Motivation and emotion/Book/2023/Medial forebrain bundle, motivation, and emotion

Overview


Motivation and emotion are pivotal components to our behaviour, and how we perceive certain experiences, and achieve tasks and goals in our life. But what influences out motivation and emotion? How does biology and the mechanisms in our brain influence this? The medial forebrain bundle provides substantial insight into how the connection between the structures of our brain can influence our motivated behaviours and our emotional regulation.

This chapter discusses the mechanisms, theories, and pathways on how the medial forebrain plays a role on the function of motivation and emotion. Focussing on the reward system, functions of the brain, the connections and pathways in the brain, mood affective disorders, and electrical stimulations effect on these process through targeting the medial forebrain bundle.

Focus questions:


 * What is the function of the medial forebrain bundle?
 * What parts of the brain are associated with motivation and emotion?
 * How does the medial forebrain bundle have an influence on pathways in the brain? What are these pathways?
 * What treatments are used to influence the medial forebrain bundle?

Function
The medial forebrain bundle carries critical signals for fuelling motivated behaviours (MacNiven et al., 2020). As well as this the medial forebrain bundle plays an important role in motivation and reward, as well as functioning the centre role in human affective disorders (Cienan et al., 2018).

Principle components
The principal components of the medial forebrain bundle (MFB) consist of the ventral tegmental area, the basal forebrain, and the mesolimbic dopamine system (Koob et al., 2013). The ventral tegmental area (VTA), nucleus accumbens (NAcc), and prefrontal cortex (PFC) are essential components linked to initiating motivated behaviour and experiencing pleasure. These components are linked through the medial forebrain bundle (Bracht et al., 2022). Furthermore, there are two distinct tracts within the MFB. This includes the infero-medial branch (imMFB) and superolateral branch (sIMFB) (Sobstyl & Stapinska-Syniec, 2021).

Ventral tegmental area
The ventral tegmental area is in the midbrain and is responsible in {{awkward} controlling behaviours which include reward processing, stress modulation, aversion, and drug addiction (Cai & Tong, 2022). The VTA plays a significant role in reward and motivation (Bouarab et al., 2019).

Basal forebrain
The basal forebrain consists of the nucleus accumbens (NAcc), olfactory tubercle, frontal cortex, and amygdala (Koob et al., 2013). The nucleus accumbens plays a vital role in motivation and reward processing, dysfunction in the NAcc has been associated to substance abuse and depression (Chen et al., 2023). The frontal cortex mediates motivation to shape human behaviour and cognitive control, furthermore it is observed that the medial frontal areas are involved in motivational aspects of behaviour (Bahlmann et al., 2015). The prefrontal cortex, which is an area of the frontal cortex, plays a critical role in emotion regulation and the capacity to modulate emotions (Davidson, 2022). The amygdala has an important role in controlling motivational behaviours such as reward-related motivation (Dutta, 2021). The amygdala is also a major processing centre for emotions (Cleveland Clinic, 2023).

Mesolimbic dopamine system
The mesolimbic dopamine system is a central nervous system circuit component of the medial forebrain bundle which is the dopamine connection between the VTA and basal forebrain (Koob et al., 2013). Dopamine is a neurotransmitter that is responsible for allowing you to feel pleasure, motivation, and satisfaction (Health Direct, 2021). Dopaminergic inputs from the ventral tegmental area innervate brain regions including the prefrontal cortex, nucleus accumbens, and amygdala, which are involved in motivational functions and reward processing behaviour (Serafini et al., 2020; Mitchell et al., 2021).

Monoamine neurotransmitters
Jiang et al., (2022) discuss the "new three-dimensional model" which proposes that there are three monoamine neurotransmitters that mediate emotion. These neurotransmitters include dopamine, norepinephrine, and serotonin. Projections from the medial forebrain bundle plays a role in mediating monominergic pathways (Bertalan, 2021). Dopamine plays a pivotal role in the brains reward system process (Varanasi, 2022). When the brain is presented with rewarding stimuli, higher levels of dopamine are released. As well as this, dopamine carries signals which affect emotion. Dopamine can be referred to as the "happy hormone" as it is activated in the presence of pleasurable experiences (Charron, 2022). Norepinephrine is responsible for emotions including fear and anger which trigger the "flight or fight" mode (Wang et al., 2020). Norepinephrine has also been observed to play a key role in the interactions between, arousal, motivation, and relapse (Espana et al., 2016). Serotonins role in the body works to regulate mood and cognition. It functions as a natural mood stabiliser through reducing feelings of anxiety and depression (Kilburn, 2018). Therefore, when we have low levels of serotonin in our system individuals can tend to have low moods, increasing levels of anxiety or depression. When there are increased levels of serotonin in our system, this can affect motivation through being a neurotransmitter that mediates brain communication (Neuroscience, 2018).

Motivation in psychology
Motivation is referred to as the drive to achieve your goals or needs (Health Direct, 2022). When we are motivated, we are induced into our actions or thoughts by either the drive of a motive, or the gain of a reward or incentive toward the end state (Souders, 2019). Motivation is an essential behavioural commodity in individuals lives. This helps us to achieve our goals, desires, and access what we need. Understanding what drives motivation, and the key biological components, gives a better understanding to how this may be reduced in individuals and how it can be enhanced in behaviour. Three major components of motivation include activation, persistence, and intensity. Activations refers to our decision to make a behaviour, persistence means our continued effort towards a goal even when challenges occur, and intensity is our concentration and stamina that goes into achieving our goal (Cherry, 2023). Instinct theory of motivation views that biological and genetic programming to be the cause of motivation (Sincero, 2012). Motivation can be categorised into two motivational types, intrinsic and extrinsic. Intrinsic motivations means the motivation come from within and is driven by personal goals or rewards. Extrinsic motivation refers to external factors which drive us to achieve something eg. money (Waters, 2022).

Emotion in psychology
American Psychological Association (2022) define emotion as conscious mental reactions which are subjectively experienced as strong feelings which are usually directed toward a specific object and are typically accompanied by physiological and behavioural changes in the body. UWA (2019) describe the process of emotion being made up of three parts, subjective experience, physiological responses, and behavioural responses. Subjective experience refers to a stimulus or experience which provokes emotion for an individual, and these emotions which arise from the stimulus may vary for different individuals. Physiological responses are the autonomic nervous systems response to the emotion one may be experiencing, eg. your heart beats faster when you feel fear. Lastly, behavioural responses are the expressions of the emotion that one is feeling, this could be smiling when you are happy, or crying when you are sad (UWA, 2019).

The medial forebrain bundle on motivation
Medial forebrain lesions can decrease motivation, however electrical stimulation of the MFB can energise motivated behaviour (MacNiven et al., 2020). The medial forebrain bundle has been discussed to be confluent with the reward-SEEKING system (Coenen et al., 2012).

Reward system


The brains reward system controls motivation whether it is intrinsic motivation or extrinsic motivation, this is due to the reward system influencing the level of dopamine in the body which drives motivation (Waters, 2023). The medial forebrain connects the VTA and NAcc which work to produce motivational behaviours. The ventral tegmental area processes information to ensure the feeling of satisfaction; it then proceeds to forward this information to the nucleus accumbens using dopamine. When there is an increase of dopamine in the VTA, this concurrently increases the NAcc (Wilde, 2009). This process works through the medial forebrain bundle. These structures are activated in response to individuals anticipating or thinking about a reward, however they are not activated when individuals are enjoying the reward (Waters, 2023). Therefore, these structures and system could potentially produce motivating behaviours in individuals.

Key pathway
The reward system pathway (Guy-Evans, 2023):

1. Perception of a rewarding stimulus

The process of this system initiates when the brain encounters a rewarding stimulus. The amygdala also plays a role in this step as it adds emotional association to the stimulus

2. Activation of the ventral tegmental area (VTA)

Upon the perception of a rewarding stimulus, the VTA is stimulated to produce and release dopamine

3. Dopamine transmission to the nucleus accumbens (NAcc)

After dopamine is released from the VTA, it travels through the mesolimbic dopamine system to the NAcc. When dopamine reaches the NAcc, it binds to receptors of neurons. This changes neural activity which promotes the feelings of pleasure and reward

4.    Reinforcement learning and behavioural response

After the rewarding experience, the prefrontal cortex comes into play, assessing the event. The PFC associates the pleasure from the NAcc to the original stimulus.

The stronger the pleasurable response is from the NAcc, the stronger the reinforcement signal sent to the PFC is. Therefore, individuals may become more motivated to produce behaviours or tasks when they had a strong pleasurable response previously before.

5.    The role of feedback

Continuous feedback loops are present between the NAcc, VTA and PFC. These loops play a role in adjusting and refining behaviours that optimise reward-seeking actions. If an expected reward doesn’t result in the anticipated pleasure, this feedback loop can reduce motivation to pursue this reward again in the future.

Mood affective disorders
Mood disorders are mental health conditions that primarily affect individuals emotional state (Cleveland Clinic, 2023). Obsessive compulsive disorder (OCD) and depression are two mental health conditions (Lebow, 2022). These mood disorders influence ones emotional state or mood by being distorted or inconsistent, which interferes with ones ability to function (Mayo Clinic, 2021). The medial forebrain bundle plays a vital role in the interconnection and pathways of the brain which mediate emotion regulation. When these pathways are influencing the interconnection negatively, this could promote the progression of mood affective disorders.

Depression
Depression is a disorder of impaired emotion regulation (Joormann & Gotlib, 2010). Two distinct symptoms within the diagnosis of depression include depressed mood (reduce motivation) and the loss or reduction in activities which were previously pleasurable (anhedonia). Data has strongly suggested that these components are modulated by the dopaminergic projections of the “reward” network which originates in the midbrain, passing through the medial forebrain bundle (Dobrossy et al., 2020).

Bracht et al, (2022) conducted a study which aimed to investigate the associations between the structural connectivity of medial forebrain bundle, functional connectivity (FC) of the VTA, anhedonia, and depression severity. It was found that there was an increase of functional connectivity between the VTA and right medial prefrontal cortex in patients with depression. The severity of depression was associated with fewer tracts in the superolateral medial forebrain branch (sIMFB). These findings concluded that there was reduced structural connectivity of the sIMFB in individuals with depression. Furthermore, increases in functional connectivity between the ventral tegmental area and prefrontal cortex may be associated with anhedonia.

Obsessive compulsive disorder (OCD)
Obsessive compulsive disorder (OCD) consists of patterns of unwanted thoughts and behaviours, leading to significant emotions of distress and anxiety (Mayo Clinic, 2020). Studies have suggested that the prefrontal cortex, basal ganglia and the thalamus play a pivotal role in the pathogenesis of OCD (Huey et al., 2015). The pathways via the medial forebrain bundle to these structures may play a role in the connectivity and communication, influencing obsessive compulsive symptoms.

What is electrical stimulation
Electrical stimulation of the MFB has been reported to induce pleasure sensations (Farakhor et al., 2019). Electrical stimulation of the ventral tegmental area or the medial forebrain bundle and its projection target the nucleus accumbens, have demonstrated to improve depressive symptoms in individuals affected by major depressive disorder (Settell et al, 2017). Electrical stimulation includes therapies including electroconvulsive therapy, transcranial magnetic stimulation, and deep brain stimulation (National Institute of Mental Health, n.d.).

Deep brain stimulation
Deep brain stimulation (DBS) is a treatment which involves implanting electrodes within areas on the brain. DBS is a promising modality on the treatment of depression and obsessive-compulsive disorder (Dobrossy et al., 2022). These electrodes produce electrical impulses that affect brain activity (Mayo Clinic, 2023). DBS to the super-lateral branch of the MFB has become an effective treatment in for resistant depression, leading to antidepressant effects (Fenoy et al., 2022).

Oldani et al, (2021) conducted a 5 year follow up experimental research design which aimed to measure the effect which deep brain stimulation had on reducing depressive and obsessive compulsive symptoms in the patient. The patient presented with symptoms of treatment resistant depression and obsessive compulsive disorder. The methods consisted of performing deep brain stimulation on the superolateral branch of the medial forebrain (siMFB). This is the area which is responsible for the interconnection of brain structures including the nucleus accumbens, ventral tegmental area, the amygdala, and portions of the hypothalamus which has been mentioned to play vital roles in the regulation of the reward pathways. Researchers discussed how DBS was a successful approach in reducing these symptoms, particularly of obsessive compulsive symptoms. Similarly, another study conducted by Senova et al., (2019) discuss how nucleus accumbens (NAcc) - deep brain stimulation is an effective treatment in reducing the symptoms of obsessive compulsive disorder through decreasing excessive fronto-striatal connectivity between the NAcc and the medial and lateral components of the prefrontal cortex.

Furthermore, Rymaszewska et al. (2022) conducted a case study which considered different alternative treatments of treatment resistant depression. Three forms of electrical stimulation were used on the patient to measure the effectiveness of the treatment. Electroconvulsive therapy was found to only be efficient for a time of 4 months. Transcranial magnetic stimulation which was found to only improve their insomnia. Deep brain stimulation of the medial forebrain bundle was the third method, and results demonstrated that after 20 months of treatment, a reduction of over 80% of depressive symptoms severity had occurred.

Conclusion
Overall, the medial forebrain bundle has been demonstrated to influence our motivation and emotion through its process of connecting pathways between brain structures. The MFB influences motivation through connecting structures like the ventral tegmental area and nucleus accumbens to activate our reward system, driving our motivated behaviours.

The MFB impacts our emotion through potentially causing hyperactive functional connectivity through structures of our brain, including the ventral tegmental area and the prefrontal cortex, promoting depressive symptoms and obsessive compulsive tendencies. However, work has demonstrated the effectiveness of electrical stimulation, more predominantly deep brain stimulation, in neutralising this connectivity and regulating these behaviours.

The neuroscience of the medial forebrain is yet to need more literature and investigation surrounding its impact on human behaviour. Although, it is apparent that the MFB plays a key role in regulating our reward activating structures and impact on emotional regulation.