reward system Flashcards
1. There are multiple “reward systems” in the brain, expressed as parallel systems. 2. Two major systems can be termed “appetitive” (responding for future reward) and “consummatory” (responding for present reward). 3. Dopamine seems to have a role in the ability of rewarding stimuli to be reinforcing (that is, bring about learning via LTP). This may be particularly important in the dorsal striatum (caudate/putamen), and form the basis of addictive behaviors. 4. Dopamine, particularly in t
two points of view on rewards
- reward and pleasure: pleasurable sensations serve as their habitual guides
- behaviorism: no description of feelings
differences between the “reward” and “reinforcement” view
reward view: feel good or has hedonic value
reinforcement view: doesn’t talk about unobservable such as “ feel good”; not cognitive but purely behavioral - if a stimulus produced by a behavior results in a higher probability of the behavior occurring again, the stimulus is a positive reinforcer
the experiment first showed the existence of reward system
Olds and Milner found that the rat “hung out” around an area of a table-top where he received stimulation
They placed this rat into a “Skinner box” and demonstrated that the rat would press a lever to stimulate its own brain
The phenomenon is called “intracranial self-stimulation” or “ICSS”
three levels of explanation why ICSS is rewarding?
evolutionary biology level: activation of species-specific neural systems for “survival” behaviors, developed via natural selection, is rewarding
neural systems level: there is a single, all-purpose “reward system” in the brain
nerve cell level
examples of evolutionary explanation
sexual behavior: rats appear to like stimulation which makes them mate
aggression: cats appear to like stimulation which evokes predatory attack
The explanation is that activation of brain systems important for survival of the individual and/or the species is rewarding in and of itself
evidence of neural level explanation
- Papez and limbic system
Early work showed that ICSS could be found with electrodes placed in structures of the limbic system, including the: hypothalamus, septum, hippocampus, amygdala, cingulate cortex - neurochemist
Stein demonstrated increases in rates of hypothalamic ICSS in rats systemically injected with methamphetamine. Subsequent work showed this happened via an amphetamine-evoked increase in dopamine transmission;
strong ICSS from the dopamine cell bodies in the ventral midbrain;
dopamine in intravenous self-administration (IVSA) of amphetamines and cocaine;
the union of the data from ICSS and from IVSA produced an offspring: the dopamine hypothesis of reward
what is the dopamine hypothesis of reward
stimuli which evoke dopamine release, particularly in the nucleus accumbens, are rewarding
some problems faced with this hypothesis and what is the conclusion?
- lateral hypothalamic ICSS is not affected by massive accumbens damage
2/3
conclusion: it appears that activation of hypothalamus-based neural systems in rewarding, independent of other limbic or striatal structures
Some investigators were using reward other than ICSS, in particular food reward, and gained insight into the role of dopamine in “reward” and “reinforcement”
Schultz: correlates of DA cell firing with learning in monkeys. DA cell bodies fire only when the reward is “unexpected”. This firing mode is termed “phasic”
“global” simultaneous dopamine influence on many medium spiny neurons in the dorsal (caudate/putamen) and ventral (accumbens) striatum
Schultz favors reinforcement over reward: “ it appears that dopamine, released in phasic mode of DA neuron firing, is a “reinforcement”, not a “reward” transmitter, and may work by enhancing LTP in striatum, accumbens and prefrontal cortex.”
Hebb’s idea of a cellular mechanism of learning
Hebb’s idea was that if a synaptic terminal and its postsynaptic membrane are simultaneously active, communication across that synapse will permanently strengthen, which is a precursor to the theory of LTP
the appetitive and consummatory division of motivated behavior
appetitive reward: the pleasure of the hunt
consummatory reward: the pleasure of achievement and satisfaction
neural mechanism of appetitive and consummatory rewards
consummatory: probably based on the activation of hypothalamic mechanism
appetitive: based on the activation of striatal, particularly accumbal, mechanisms
example of appetitive behavior
striatal areas receive both a dopamine input from the midbrain and a glutamate input from forebrain regions
an example: decrease in fixed ratio 16 lever-pressing with food reward by injection of the GABA agonist muscimol in the basolateral amygdala
explanation of the BLA experiments
A “disconnection” experiment showed that the decrease in FR responding after BLA inactivation was related to BLA projections to the ipsilateral accumbens (BLA-BLA, NA-NA, ipsi-contra, IPSI-IPSI)
Therefore, a BLA-accumbens pathway must be active in both hemisphere for normal FR responding
explanation of decrease of FR in BLA experiment
a dose of muscimol in BLA which decreases FR responding for food has no effect on intake when a rat simply eats the same food. So it can’t be affecting the “primary reward” of the food itself. The decrease of FR responding for the food would seem due to a decrease in the “incentive reward” of the food
