Motivation and emotion/Book/2014/Dopamine and motivation
What is the role of dopamine in motivation?
The question about why we feel inclined to behave in certain ways or seek certain things has resulted in research on how the brain develops its motivations. One particular aspect that has shown to have an association is Dopamine activity. The neurotransmitter has been shown to facilitate behaviours via different theories of rewarding the brain. This association takes into account some basic learning principals and also new relevant theories. These theories have been shown the help explain why behaviour, that can be detrimental to a person, is sought after . Below is a foundation of dopamine function into motivation and an insight into why people are motivated to do certain things .
Dopamine - function and structures
Dopamine is a neurotransmitter that resides in the midbrain and the basal fore brain. Dopamine can be classified as a monoamine. It is a catecholamne transmitter which is a type of monamine. Other types of neurotransmitters from this classification include epinephrine, norepinephrine, histamine and serotonin. Receptors for dopamine are named from D1 to D5, each responsible for different responses. Dopamine is known to facilitate both movement and emotional responses, but has also been regarded as the ‘reward system’ as it facilitates behaviour in accordance to rewards. Decreases in dopamine activity have been linked to the onset of Parkinsons’ disease and also to addictive behaviours such as substance dependence (Badgaiyan, Fischman & Alpert, 2009)
Dopamine has been shown to control a variety of functions such as positive reinforcement, food intake, emotion, cognition, locomotor activity and endocrine regulation (Missale, Nash, Robinson, Jaber & Caron, 1998). More specifically, the midbrain system involving dopamine is part of the ventral tegmental area (VTA) that is closely related to, and affects, the limbic system. In conjunction with the hypothalamus, the limbic system is known for the initiation of basic survival needs such as escape from danger and food-seeking. It has been shown to initiate drives and motivations in conjunction with motor functioning (Mogenson, Jones & Yim, 1980)
Dopamine receptors are responsible for the variety of functions, facilitating both inhibiting and disinhibiting actions. For the reward pathway, D1 and D2 receptors are responsible with both acting as agonists and antagonists . For example, in self-drug use, the D2 receptor mediates the stimulant drug reinforcement while the D1 receptor provides no antagonist and as permissively (Missale et al, 1998) . Roles of D3 and D4 receptors in the limbic region are still unknown (Missale et al, 1998) .
Motivation - intrinsic and extrinsic
Motivation is being activated and energised towards an end goal (Ryan & Deci, 2000). It is the driving force that pushes a person forward in an attempt to gain or achieve something, wetherthat is basic primal natures like food or sex, or larger goals such as good grades or a promotion in one’s career. Motivation, however, can be split into different categories and is theorised to develop in different ways.
Intrinsic motivation is the desire of behaviour in reward of internal satisfaction with no influence of environmental rewards (Ryan & Deci, 2000) . These motivations explain behaviours that stimulate the person in ways that have no reward, such as curiosity, fun or the challenge. An example is an artist painting with no promise that art would be sold or any gratification for the work. Throughout life, intrinsic motivation plays a large role in facilitating our cognitive, social and physical developments. As humans progress, learning and growing helps develop a person’s skills and find passions in life.
Authors have argued whether the drive comes from a person being satisfied with the work done and thus provides a rewarding factor, or whether it is just the interest of the task. In relation to Operant Behaviour Theory (Skinner, 1953) every behaviour is said to be acted based on the assumption a reward will follow, which in case of intrinsic motivation, outlines satisfaction as the main driving force. However, Learning Theory (Hull, 1943) contrasts this theory by stating that motivations come from physiological drives, and thus intrinsic motivations are that which fulfil these needs and provides satisfaction.
Extrinsic motivation comes from the drive of achieving a goal or behaviour in desire of an external reward. In accordance with Self-Determinism Theory (Deci & Ryan, 1985), extrinsic motivation can vary across reasons for different behaviour. An example would be a person achieving an activity to avoid punishment, as a child at school would do homework, but another child may complete their homework as they value good grades. However, it is noted that possession of just extrinsic motivation can be detrimental. Intrinsic motivation is related more to obtaining a higher performance, whereas extrinsic can reduce sustained performance (Curry, Wagner & Grothaus, 1990)
Theories of dopamine as a motivational factor
Now that dopamine as a neurotransmitter and motivation aspects have been explained, the role of dopamine and motivation combined is of more focus. Aspects of pleasure from dopamines’ function can simply be related to motivation, as something that feels good would be more sought out. Is it this simple however? Does this explain why drug addicts seek more drugs during addiction? What about motivation for mundane tasks such as homework or cleaning?
The Hyodenia Hypothesis proposed by Wise (1980) states that dopamine specifically located in the nucleus accumbens is the ‘pleasure neurotransmitter’. Wise states that the dopamine is translated into hedonic messages that we interpret and experience as pleasure, such as euphoria. This hypothesis has been supported via neuroscience studies that show this area of brain in conjunction with dopamine function plays a large role in psychostimulant activation and psychomotor stimulant reinforcement. More specifically, it is this area that is shown to be activated via abuse of certain drugs and thus produces a reinforcing effect. This theory has gained lots of popularity and leads to explain the rewards experienced by things such as sex, food, social and cognitive rewards. Neuroimaging studies have supported this theory, showing pleasure via food or drugs is correlated with the receptor occupancy of dopamine in the ventral striatum (Berridge, 2007). So if this hypothesis shows how we experience pleasure from behaviours, how then are the pleasures learned?
If we consider basic learning principals of operant conditioning proposed by Skinner, we assume that behaviours are executed because of reward. Has this theory got applicability all the way down to neurobiological levels? Research has shown that interference in the transmission of dopamine will interfere with the reinforcements of learning while enhancing the release of dopamine acts as a reinforcer . Dopamine has been shown to be crucial for motivation to seek rewards . Schultz (1998) proposed a hypothesis that stated that ‘neurons that fire together, wire together, but as long as they get a burst of dopamine’. In other words, dopamine adjusts the strengths of synaptic connections between certain neurons and that these neurons will only fire together if dopamine is present. This shows that dopamine can change and influence the synaptic plasticity, and therefore create synaptic plasticity that is specifically correlated with reward seeking behaviour (Bromber-Martin, Matsumoto & Hikosaka, 2010).
However, as the mind is now expecting a reward for certain behaviour, does this prediction have an effect on the dopamine release itself? This aspect proposed by Schultz (1998) examines the prediction of rewards effect of dopamine levels. Schultz states that a ‘reward prediction error’ occurs that takes into account the reward received for the behaviour versus the predicted reward that is expected to occur. As seen in Figure 1, if the reward received is larger than what was predicted the dopamine neurons are more excited and the opposite occurs as well, if reward is smaller than predicted the neurons are phasically inhibited. It is also noted that if a reward is ‘cued’ and the reward size is fully predicted then no effect is produced (Bromberg-Martin et al., 2010). This has led to a number of understandings about how dopamine affects our behaviour. Studies have utilised this information and shown that these predictions of reward match preferences in behaviour. It explains why larger rewards are more preferable than small ones, probable vs improbable ones and immediate rewards over delayed ones (Bromberg-Martin et al., 2010) .
Following the theories of dopamine’s role in reward and prediction, speculation over theories has come into question. Reasons being that research has shown that certain stimuli that are not associated with reward or any prediction of reward can still trigger the dopamine system . These stimuli can include salient or novel stimulus or stimulus that resembles another stimuli that does predict rewards (Kakade & Dayan, 2002). Another aspect that the theory may not be sound is dopamine release has been shown to be associated with a set of motor effects, that are irrelevant or detrimental to delivery of said reward . These motor effects are independent from any prediction or action (Kakade & Dayan, 2002). One main stimuli that seems to present this effect of the ‘novelty’ of a behaviour. The theory around why this stimulus causes dopamine activity is that the novelty is a surrogate reward and as such the novelty in itself is the rewarding factor .
Wise (2004) outlines multiple hypotheses about how dopamine effects motivation. Due to the large majority of theories Wise has included, we will only discuss three. Components of these theories are mentioned above, but categorising and providing support has made it easier to understand the functions dopamine plays.
The dopamine hypothesis of reinforcement
Reinforcement being the operative word, Wise (2004) describes this as the ‘stamping-in’ of stimulus associations and response habits that follow the acquiring of a reward. Support for this hypothesis follows animal studies that adopt the classic lever push to obtain food. It was found that impairment of dopamine function during these tests resulted in the animal not pushing the lever. This is also apparent when the animal is in an environment that they normally experience a reward and thus prefer that environment. Blocking the dopamine function leads the animal to not prefer this environment . Therefore the reinforcing stimuli and events do not reinforce the behaviour or associations with reward when the function is disabled (Wise, 2004).
The dopamine hypothesis of reward
This hypothesis is similar to the hypothesis of reinforcement, however it argues that reinforcement affects the still-active memory trace of the behaviour and not the specific behaviour itself. It’s shown that in addition to reinforcing effects, rewards stimuli have a proactive drive effect and cause motivational arousal. It also increases the chance of response initiation if the reward has not been earned. In the case of the animal studies using the lever, the post-response stimulation of the reward energises the animal before and during the following lever presses after the initial one (Wise, 2004)
The dopamine hypothesis of incentive motivation
- This theory focuses on the acquired motivational importance that has been acquired for a neutral stimulus through previous associations of reward (Wise 2004) . An example of how this occurs is the colour yellow on a banana. Until a banana has been eaten, yellow remains a neutral stimulus and holds no value. Once taste as been acquired, yellow becomes associated with the consequences of the tatse and thus becomes an incentive-motivational stimulus. Basically, they are learned predictors or reward. Support follows the same principals mentioned in the previous hypothesis surrounding animal studies and the blocking of dopamine leads to no association and for previous acquired associations to be lost (Wise, 2004)
As we have established that certain stimuli stimulate dopamine activity via associations, we will now focus around the specifics of these stimuli that warrant the explanation of motivational factors surrounding dopamine. As discussed, in the hypothesis of incentive motivation, the pairing of a reward with a stimulus give that stimulus value. Values of the stimuli are in turn what make motivation and the predicted consequences of those stimuli. These motivational stimuli can be primary or secondary, as mentioned above with basic desires such as survival instincts are classified as primary as they are genetically predetermined. Secondary associative learning is used to explain this mechanism and how a stimulus with non desirable characteristics can then gain motivational value (Di Chiara, 1999). Another aspect is instrumental learning, by which a person learns the ability to control the occurrence of biologically significant events (Di Chiara, 1999). An example would be the learning of stimulus-response relationships following behaviours, such as administration of drugs. Once relationships have been learned between stimulus and response, that cognitive representations of the value of motivational qualities of the stimulus via incentive learning (being reexposed to the stimulus). After repeated exposures, habit learning plays a role in that the relationships can become strengthened so that responding to the stimulus is independent of the outcome (Dickinson & Balleine. 1995)however are drawn from acquired learning from the predictive association with the primary stimuli as mentioned above in the hypothesis of incentive motivation (Di Chiara, 1999). This is where
Dopamine in behaviour
Drug abuse - cocaine
Drug addictions have been important in the study of dopamine and its function towards motivation. One of the most studied drugs in this field is cocaine as it has been shown to be one of the most reinforcing drugs (Volkow, Fowler, Wang & Swanson, 2004). It functions by increasethe amount of dopamine by blocking the reuptake transporters that remove excess dopamine. Expectancy of cocaine leads to higher dopamine activity. The prediction of reward can affect the size of the reward itself. It has been shown that if cocaine is given in an environment that the subject has previously used cocaine, the dopamine increase is larger and also increases it’s reinforcing effects. (Volkow et al, 2004). In terms of addiction, the ongoing abuse of cocaine results in neurobiological chances that disrupt the circuits that are specifically regulated by dopamine. The surrounding theory of this addiction is the ‘incentive sensitization’ hypothesis which states there is an increase ‘want’ for the drug, but not necessarily an increase in the ‘liking’ of the drug. It is proposed that due to chronic exposure dopamine levels deplete and thus affect reward, saliency and motivation . The drug is sought after to compensate for this depletion (Volkow et al, 2004). This hypothesis can explain the motivational factors behind addiction and dopamine activity in the brain, as motivation for the ‘reward’ and also motivation for the compensation of depleted dopamine levels are shown.
Sexual behaviour has also been a researched topic surrounding dopamine. This would be another seemingly obvious association. Sex falls under basic survival instincts. If we relate dopamine into thisit incorporates the theories showing dopamine reward as incentive for motivational behaviour in regards to sex. During copulation and also during a period of pre-copulation of rats, dopamine levels have been shown to increase (Melis & Argiolas, 1995). This can further be supported by looking at side effects of dopamine stimulating drugs that help with Parkinson's and schizophrenia. In treatment of Parkinson's where pharmaceutcals were prescribed to mediate these levels, sexual libido was shown to increase. In some treatments for schizophrenia, drugs that antagonised the release of dopamine were shown to decrease libido and impair sexual function. Specifically , the medial preoptic area of the brain has been shown to be of a large importance in the facilitation of male sexual behaviour. In human studies, damage to this particular area has shown an onset of sexual dysfunction. Animal studies have shown electrostiumlation of this area can affect the time to reach ejaculation in rats (Dominguez & Hull, 2005). It has also been shown that mediate the levels of dopamine in the medial preoptic area effects sexual motivation, be that the desire to be sexually active with another person (Dominguez & Hull, 2005). This further contributes to the role dopamine plays in motivation.
Genetics in addiction
In relation to addiction, the debate about why some people are more inclined to become addicted to a substance or behaviour comes into play. It is established that dopamine effects behaviour in accordance with reward, so why aren’t we all abusing cocaine and seeking sex every day? Evidence in accordance with gambling addictions has shown that the D2 receptor, which plays a large role in the mediation of dopamine, may not be as present in some brains as others. This trait is genetic and known as the D2A1 allele receptor gene. Research has linked this gene and lack of D2 receptors as a significant risk factor in pathological gamblers (Blaszczynski & Nower, 2002). Therefore it has been hypothesised that possession of this gene leads an individual to seek, or for the sake of this subject, motivated to participate in pleasure-generating activities. This then puts the subject at a high risk of impulsive, compulsive and addictive behaviours .
Table 1. What excess or deficient levels of dopamine can result in and what is considered normal.
|Anxiety||Depression||Feelings of wellbeing, satisfaction|
|Compulsions||Anhedonia||Pleasure, reward in accomplising tasks|
|Sexual Fetishes||Lack of ambition and drive||Healthy libido|
|Sexual addiction||Inability to "love"||Good feelings toward others|
|Unhealthy risk-taking||Low libido||Healthy bonding|
|Gambling||Erectile dysfunction||Healthy risk taking|
|Compulsive activities||No remorse about personal behaviour||Sound choices|
|Psychosis||Social Anxiety Disorder||Maternal/Parental love|
Source: Health-science-spirit.com, 2014
Dopamine has been shown to play a large role in motivation. As some of these theories vary, supporting evidence shows that an association is evident. The value of reward in any form plays a role in behaviour; it is just where the reward lies that is debated. Associated stimuli with reward or a combination of these stimuli with aspects of initial reward and reinforcing factors are believable explanations about why people are motivated or how they become motivated. The real life applications of these theories are quite large, as problem behaviours of drug addiction and gambling cost society greatly. Through the understanding of dopamine's function and possibilities of lacking in D2 receptors we are able to understand these behaviours more and thus plan ways in which to overcome these obstacles. Regardless of theories however, the association between dopamine activity and motivation seems to be very well established and accepted.
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