Brain stimulation reward (BSR) is a pleasurable phenomenon elicited via direct stimulation of specific brain regions, originally discovered by James Olds and Peter Milner. BSR can serve as a robust operant reinforcer. Targeted stimulation activates the reward system circuitry and establishes response habits similar to those established by natural rewards, such as food and sex. Experiments on BSR soon demonstrated that stimulation of the lateral hypothalamus, along with other regions of the brain associated with natural reward, was both rewarding as well as motivation-inducing. Electrical brain stimulation and intracranial drug injections produce robust reward sensation due to a relatively direct activation of the reward circuitry. This activation is considered to be more direct than rewards produced by natural stimuli, as those signals generally travel through the more indirect peripheral nerves. BSR has been found in all vertebrates tested, including humans, and it has provided a useful tool for understanding how natural rewards are processed by specific brain regions and circuits, as well the neurotransmission associated with the reward system.
Example frequency-rate plot for 10 trials over increasing stimulation frequencies. Hypothetical data (which reflects established concepts in ICSS procedures) is plotted as the percent maximum responses for any given trial during a sequence. The blue line represents hypothetical data at baseline, while the black line represents hypothetical data following administration of 10 mg/kg cocaine to the subject. This reflects the characteristic "left-shift" of the frequency-rate curve following administration of a drug that increases dopaminergic transmission in the reward pathway.
The reward system is a group of neural structures responsible for incentive salience, associative learning, and positively-valenced emotions, particularly ones involving pleasure as a core component. Reward is the attractive and motivational property of a stimulus that induces appetitive behavior, also known as approach behavior, and consummatory behavior. A rewarding stimulus has been described as "any stimulus, object, event, activity, or situation that has the potential to make us approach and consume it is by definition a reward". In operant conditioning, rewarding stimuli function as positive reinforcers; however, the converse statement also holds true: positive reinforcers are rewarding.
Tuning of appetitive and defensive reactions in the nucleus accumbens shell (above). AMPA blockade requires D1 function in order to produce motivated behaviors, regardless of valence, and D2 function to produce defensive behaviors. GABA agonism, on the other hand, does not requires dopamine receptor function (below). The expansion of the anatomical regions that produce defensive behaviors under stress, and appetitive behaviors in the home environment produced by AMPA antagonism. This flexibility is less evident with GABA agonism.
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