chapter 18 notes

chapter 18 notes - CHAPTER 18: Drug Abuse Positive...

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CHAPTER 18: Drug Abuse Positive Reinforcement: If a behaviour is regularly followed by an appetitive stimulus, that behaviour is reinforced. If an addictive drug is taken by fast-acting route (ie IV), the last action (injecting) is particularly reinforced. This reinforcement is powerful and immediate. Effect of a reinforcing stimulus is greatest if it occurs right after a behaviour/ response. Reason: instrumental conditioning involves learning about consequences of behaviour; if cause and effect times are separated, the “learning” is less effective. Logan: hungry rats trained to run through a maze to one of 2 corridors- one where they were given more food but w/ slight delay, or one where they were given less but immediately. Immediacy of reinforcement took precedence over quantity; rats ran to the latter corridor. Drug users prefer heroin to morphine simply because it has a more rapid effect; a “rush”. The use of methadone for opiate addiction and nicotine patches are based on this principle. Our reinforcement system has evolved to deal w/ immediate effects of behaviour, which is why addictive drug effects can often overpower recognition of long-term aversive effects. Cognitive processes of expectation and memory play an important role: if person takes a pill to feel euphoric ten mins later, they remember taking the pill and have an idea of its effects; if a rat is given the same pill, it will feel the euphoric effects, but they won't reinforce any behaviour. Neural mechanisms of positive reinforcement: All natural reinforcers (food for hungry animal, sexual contact, etc) cause release of dopamine in the nucleus accumbens (not the only effect of reinforcing stimuli, but a necessary one). Different addictive drugs stimulate release of dopamine in different ways. Process of addiction begins in mesolimbic dopaminergic system and produces long-term changes in other brain regions that receive input from these neurons. First changes take place in the ventral tegmental area: single administration of an addictive drug increased strength of excitatory synapses on dopaminergic neurons of the VTA in mice. As a result, increased activation is seen in many regions that receive dopaminergic input from the VTA, including the nucleus accumbens (in ventral striatum). Many important behavior-affecting changes occur in the dorsal striatum (caudate/putamen). (These are all in the basal ganglia, which plays a critical role in instrumental conditioning). Early reinforcing effects that take place in the ventral striatum (nucleus accumbens) encourage drug-taking behavior, but changes that make the behavior habitual involve the dorsal striatum. Infusion of dopamine antagonist into dorsal striatum suppressed rat's lever presses that had been
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This note was uploaded on 12/18/2011 for the course PSYC 211 taught by Professor Yogitachudasama during the Winter '09 term at McGill.

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chapter 18 notes - CHAPTER 18: Drug Abuse Positive...

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