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Lecture 5

PSYC62 - Lecture Five.docx

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University of Toronto Scarborough
Suzanne Erb

Lecture Five Outline: Neurobiology of brain reward Addiction o Dependence o Tolerance o Sensitization Neurology of Brain Reward What is Reward? Reward in the context of learning o When a behaviour is followed by a consequence that strengthens the future likelihood of the behaviour occurring again o Produces pleasure o Stimuli that may be neutral under normal circumstances can become capable of producing pleasure when paired with another stimulus (Pavlovian) Gains incentive properties and induces approach behaviour by virtue of it producing a pleasurable state Different Types of Rewards Natural Rewards o Food, water, sex, nurturing, etc Artificial Rewards o No adaptive value o Can strongly control behaviour Does so by acting on the same structures that natural rewards work on Drugs, money (gambling), etc Olds and Milner (1954) o Discovered that electrical stimulation of certain areas of the brain was rewarding and reinforcing o Brain stimulation reward procedure o If an electrode is implanted in the brain reward circuit, they will work to apply stimulation in that brain region Brain Stimulation Reward Sites Include: o Medial forebrain bundle (MFB) Primary o Ventral Tegmental Area (VTA) Reward o Nucleus Accumbens Pathway o Frontal Cortex o Lateral Hypothalamus o Substantia Nigra o Striatum, amygdala, hippocampus, thalamus, cerebellum o Locus coeuleus DA hypothesis o Very influential o The DA hypothesis of reward has instigated a considerable amount of research over the last few decades o However, much of that research can lead one to conclude that its oversimplified to consider DA as the sole mediator of primary reward, or primary motivation for natural stimuli such as food, water, and sex o DA transmission within the reward circuit, especially the nucleus accumbens reinforces a behaviour o Drugs of abuse can hijack this system and produce pathological changes that lead to addiction o All drugs of abuse share the ability to dramatically increase extracellular levels of dopamine in the nucleus accumbens (NA) o Dopamine isnt just a pleasure chemical Effects of a saline injection or cocaine o Cocaine produces a dramatic effect o If we substitute morphine, alchohol, etc, there would still be an increase in the dopamine in the NA o Evidence of its reinforcing effect Imaging studies are often used to support the idea that nucleus accumbens mediates pleasure But imaging research also demonstrates that the nucleus accumbens responds to: o Stress o Aversion o Hyperarousal/irritability NA lights up during pleasure but also during stress Thus, concepts about role of DA in drug-induced euphoria, dependence, and addiction have undergone revision Animal studies indicate that dopamine neuron activity isnt just tied to positive reinforcing stimuli If you have a drug thats reducing anxiety, we would expect an increase in dopamine if its a pleasure chemical. But in fact, the drug is associated with reducing DA transmission Drugs that induce anxiety are associated with DA increases We have to move beyond the DA synapse view of drugs, pleasure and reinforcement o Its not true that the more DA you have in the NA synapses, the greater the rewarding results Addiction A related set of terms describes behaviour patterns of excessive or chroni
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