stimulation of the mpfc as a whole produces antidepressant effects. this effect appears localized to the rodent homologue of the pgacc (the prelimbic cortex), as stimulation of the rodent homologue of the sgacc (the infralimbic cortex) produces no behavioral effects. furthermore, deep brain stimulation in the infralimbic cortex, which is thought to have an inhibitory effect, also produces an antidepressant effect. this finding is congruent with the observation that pharmacological inhibition of the infralimbic cortex attenuates depressive behaviors. schizophrenia schizophrenia is associated with deficits in motivation, commonly grouped under other negative symptoms such as reduced spontaneous speech. the experience of “liking” is frequently reported to be intact, both behaviorally and neurally, although results may be specific to certain stimuli, such as monetary rewards. furthermore, implicit learning and simple reward-related tasks are also intact in schizophrenia. rather, deficits in the reward system are apparent during reward-related tasks that are cognitively complex. these deficits are associated with both abnormal striatal and ofc activity, as well as abnormalities in regions associated with cognitive functions such as the dorsolateral prefrontal cortex (dlpfc). attention deficit hyperactivity disorder in those with adhd, core aspects of the reward system are underactive, making it challenging to derive reward from regular activities. those with the disorder experience a boost of motivation after a high-stimulation behaviour triggers a release of dopamine. in the aftermath of that boost and reward, the return to baseline levels results in an immediate drop in motivation. impairments of dopaminergic and serotonergic function are said to be key factors in adhd. these impairments can lead to executive dysfunction such as dysregulation of reward processing and motivational dysfunction, including anhedonia. history skinner box the first clue to the presence of a reward system in the brain came with an accidental discovery by james olds and peter milner in 1954. they discovered that rats would perform behaviors such as pressing a bar, to administer a brief burst of electrical stimulation to specific sites in their brains. this phenomenon is called intracranial self-stimulation or brain stimulation reward. typically, rats will press a lever hundreds or thousands of times per hour to obtain this brain stimulation, stopping only when they are exhausted. while trying to teach rats how to solve problems and run mazes, stimulation of certain regions of the brain where the stimulation was found seemed to give pleasure to the animals. they tried the same thing with humans and the results were similar. the explanation to why animals engage in a behavior that has no value to the survival of either themselves or their species is that the brain stimulation is activating the system underlying reward. in a fundamental discovery made in 1954, researchers james olds and peter milner found that low-voltage electrical stimulation of certain regions of the brain of the rat acted as a reward in teaching the animals to run mazes and solve problems. it seemed that stimulation of those parts of the brain gave the animals pleasure, and in later work humans reported pleasurable sensations from such stimulation. when rats were tested in skinner boxes where they could stimulate the reward system by pressing a lever, the rats pressed for hours. research in the next two decades established that dopamine is one of the main chemicals aiding neural signaling in these regions, and dopamine was suggested to be the brain's "pleasure chemical". ivan pavlov was a psychologist who used the reward system to study classical conditioning. pavlov used the reward system by rewarding dogs with food after they had heard a bell or another stimulus. pavlov was rewarding the dogs so that the dogs associated food, the reward, with the bell, the stimulus. edward l. thorndike used the reward system to study operant conditioning. he began by putting cats in a puzzle box and placing food outside of the box so that the cat wanted to escape. the cats worked to get out of the puzzle box to get to the food. although the cats ate the food after they escaped the box, thorndike learned that the cats attempted to escape the box without the reward of food. thorndike used the rewards of food and freedom to stimulate the reward system of the cats. thorndike used this to see how the cats learned to escape the box. other species animals quickly learn to press a bar to obtain an injection of opiates directly into the midbrain tegmentum or the nucleus accumbens. the same animals do not work to obtain the opiates if the dopaminergic neurons of the mesolimbic pathway are inactivated. in this perspective, animals, like humans, engage in behaviors that increase dopamine release. kent berridge, a researcher in affective neuroscience, found that sweet (liked ) and bitter (disliked ) tastes produced distinct orofacial expressions, and these expressions were similarly displayed by human newborns, orangutans, and rats. this was evidence that pleasure (specifically, liking) has objective features and was essentially the same across various animal species. most neuroscience studies have shown that the more dopamine released by the reward, the more effective the reward is. this is called the hedonic impact, which can be changed by the effort for the reward and the reward itself. berridge discovered that blocking dopamine systems did not seem to change the positive reaction to something sweet (as measured by facial expression). in other words, the hedonic impact did not change based on the amount of sugar. this discounted the conventional assumption that dopamine mediates pleasure. even with more-intense dopamine alterations, the data seemed to remain constant. however, a clinical study from january 2019 that assessed the effect of a dopamine precursor (levodopa), antagonist (risperidone), and a placebo on reward responses to music – including the degree of pleasure experienced during musical chills, as measured by changes in electrodermal activity as well as subjective ratings – found that the manipulation of dopamine neurotransmission bidirectionally regulates pleasure cognition (specifically, the hedonic impact of music) in human subjects. this research demonstrated that increased dopamine neurotransmission acts as a sine qua non condition for pleasurable hedonic reactions to music in humans. berridge developed the incentive salience hypothesis to address the wanting aspect of rewards. it explains the compulsive use of drugs by drug addicts even when the drug no longer produces euphoria, and the cravings experienced even after the individual has finished going through withdrawal. some addicts respond to certain stimuli involving neural changes caused by drugs. this sensitization in the brain is similar to the effect of dopamine because wanting and liking reactions occur. human and animal brains and behaviors experience similar changes regarding reward systems because these systems are so prominent.