Motivation and emotion/Book/2022/Reward system, motivation, and emotion

Overview

 * Discovering the reward centre

Understanding of the reward system first came from an experiment by Olds and Milner (1954) conducted on rats. They placed rats in a Skinner box (see Figure 1) with a lever that was connected to electrodes in their brain. They discovered areas of the brain that were considered rewarding or punishing based on how often the rats would push the lever. They found that rats would push the lever up to 2000 times per hour to receive this stimulation when it was connected to the septal region of the brain and later on the nucleus accumbens (Olds, 1956). Olds and Milner had discovered what is now known as the reward centre of the brain.

The reward system is an organisation of pathways in the brain responsible for motivational drive, positive emotions, and associative learning (Schultz, 2000). When a behaviour, stimulus, or event is rewarding, the reward system generates positive learning, elicits approach behaviour and results in positive emotions, such as pleasure and desire (Schultz, 2017). The reward system is intricately tied to both motivation and emotion, creating the neuronal signatures for positive emotions and motivational action.

Motivation is the state of wanting, an internal process that produces goal-directed effort (Baumeister, 2016). Emotions are a response to stimuli that involve physiological arousal, subjective feelings, a motivational purpose and expressive behaviours (Reeve, 2018, pp. 9). Emotions can be considered a subset of motivation, with emotions being one type of motive. Emotion researchers often conceptualise emotion as motivational states aimed at initiating behaviour (DeSteno et al., 2004). Emotion provides feedback for the motivational system, showing how well or poorly the behaviour is going.

The reward system is a group of subcortical brain structures communicating through the dopamine network (Reeve, 2018, pp. 53). Dopamine is the major neurotransmitter impacting the reward system, and heavily involved in motivation and emotion. Communication occurs through several dopaminergic pathways in the reward system, leading to emotional valence and goal-directed behaviour (Hauser et al., 2017). The prefrontal cortex forms part of this network, with top-down cognitive effects, like expectation, influencing the reward system. The reward system forms the beginning of positively-valanced emotions through the release of dopamine. It forms a feedback loop, known as the pleasure cycle, consisting of wanting, liking, and learning, and leads to motivated action. While the reward system is most often an adaptive mechanism, it can become dysfunctional, leading to a range of mental and behavioural disorders. Knowing how the reward system informs our emotions and behaviour provides a blueprint for maintaining a functional reward system.


 * What parts of the brain make up the reward system?
 * What role does the reward centre play in motivation and emotion?
 * What are the consequences of a dysfunctional reward system?

Reward system
The reward system is made up of brain structures, the neurotransmitter dopamine, and the networks they communicate through. Some of the parts play vital roles and are associated with the reward system like the ventral striatum and nucleus accumbens. The amygdala and hypothalamus play more of a supporting role, yet are vital to the proper functioning of the system. Dopamine is central to the reward system, with anything rewarding triggering a release of dopamine in the brain (Sabatinelli et al., 2007). The reward system is often named the dopaminergic system, with the main pathways in the reward system being labelled the dopaminergic pathways. All of the brain areas associated with the reward system play a role in either synthesising, encoding, or passing along the message dopamine has produced.

Neuroanatomy


The reward system begins in the ventral tegmental area (VTA) where dopamine is manufactured. Dopamine is released along several dopaminergic pathways to cortical and limbic areas of the brain (see figure 2) (Love, 2014). The nucleus accumbens is a vital part of the reward system, with dopamine release in this area resulting in the experience of pleasure (Peciña & Berridge, 2005). In the prefrontal cortex the conscious experience of reward is felt, and in the orbitofrontal cortex the learned value of a reward is stored for the future (Reeve, 2018, pp. 53). The basal ganglia receives excitatory signals from the VTA, and then initiates motivated action through the supplemental and presupplemental motor areas (Reeve, 2018, pp. 53).

Table 1. Anatomy of the reward system (Table adapted from Reeve, 2018, pp. 55)

Major dopaminergic pathways


The reward system contains four major dopaminergic pathways (see Figure 3). Each pathway has a unique function, with dopaminergic neurons being sent along each to influence emotion, motivation, learning and behaviour. The first two are classified under the mesocorticolimbic pathway, and are directly related to the feeling of reward. The other two pathways are still dopaminergic, but are indirectly related to the reward system.

Mesolimbic pathway
The mesolimbic pathway is the core of the reward system. The pathway starts in the VTA with the manufacturing of dopamine which is then projected into the ventral striatum, where the nucleus accumbens and olfactory tube are situated. In the nucleus accumbens the biology of reward is created, and the experience of liking and pleasure occur (Reeve, 2018, pp. 53). Activation of this area causes dopamine levels to rise, increasing the motivation to repeat the behaviour that triggered the initial dopamine release. The nucleus accumbens is connected with the amygdala, hippocampus, hypothalamus, and other limbic structures that help evaluate, respond and learn from the stimulus and associated rewards.

Mesocortical pathway
The mesocortical pathway also begins in the VTA and projects into the cortical area of the brain. This pathway connects to the frontal lobes of the brain and is responsible for the conscious experience of pleasure (Hauser et al., 2017). It is important for the normal functioning of the dorsolateral prefrontal cortex (Bidwell et al., 2011). The cortical part of the brain is then involved in cognitive control, motivation, and emotional response.

Nigrostriatal pathway
This pathway starts in the substantia nigra and projects to the caudate and putamen in the basal ganglia. This pathway is responsible for planned motor movements. Degeneration of neurons in the substantia nigra is implicated in Parkinson's disease, leading to a reduction in dopamine production and the associated motor deficits (Kordower et al., 2013).

Tuberoinfundibular pathway
Dopamine neurons in this pathway project from the arcuate nucleus to the infundibular region of the hypothalamus (Gudelsky, 1981). Dopamine release in this part of the brain inhibits the pituitary gland from secreting the hormone prolactin, responsible for the production of milk, and is involved in sexual satisfaction, the immune system, and metabolism (Freeman et al., 2000).

Expectations matter
Why does new years eve always seem to be a flop, while a quiet Tuesday evening can turn into an incredible night? Why can the exact same piece of chocolate be more or less rewarding on separate occasions? The stimulus we experience does not have an exact correlation with the level of reward or pleasure that we experience. The level of reward we experience is related to our expectations about a given stimulus or event. This phenomenon is known as reward prediction error (Montague et al., 1996).


 * Reward prediction error

Reward prediction errors are the result of the difference between the rewards we predicted and the rewards we actually receive. Positive prediction error occurs when the reward we experience is better than expected. Neutral prediction error is when expectation matches up with the actual reward. Negative prediction error occurs when the reward is less than what was expected (Schultz, 2022).

Reward prediction error is the result of a top-down effect coming from the prefrontal cortex. Dopamine in the VTA increases when events turn out better than expected, and decreases when they do not meet expectations, regardless of whether the stimulus would be considered positive. This explains why the new years eve party, with high expectations, often doesn't live up to expectations, resulting in a disappointing night. While the quiet Tuesday evening, requires a far smaller reward to surpass people's expectations, and may result in a memorable night.

The anticipation of a rewarding event is the trigger for dopamine release in the VTA. A second release occurs at the time of the event, but this is less substantial than the first release (Reeve, 2018, pp. 54). This is why the anticipation of the upcoming holiday is often more rewarding than the actual holiday, or why the anticipation of the chocolate bar is more rewarding than the experience of eating it. For this reason, dopamine release is highest when rewarding events are surprising and we have not had the chance to anticipate them. Although, anticipated events can still be rewarding and result in larger amounts of dopamine release if they go better than expected (Mirenowicz & Schultz, 1994).

 {Which brain structure manufactures and initiates dopamine release? - Nucleus accumbens - Ventral striatum + Ventral tegmental area - Basal ganglia - Mesolimbic pathway
 * type=""}

{Positive prediction error occurs when the actual reward was greater than predicted, and results in increased dopamine release. + True - False
 * type=""}

The reward system's role in motivation and emotion
The reward system is fundamental to the distribution of dopamine, leading to the experience of positive emotions and driving motivated behaviour (Reeve, 2018, pp. 53-55). The amount of dopamine released in the VTA is proportionate to the intensity of positive emotion, motivation, learning, and future effort exertion (Reeve, 2018, pp. 53). This demonstrates the role of dopamine as the beginning of positive emotions, specifically the basic emotions of joy and interest, along with other secondary emotions (Reeve, 2018, pp. 340). The learning encoded from this pleasurable experience leads to a "pleasure cycle" involving three mechanisms driving motivational behaviour. Areas of the reward system are also responsible for carrying out or inhibiting motivated behaviour.

Beginning of positive emotions
A dopamine release in the VTA is the precursor for some positively valenced emotions, particularly those that lead us towards future motivated behaviour. However, there are several distinct constructs in the hedonic brain network with dopamine appearing to be more important for incentive salience (see below) and reward-based learning, and play less of a role in "liking", which translates to the hedonic experience of pleasure (Alexander et al., 2021). While there is a correlation between the amount of dopamine released in the VTA and levels of positive emotions, the state of liking is not dependant on dopamine, and it appears dopamine and the reward system are more directly related to motivated states and actions, and positive emotions help influence the levels of desire and approach behaviour (Berridge & Robinson, 2016). It may be more helpful to view the dopaminergic pathway related to reward and motivation, and the hedonic brain structures involved in the experience of pleasure as separate but interdependent constructs. Taking this view, rewards can influence the hedonic areas of the brain and lead to positive emotions such as joy, and similarly, the absence of a reward can lead to negative emotions like disappointment and anger (Sander & Nummenmaa, 2021).

Incentive salience is a desire for a reward, characterised by a physiological state of wanting that is formed in the mesolimbic system, and based on learned associations about a reward cue (Hyman 2005).
 * Incentive salience

Pleasure cycle


The reward system, made up of the dopaminergic pathways, can be broken up into a three part feedback loop known as the pleasure cycle (see Figure 4). The pleasure cycle consists of three motivational states: wanting, liking, and learning (Kringelbach & Berridge, 2017). Each state is accompanied by a behaviour that helps to achieve its goal. Wanting is a motivational state arising from a need for something, and results in the searching for this reward (Reeve, 2018, pp. 56). The next phase in the pleasure cycle, "liking", which involves consuming or experiencing the reward, and can be considered a motivational state that comes from experiencing pleasure (Berridge & Robinson, 1995). The final stage is "learning", which is accompanied by the feeling of satiation and is what contributes to the motivational state of wanting again, either immediately or sometime in the future (Reeve, 2018, pp. 56). This forms a feedback loop that reinforces itself over time.

The first experience of liking leads into the satiety phase, with strong learning, and subsequently increased wanting and seeking out of the experience in the future. Each stage relies on different structures in the brain, with wanting occurring mostly within the mesolimbic pathway, liking spread across the orbitofrontal cortex, nucleus accumbens, insula, and ventral pallidum, and the learning occurring in the orbitofrontal cortex and prefrontal cortex (Kringelbach & Berridge, 2017). When the pleasure cycle is disrupted and the wanting and liking aspects diverge, this can lead to addiction (Reeve, 2018, pp. 56).

Motivated action
The basal ganglia can be considered part of the reward system, as it takes the final steps in transferring motivated and emotional states into action. Increases in dopamine to the VTA and ventral striatum (part of the basal ganglia) induces incentive salience, whereas decreases in dopamine, particularly in the ventral striatum, reduces the desire to approach (Ikemoto et al., 2016). Once the motivation to take action has been established, the basal ganglia receives information from cortical areas of the brain in the form of action plans. Information is projected from the caudate nucleus, substantia nigra, putamen, and globus pallidus to various areas in the basal ganglia, known as the pre-supplemental, supplemental, and motor areas (Reeve, 2018, pp. 55). It is then the role of the basal ganglia to excite or inhibit the action plans received. (Pessiglione et al., 2007).

 {The state of "wanting" occurs predominantly in the mesolimbic pathway + True - False
 * type=""}

{The area of the brain that is responsible for carrying out motivated behaviours is the: - Prefrontal cortex + Basal ganglia - Ventral tegmental area - Nucleus accumbens - Mesocortical pathway
 * type=""}

Hijacked reward system
A dysfunctional reward system has been implicated in several mental disorders, including mood disorders, attention-deficit hyperactivity disorder, Parkinson's disease, and addiction (Taber et al., 2012). The reward system plays an important role in healthy motivational and emotional states. When these pathways and neurotransmitters in the brain are disrupted from normal functioning it has negative effects on individuals, leading to problems with both mood and behaviour.



Addiction
Using the pleasure cycle as a guide, a functioning reward system has both liking and wanting in tune with the other (Reeve, 2018, pp.56). Addiction occurs when wanting occurs in the absence of liking (see Figure 5), leading addicted individuals to compulsively consume without receiving any pleasure. This can be explained by reward prediction error, with the expected reward not meeting expectations of the chronic user. They produce a smaller dopamine release than previous uses and even a smaller release than when thinking about taking a substance (Reeve, 2018, pp. 54). At the same time wanting increases as the nucleus accumbens becomes highly sensitive to dopamine stimulation (Di Chiara, 1998). Another explanation for the loss of liking is due to the downregulation of dopamine receptors that occurs after chronic substance use (Volkow et al., 2010). Despite the blunting of areas in the reward system, the high levels of desire and impulsivity can be explained by the dysregulation of pathways from the prefrontal cortex and amygdala (Volkow et al., 2010).

A supernormal stimulus is a stimulus that produces a stronger effect than the stimulus for which the response mechanism evolved (Barnett, 2021). Supernormal stimuli can produce heightened levels of dopamine release in the brain, making us more susceptible to addiction. An example of supernormal stimuli are refined sugars, which were not part of a human diet until very recently, leaving us exposed to higher levels of reward and increased risk of addiction (Lenoir et al., 2007). Other examples include porn, cocaine, and the internet.
 * Supernormal stimuli

 {A sign of addiction is the loss of 'wanting' in the pleasure cycle. - True + False
 * type=""}

Conclusion
The reward system is responsible for much of the neurochemistry behind the emotions, motivations, and behaviours of daily life. The reward system produces various motivations and emotions through a number of structures in the brain communicating with dopamine along a series of pathways starting in the VTA. Communicating along these pathways, the reward system produces the biology and conscious experience of reward. It turns motivations into actions, processes and learns the reward-value of events and stimuli. The reward system releasing dopamine is the beginning of the positive emotions of joy and interest, and produces incentive salience, and the desire to seek out more of a given reward. The feedback loop, known as the pleasure cycle, occurs within the reward system and reinforces the reward value of a given event or stimulus. The reward system also turns incentive salience into motivated action by sending action plans to the basal ganglia. Disruptions along the reward system can result in adverse mood-related and behavioural problems, including several mental disorders and various forms of addiction. In summary, the reward system is a network that plays a vital role in the experience of pleasure and motivating us to seek out things that are rewarding.