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Dopamine and reward (function as a reward system= more dopamine more…

- - - - a(R-Vcs(n))
      - p error = difference between the actual and expected outcome
      - Actual= R
      - expected = VCS
      - a large prediction error in trial 0 resulted in a large amount of learning on trial n
      - smaller error on trial n resulted in smaller level of learning in trial n+1
- - - - neurons fire action potentials when event has occurred
      - best explained in terms of learning about what predicts rewards
      - three types of activity=
        
        Presentation of an unexpected or more than expected reward= activation
        
        stimuli that predict rewards ( activation)
        
        Failiure of an unexpected reward (inhibition)
    - - D'ardenne et al(2008)
        
        played a game whilst undergoing fMRI scan
        
        Task: Guess whether the number on the right of the screen would be greater than the number on the left (max 10)
        
        won $1 if they guessed correctly and lost $1 if they guessed incorrectly
        
        found activity in the VTA increased when they unexpectedly got the $1 and was greater as the probability of reward decreased
        
        no response for negative prediction errors - hotly debated!
      - recap: Behaviours are performed that maximise the amount reward received - reward prediction errors signal the contingency between behaviors and rewarding outcomes is different from the expected contingency #
      - dopamine neuron responses signal the difference between actual and predicted outcomes -learning- dopamine neurons move their response to the earliest predictor of reward
        
        dopamine neurons therefore signal other brain systems to alter behaviour to increase the probability of obtaining reward
  - - - learned a task using apple juice for rewards
      - before the task dopamine neurons responded to unpredicted rewards
        
        during learning phase , reward became increasingly predictable and neuronal activity decreased to baseline levels
        
        no phasic activity when fully predictable rewards were delievered
        
        when expected rewards were omitted -phasic decrease in activity at the time the reward was expected
        
        after learning predictable rewards elicit no change
        
        if rewards were delievered at the wrong time there is decreased activity at the time that rewards are expected followed by increased activity at the time of delievery
- - - - Ventral
        
        Nucleus accumbens
        
        dopamine dependant RPes occur here (reward predictor errors)
        
        pessiglione et al, (2006)
        
        choice between two abstract stimuli associated with different probabilities of winning £1
        
        Particpants administered with either L-Dopa or Haloperidol
        
        Haloperidol reduced the magnitude of the RPE signal and its behavioural effects
        
        L dopa- enhanced the size of the signal and its behavioural effects
        
        signal in the Ventral striatum was consistent with the RPEs
        
        how 'pure' is the encoding of RPE in the nucleus accumbens
        
        Syed et al, 2016 >>action initiation influences the dopamine signal
        
        different to classical conditioning in the Schultz's study= classical conditioning - but what if animals have to do something for a reward - does initiating an action influence dopamine release?
        
        Olfactory turbede
      - Dorsal
        
        two adjacent but anatomically separated groups of neurons
        
        the caudate
        
        the putamen
- - - - D neurons secrete dopamine to cells that have dopamine receptors ie D1, D2 etc
        
        via dopaminergic pathways
- - - - Ribas - Fernandes (2011)
      - found activity in the ACC signalled prediction errors for the individual actions leading to a goal
        
        thus prediction errors in the ACC relate to the performance of individual actions that lead to a goal
        
        SUMMARY: rpes found in several interconnected areas but have different functional and neuronal properties
        
        Dopamine neuron response may be more complicated than classical view suggests
        
        IN THE MIDBRAIN< THE STRIATUM AND THE ANTERIOR CINGULATE CORTEX
- - - - reinforcing effect of all drugs (abused) is dependant on the activation of the mesolimbic dopamine system
        
        assess the influence of mesolimbic dopamine transmission reviewed - on drug reinforcement
        
        evidence: all five classes of abused drugs increase dopamine transmission in the limbic regions of the brain -
        
        interaction between ; transporters , ionotrophic receptors and metabotrophic receptors
        
        (in psychostimulants ) behavioural pharmacological experiments indicate that increased dopamine transmissioin Is enough to provide psychostimulant reinforcement
        
        produce effects in the limbic system of the basal ganglia ,
        
        dopamine reward hypothesis :
        
        late 1970's - Fibiger, 1978 and wise, 1978
      - looked at 5 classes of drugs -
        
        ethanol
        
        cannabinoids
        
        opiates
        
        nicotine
        
        psychostimulants
      - reviewed the mecahnisms for each category and how they effect the mesolimbic dopamine transmission
  - - - over time the rats repeatedly self administer dopamine to themselves - find dopamine rewarding
  - - - some neurons treat drugs as rewards inn their own right - dopamine neurons in the ventral striatum show phasic responses to the injection of drugs and display activity during the expectation of drugs like reward
      - durgs which mimic or boost the phasic dopamine reward prediction error might generate a powerful teaching signal and might produce behavioural changes
      - dopamine neurons also code temporal information (expectation of the timing of a reward -
        
        when rats asked to wait for reward - they cant wait long enough - too much motivation for the reward and they are expecting the reward so cannot wait to press the lever
- - - - postive
        
        response to unexpected rewards
        
        outcomes unexpected and important to learn from
      - negitive
        
        response to absence of predicted reward
      - IE. monkey sees green light - gets a reward -reward is fully predicted and monkey is no longer learning-
- - - - Schultzs study = classical conditioning
      - what if animals do something for reward
- - - - midbrain to limbic system and cortex
      - REWARD PROCESSING
    - - midbrain to striatum
      - MOTOR CONTROL (influence parkinsons disease)
- - - - target stimulus (classical view - dopamine neurons should not fire here -
        
        find the target (classical view -dopamine neurons should not fire here ) but they are
        
        OUTCOME