As previously described in detail,32
feeling a normal emotion requires the identification of the emotional significance of a stimulus (appraisal), then the production of an affective state (production), which can be regulated at different levels (regulation). These three steps can be considered as being organized through two different systems, with a reciprocal functional relationship.
A ventral system (including the amygdala, insula, ventral striatum, and ventral regions of the anterior cingulate gyrus and prefrontal cortex), could be more specifically involved in the identification of the emotional significance of environmental stimuli, and the production of affective states. This system could also be in charge of automatic regulation and mediation of autonomic responses to emotive stimuli and contexts accompanying the production of affective states.
A dorsal system (including the hippocampus and dorsal regions of the anterior cingulate gyrus and prefrontal cortex), on the other hand, could be more important for effortful rather than automatic regulation of affective states, probably through executive functions, including selective attention and planning.
The basis of hedonic feelings has been more specifically studied through different paradigms. Euphoric response to dextroamphetamine,33
and even pleasurable responses to music,37
and attractive faces,39
have been associated with activity within the nucleus accumbens, ventral caudate, and ventral putamen, and, in studies devoted to the neurobiology of pleasure, with dopamine release in the ventral caudate and putamen.
The ventral striatum, and particularly the nucleus accumbens,40
may indeed have a priority role according to studies in both animals and humans, in behavioral responses to, anticipation of, and/or monitoring of errors in the prediction of reward.41,42
The nucleus accumbens appears to respond to the emotional intensity and self-relatedness of a variety of stimuli, independent of their valence,43
with both positive and negative valences possibly processed along a rostrocaudal gradient.44
The nucleus accumbens receives projections from midbrain regions (such as the ventral tegmental area), from regions involved in emotion (such as the amygdala, orbitofrontal cortex, and medial prefrontal cortex), from motor regions (such as the dorsal caudate and globus pallidus), and from regions involved in memory (such as the hippocampus).45
The accumbens also indirectly projects to cortical regions including the cingular and medial prefrontal cortex, the ventral pallidum, the thalamus, the amygdala, and the hypothalamus.46-48
Many of these regions are also implicated in emotion processing, suggesting a network of tightly anatomically and functionally connected regions.49
The orbitofrontal cortex is a nexus for sensory integration, the modulation of autonomic reactions, and anticipation in learning, prediction and decision-making for emotional and reward-related behaviours.50
Ncuroimaging studies have found that the reward value,51
and the expected reward value,52
and even the subjective pleasantness of food53
and other reinforcers are represented in the orbitofrontal cortex. The orbitofrontal cortex receives input from the five classic sensor}' modalities: gustatory, olfactory, somatosensory, auditory, and visual,54
and also receives visceral sensory information. This large variety of inputs makes the orbitofrontal cortex one of the most polymodal regions in the entire cortical mantle.53
The orbitofrontal cortex has direct reciprocal connections with other brain structures, including the amygdala, cingulate cortex, insula/operculum, hypothalamus, hippocampus, striatum, periaqueductal grey, and dorsolateral prefrontal cortex. Hence, the orbitofrontal cortex may have an important role for representing incentive salience, hedonic impact, and subjective hedonic experience, ic, constituting the link between reward and hedonic experience.53
It has been shown that the human amygdala is a key structure for extracting the affective significance from external stimuli,55
responds preferentially to emotionally valenced faces, for fearful but also for happy faces, and rapidly habituates to them.56
According to discrepant findings,57,58
the amygdala could be considered as reacting more intensively for negative stimuli, explaining its major function in fear and anxiety.
The anterior cingular cortex was not activated by transient happiness induced by recalling positive life events and looking at happy human faces.59
On the other hand, individual differences in the ability to accurately detect emotional signals, interoceptively or exteroceptive ly, may be at least in part a function of the degree to which the anterior cingulate cortex participates in the experiential processing and response to emotion cues.60
The ventromedial prefrontal cortex (VMPFC) has been implicated21
in the generation of an abstract representation of the rewarding value of a stimulus by attending to its context,61
and the learning of contingencies based on the outcome of a rewarding situation.35
By contrast, lateral areas of the ventral prefrontal cortex may be less involved in hedonic emotions, responding to avcrsive rather than rewarding stimuli.62,63
Some other regions might have a more obvious role in negative and/or distressing emotions rather than hedonic experiences, such as the insula.64,65
Recall-generated sadness was associated with significantly greater increases in activity in the vicinity of the anterior insular cortex, suggesting that this region participates in the emotional response to potentially distressing cognitive or interoceptive sensory stimuli.