BEHAVE CHAPTER 2 (The Mesolimbic / Mesocortical Dopamine System
BEHAVE CHAPTER 2
The Mesolimbic / Mesocortical Dopamine System
- Arises from a region near the brain stem called the ventral tegmental area("Tegmentum")
- Organization of this circuitry;
-Tegmentum sends projections to accumbens
- A key target of dopaminergic neurons is accumbens
- Dopaminergic system is about reward, various pleasurable stimuli activate tegmental neurons, triggering their release of dopamine.
- Dopamine is about anticipation of the reward rather than reward itself
- It also fuels goal-directed behaviour that is necessary to get the reward
- It’s about the motivation arising from those dopaminergic projections to the PFC that is needed to do the harder thing (i.e., to work)
- Dopamine is not about the happiness of reward. It’s about the happiness of pursuit of reward that has a decent chance of occurring. This is central to understanding the nature of motivation, as well as its failures (e.g., during depression, where there is inhibition of dopamine signaling due to stress, or in anxiety, where such inhibition is caused by projections from the amygdala)
- It also tells us about the source of the frontocortical power behind willpower. In a task where one chooses between an immediate and a larger delayed reward, contemplating the immediate reward activates limbic targets of dopamine (the mesolimbic pathway), whereas contemplating the delayed reward activates frontocortical targets (the mesocortical pathway). The greater the activation of the latter, the more likely there’ll be gratification postponement.
- Humans delay gratification for very long times. We also use dopaminergic power to motivate us to work for rewards that come after we are dead - being a war hero, caring about global warming for the future.
Dopaminergic Activation is caused by;
- Drugs like cocaine, heroine and alcohol
- Sex and food in every species examined
- Pleasurable aesthetics (when you hear new music, more accumbens activation more you like it)
- Artificial cultural inventions(males looking at sport cars)
- Schadenfreude - gloating over someone's fall
- Gambling - intermittent reinforcement is most effective
- Chronic stress or pain depletes dopamine and decreases the sensitivity of dopamine neurons to stimulation, causing the defining symptom of depression—“anhedonia,” the inability to feel pleasure.
- Following a reward, the dopamine system codes for discrepancy from expectation—get what you expected, and there’s steady-state dopamine. Get more reward or get it sooner than expected, and there’s a big burst; less or later, a decrease. Some tegmental neurons respond to positive discrepancy from expectation, others to negative; appropriately, the latter are local neurons that release the inhibitory neurotransmitter GABA. Those same neurons participate in habituation, where the reward that once elicited a big dopamine response becomes less exciting.
- An emptiness comes from combination of unnatural sources of reward and the inevitability of habituation; this is because unnaturally strong explosions of synthetic pleasure evoke unnaturally strong degrees of habituation
Anticipation requires learning. This explains context-dependent craving in addiction.(sigaranın sabah kalkınca ya da yemekten sonra değerli olması)
Can a reliable cue of a reward become rewarding itself? A light in the left side of a rat’s cage signals that lever pressing will provide food on the right side. Rats will hang out on the left side of the cage, because it feels nice to be there. The signal has gained the dopaminergic power of what is being signaled. Similarly, rats will work to be exposed to a cue that signals some kind of reward is likely, without knowing what or when. This is what fetishes are, in both the anthropological and sexual sense.
- What happens if the post–light cue release of dopamine doesn’t occur? Crucially, the monkey doesn’t press the lever. Similarly, if you destroy its accumbens, a rat makes impulsive choices, instead of holding out for a delayed larger reward.
- Activate the vmPFC or inactivate the dlPFC, and short-term reward becomes more alluring. Accumbens of impatient people underestimates the magnitude of the delayed reward, and their dlPFC overestimates the length of the delay.
Differences in Humans For Self-Control
- Our dopaminergic system, frontal cortex, amygdala, insula and other members code for differing aspects of reward magnitude, delay, and probability with varying degrees of accuracy, all influencing whether we manage to do the harder thing. Individual differences among people for gratification postponement arise from variation in the volume of these individual neural voices. (For example, there are abnormalities in dopamine response profiles during temporal discounting tasks in people with the maladaptive impulsiveness of ADHD. Similarly, addictive drugs bias the dopamine system toward impulsiveness.)
- Low levels of serotonin in the brain is associated with elevated levels of human aggression, ranging from hostility to overt violence.
- Low serotonin didn’t predict premeditated, instrumental violence. It predicted impulsive aggression, as well as cognitive impulsivity (trouble inhibiting a habitual response). Other studies linked low serotonin to impulsive suicide.
- In both animals and humans, decreasing serotonin increases behavioral and cognitive impulsivity.
- Increasing serotonin signaling did not lessen impulsiveness in normal subjects, but it did in subjects prone toward impulsivity, such as adolescents with conduct disorder.
- Working memory, executive function, postponement, long-term planning, regulation of emotions, reining in impulsivity
- Makes you do the harder thing when it is the right thing to do
- Most recently evolved region in the brain
- Not developed fully until midtwenties
Prefrontal Cortex (PFC)
- Newest Part of the Frontal Cortex
- PFC is the decider, whether to say something or not, coke or pepsi, usually decisions between emotions and cognition
- Essential for categorical thinking
- Frontal cortex tracks rules, learned at age 3 not to pee when you feel like it, increases influence over neurons that regulate bladder.
- Silences during orgasm.
Dorsolateral Prefrontal Cortex (dlPFC)
- Decider of deciders, the most rational, cognitive, utilitarian, unsentimental part of PFC.
- Most recently evolved, last to mature.
Humans with dlPFC damage;
- Impaired in planning
- Impaired in gratification postponement
- Perseverate on strategies that offer immediate reward
- Show poor executive control over their behaviour
- Without the dlPFC the metaphorical superego is gone, resulting in individuals that are hyperaggressive, hypersexual ids
Ventramedial Prefrontal Cortex (vmPFC)
- It is about the impact of emotion when decision making.
- Many of our best and worst behaviours are result of interactions of vmPFC with dlPFC and the limbic system
Humans with vmPFC damage;
- Intelligence and working memory remains normal
- Have difficulties making social/emotional decisions
- Damaging vmPFC means removing limbic input to the PFC, which eliminates gut feeling and makes decisions harder
- Poor judgment on choosing friends and partners,
- Don't shift behaviour based on negative feecback.
- Without vmPFC, behaviour is inappropriate in a detached way
- Seeing someone after a long time says, "You gained weight", when people are shocked, with calm puzzlement: "But it's true"
Frontotemporal Dementia (FTD)
- People with FTD exhibit behavioral disinhibition (stealing, hypersexuality, aggression, gambling) and socially inappropriate behaviours and apathy.
- Social and behavioral disinhibition also occur in individuals with stroke damage in the frontal cortex - for example; sexually assaultive behaviour in octogenerians.
- Self control is finite, after making the frontal cortex work hard, performance of subsequent tasks decline
- Increase cognitive load on frontal cortex, afterward subjects become less prosocial, less helpful, more likely to lie
- Increase cognitive load with a task requiring difficult emotional regulation and subjects cheat more on diets
- When you learn something (like playing the piano, getting good at a sport), it is transferred to more reflexive regions of the brain, like the cerebellum. - body knows what to do without thinking.
- Soon after potty-training, doing the harder thing with our bladder muscles become automatic, likewise with other demanding frontal tasks.(learning to drive)
During REM sleep and dreaming frontal cortex is hypofunctional - hypersexuality, outburst of emotions, illogical acts. While dreaming, frontal cortex goes offline.
A subject plays a game with two people and is manipulated into feeling that she is being left out. This activates her amgydala. Soon, her PFC activates and rationalizations kick in -"This is just a game, I have friends". and the amygdala et al. quiet down. What if frontal cortex is not fully developed? Amygdala is increasingly activated, the person feels increasingly distressed. This is a typical teenager.
Autonomic Nervous System
Sympathetic Nervous System (SNS)
Mediates 4F's: fear, fight, flight, fuck
Speeds up the hearts
- Part of the Limbic System
- Aggression, fear and anxiety
- Sensitive to unsettling circumstances that are social
- Linked to social uncertainty
- Key role in perceiving fear-provoking stimuli and expressing fear
- Plays a logical role in social and emotional decision making
- Helps mediate inner and learned fear
Conditioning - Rat experience - shock after a tone
After some time, sound of tone causess release of stress hormone
Synaptic Changes - After conditioning, the synapses coupling the BLA and central nucleus neurons become more excitable, amount of receptors for excitatory neurotransmitters in dendtric spines in these circuits increase.
Conditioning increases "growth factors" which prompt the growth of new connections between BLA and central amygdala neurons.
When conditions change, the fear response gradually abates.