reinforcement - drugs & addiction

• Reinforcement

  Many brain areas & neurotransmitter systems are identified with reward - ventral tegmentum & dopamine in particular
• reinforcement vt

  • Electrical microstimulation of VT (lower midbrain) can have reinforcing effects on behaviours, similar to natural reinforcers
  • VT projects to nucleus accumbens, prefrontal cortex, hippocampus & amygdala (limbic system) - Mesolimbic Dopaminergic System
• reinforcemt 3

  Extracellular dopamine concentration (rats) & fMRI activation (humans) increased in Nucleus Accumbens : VT stimulation – rats;  money reward - humans
• reinforcement - Schulz et al 1993

  • Dopamine neuron respons to reward during period when monkey learning task
  • performance not yet reliable = reward is ‘unexpected'
  • Once monkey can reliably perform task reward becomes ‘expected’ & DA neuron stops responding to reward
• reinforcement - signals

  • Unexpected reward → strong positive dopamine signal
  • Declines with repeated presentation & learning,
  • Eventually presentation of predicted reward → no signal
  • dopamine in Nucleus Accumbens surges in response to the predictive stimulus
• reinforcemnt - signal 2

  • Omission of predicted reward = suppression of dopamine signal
  • Response of midbrain dopaminergic neurons represents learning signal that codes for errors in prediction of reward
• reinforcement - signal 3

  • Learning associations between 2 non-reinforcing stimuli = linked to enhanced dopamine release in NA, - also receives excitatory input from medial prefrontal cortex
  • Mesolimbic dopamine system involved in modulation of associative learning - not only reinforcement
• reimforcement

  • dopamine antagonists block reinforcement learning
  • dopamine modulates long-term potentiation
  • D1 receptor activation enhances strong excitatory input and reduces weak input.
  • dopaminergic reward systems = powerful modulator of learning
  • modulate how learning instantiated in brain as memory