyer, K. N., Krach, S., Kohls, G., Rademacher, L., Irmak, A., Konrad, K., Kircher, T. & Gründer, G. (2009). Anticipation of monetary and social reward differently activates mesolimbic brain structures in men and women. Social cognitive and affective neuroscience, 4 (2), 158-165.