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= Temporal Discounting in the Brain = Temporal discounting is a behavioural measurement used in a variety of social science and economic settings. Refer to the finding that a larger reward in the future is less attractive than an immediate albeit smaller reward now.

Background
Decision-making behaviour has many applications for real-life experiences like judgement, self-projection, personality and situational influences. Research from a variety of different sectors tells us that choice-preference is a subjective characteristic for each person. In psychology, research on this phenomenon has been applied to a wide spectrum of applications from clinical populations (addiction) to developmental disorders (ADHD) and further.

Gratification and waiting are the two main systems that dictate temporal choices. These are mutually incompatible and in conflict during delay decisions and relate to an individual’s subjective value of a reward and their cognitive control ability. Neural correlates coincide with this view since areas relating to intertemporal choice, specifically the ventral striatum and prefrontal cortex, are shown to have functional differences in “impulsive” and “patient” individuals. Now intertemporal decision-making is being used in neuroimaging to investigate impulsiveness, cognitive control and variability in different populations across fields.

Theoretical Approaches
There is much speculation on which model best accounts for differences found in intertemporal variation. Some models relate to a general mechanism of reward whilst others focus on valuation systems or delay functions. Each of the distinct modelling systems point to different aspects of cognitive strategies involved in decision-making over time delay.

Brain structures
Two general systems have been related to the two systems in competition during delay tasks which can reliably predict an individual’s choice. The immediate gratification system is shown in regions of more primitive limbic system whereas the rational judgement is active in more evolved frontal cortex. Respectively associated with immediate rewards preference and delayed options.

Ventral striatum (VS) is considered part of the subjective valuation system. Activity is more pronounced during valuation processes like those in delay decisions. It is associated with higher impatience and steeper discounting rates. In addition, it has function links to the dopaminergic system which is also implicated in decision making processes. In some studies VS activation is higher during shorter delays.

Medial orbitofrontal cortex (mOFC) is also suggested to play a part in the same valuation system as the VS as well as integrating costs, such as time, into this value process. It is activated across different domains regardless of task variation, suggesting it has a role in comparison between choices.

Dorsolateral prefrontal cortex (dlPFC) is implicated in cognitive control. Studies have shown higher activation in the dlPFC is linked to greater exertion of cognitive control and thus is associated with postponing a reward.

Regional-functional overlap
The regions discussed above are linked to other processes which point to intersections between these functions and decision-making processes. The PFC and OFC are part of an extended network in prospective representation and episodic imagery, along with the hippocampus and amygdala, damage to any one of these regions can have detrimental effects on memory as well as decision making abilities. mPFC has a large overlap with decisions relating to risk and gambling, and whether these two behaviours are dissociable from delay decision is still debateable.

Applications
Decision-making tasks such as temporal discounting provide a representation of the valuation and self-control systems discussed above in certain populations and their corresponding neural correlates in comparison to healthy subjects.

Health Obese individuals have been found to discount more than healthy individuals.

Addiction Drug dependent populations show a higher discount rate than healthy population. Research in this field also have applications to treatment and intervention programmes. Studies have found substance-dependent individuals discount more when in a deprived state, whilst ex-users display lower discount rates relative to addicted states and are comparable to healthy populations. Discount rates have also been linked to relapse behaviour and abstinence in predicting recovery success.

Development and Lifespan Maturation of the different cortical areas relate to some behaviour differences observed between adults and teenagers. Prefrontal cortex typically develops relatively later during adolescence while deeper limbic systems reach full maturation earlier in life. This can account for the tendency for teenagers to discount more steeply and choose instant gratification more frequently than adults and suggests that they may be particularly at risk of developing impulse disorders such as addiction.

Regional overlap
The brain structures involved in delay decisions are often implicated in larger systems and thus simply “share” their function with decision making processes. Making it very difficult to study in isolation and to draw general conclusions. The dopamine system is applicable in many reward processes relating to decision making, gratification and reward; it is also implicated in addiction and some clinical disorders

Individual Differences
Genetic differences The dopamine system is a key player in the valuation of delayed rewards. Research has found variation in catechol-O-methyltransferase gene can produce differences in dopamine concentration in the prefrontal cortex. Carriers of met/met variation show higher concentration of dopamine levels and discounted less than val/val carriers.

Trait-state dependent Research has found that variability can occur even within individuals across different testing sessions. This context-dependent difference also changes between people as some people may be more impulsive or more patient than others.

Intellectual Ability Some studies have suggested that IQ may relate to decision-making abilities as people with higher IQs generally discount less frequently and are seen as more patient.

Practical
Alongside interindividual variability, there is also variation within the literature itself which is problematic for future research and is difficult to summarise from. The majority of neuroscience research is conducted on animals using different rewards and typically much shorter delays making it difficult to generalise findings to human populations. Studies are often difficult to conduct due to the large overlap in processes and neural mechanisms. This makes it difficult to create a sole dissociation of a specific underlying feature involved in the decision process, a particular network or region-of-interest in the brain or even to isolate behaviour between testing sessions. Whilst it is unlikely to produce a dissociation, the processes underlying temporal decisions is still noteworthy to investigate as the factors that influence the behaviour of decision makers under time delay have implications for a vast area of research and for real-life phenomena. There is a lot of overlap with more general decision-making behaviour such as gambling and business decision-making.