Neural Correlates of Happiness: Some Insights

Abstract

Happiness is a subjective feeling of positive emotion. Happiness has been closely linked to pleasure and pleasant feelings. The meaning of the word “happiness” has multiple meanings which include good fortune or luck in life or in a particular affair (success, prosperity) and a state of pleasurable content of mind, which results from success or the attainment of what is considered good, which also includes successful or felicitous aptitude, fitness, suitability, or appropriateness. The psychological process of reward is closely linked to pleasure or liking, psychological and neuroscience studies have focused on the neural circuitry that processes reward and pleasure to understand how happiness is generated in the brain and also to elucidate which brain structures participate in generating pleasure and happiness.[1] The neuroscience of happiness is still in nascent stages with multiple circuits that have not been identified as yet. The role of specific structures has not yet been properly elucidated as well. Key brain structures or hotspots for pleasure have been identified using functional brain imaging techniques such as positron emission tomography and functional magnetic resonance imaging.[2] The article is an overview of the various neural substrates that may be involved in the generation and maintenance of happiness in humans. We shall first consider brain structures that presumably participate in generating feelings of pleasure and we shall also discuss important brain areas, including the basal ganglia, orbitofrontal cortex, anterior cingulate cortex, and the insula. TOWARD A NEUROBIOLOGY OF HAPPINESS Recent advances in social cognitive and affective neuroscience have revealed brain networks linked to pleasure and reward. The brain areas activated coincident with pleasure and positive emotions are widespread across the brain. These areas include the orbitofrontal cortex, the cingulate cortex, the medial prefrontal cortex, the insula, the nucleus accumbens, the ventral pallidum, the substantia nigra, and the ventral tegmental area.[3] These areas are often called hedonic hotspots as they light up during pleasurable activities in functional neuroimaging studies. A reward activates the reward system or hedonic hotspots and generates feelings of pleasure or positive emotions (i.e. liking) and also activates motivational systems and produces incentive behavior (wanting).[4] The electrical stimulation of subcortical structures, such as the nucleus accumbens, the lateral hypothalamus, and the ventral tegmental area, has been known to produce strong incentive behaviors in animal studies.[5] The orbitofrontal cortex, the insula, and the medial prefrontal cortex are considered to participate in liking and pleasure.[6] DOPAMINE – THE PLEASURE NEUROTRANSMITTER Behavioral studies have established that subcortical brain structures along the medial forebrain bundle produce the brain stimulation reward effect. There are multiple neurotransmitters or neurochemicals that play a role in this action. Dopamine is the main neurotransmitter that contributes to the reward effect. It has been shown that the administration of dopamine and prodopaminergic drugs modulate the reward effect. Brain mapping studies showed that the brain structures associated with the reward effect correspond to the structures where dopamine neurons exist, or dopamine fibers are present or terminate. The medial forebrain bundle includes massive dopamine fibers. Both the mesocortical dopamine fibers and the nigrostriatal dopamine fibers pass through the lateral hypothalamus, which is a part of the medial forebrain bundle and the most effective brain area of the reward circuit. The activation of the dopamine system could produce the reward effect and generate pleasant and happy feeling in animals and humans.[7] THE BASAL GANGLIA AND REWARD SYSTEMS In the basal ganglia, the nucleus accumbens and the ventral pallidum have been considered to participate in the mechanisms of liking that cause pleasure and reward. These areas enhance liking responses to sensory stimuli that produce pleasant feelings such as a sweet taste or opioid stimuli. The reward system contains distinguishable functional components such as liking (pleasure/palatability) and wanting (appetite/incentive motivation). Liking is mediated by neurotransmitter systems such as opioid systems and anatomical structures such as the ventral pallidum, whereas the mediation of wanting involves mesocortical dopamine systems and a division of the nucleus accumbens. Opioid stimulation in the nucleus accumbens or the ventral pallidum enhances hedonic liking responses to a sweet taste and appetitive wanting responses to a food reward.[8] Thus, the ventral pallidum and the nucleus accumbens seem to participate in liking mechanisms that cause pleasure responses. Therefore, these brain structures can be considered subcortical pleasure centers or hotspots and are important when considering the neurobiology of happiness. Experimental results suggest that the ventral pallidum participates in enhancing liking responses, whereas the nucleus accumbens participates in enhancing wanting responses.[9] ROLE OF THE ORBITOFRONTAL CORTEX IN HAPPINESS The orbitofrontal cortex contains the taste area and taste has been shown to represent the reward value of taste, in that neurons responded to food only when the animal was hungry. It also contains the olfactory area, and this area represents the reward value of odor, and olfactory neurons in this area respond to the smell of a food. The orbitofrontal cortex is also the decision-making area of the brain and plays a role in various reward and pleasure-based decisions that may be made by the subject. There is no neuroimaging evidence indicating that brain activity in the orbitofrontal cortex is correlated with a subjective report of pleasure or pleasant feelings.[10] Studies indicate that the orbitofrontal cortex represents not only the reward value of food but also the subjective pleasantness when the food is consumed. Several recent studies have shown that the encoding of pleasure reaches the orbitofrontal cortex. Neuroimaging studies have revealed correlations between brain activation and subjective pleasantness in human subjects. Cerebral blood flow changes were observed in the orbitofrontal cortex, in addition to the ventral striatum, the amygdala, and the ventromedial prefrontal cortex in cases of reward and pleasure.[11] The orbitofrontal cortex is an important brain region for processing pleasantness and unpleasantness. In a number of neuroimaging studies, there have been two distinct trends of neural processing in the orbitofrontal cortex. One trend is a mediolateral distinction. The medial orbitofrontal cortex is related to positive emotion. Medial orbitofrontal activity has been shown to reflect the reward value of many different food or liquid rewards. The medial orbitofrontal cortex was activated to the pleasantness of stimuli, such as whether the taste and smell of stimuli are pleasant and the degree of this pleasantness.[12] On the other hand, the lateral orbitofrontal cortex is related to negative emotion. Lateral orbitofrontal activity is known to reflect the unpleasantness of stimuli, such as monetary loss or painful touching. It has been shown that an abstract-concrete gradient is present along the posterior–anterior axis. More complex or abstract reinforcers (e.g., the gain or loss of money) are represented more anteriorly in the orbitofrontal cortex, whereas simpler and concrete reinforcers (e.g., a good taste or pain) are represented more posteriorly. Thus, within the orbitofrontal cortex, the medial-lateral hedonic gradient interacts with the posterior–anterior abstract-concrete gradient.[13] Although there is some evidence that orbitofrontal activity encodes the subjective experience of pleasure or happiness, it is not yet clear whether orbitofrontal activity actually causes positive emotion or pleasure. Damage to the orbitofrontal cortex impairs pleasure-related decision-making. Some caution is required when we consider whether the orbitofrontal cortex generates positive emotion or pleasure.[14] THE INSULA AND HAPPINESS Almost all recent imaging studies of emotion show activation of the anterior insula in human subjects experiencing a variety of emotional feelings, including maternal and romantic love, anger, fear, sadness, happiness, sexual arousal, disgust, aversion, unfairness, inequity, indignation, uncertainty, disbelief, trust, empathy, and beauty.[15] Activation of the left anterior insula was reported, whereas subjects were either seeing or making a smile. Activation in the insula was found when subjects listened to happy voices and was associated with hearing pleasant music. Further, selective activation of the left insula was found when subjects were experiencing joy. Thus, the anterior insula apparently is activated in association with a variety of subjective feelings including happiness or pleasant feelings.[16] THE ROLE OF THE MEDIAL PREFRONTAL CORTEX Recently, the medial prefrontal cortex including the anterior cingulate cortex has been considered to be an important brain structure for social cognition processes, such as self-reflection, person perception, and making inferences about other’s thoughts. However, the medial frontal cortex is not a uniform area. This area has been divided into several functionally different subregions.[17] Functional brain imaging studies show that the anterior dorsal cingulate sulcus is active when the link between an action and reinforcement is critical, especially during error detection, or when action selection is guided by the expectation of a reward. This region is also important for detecting when actions are potentially in conflict. In contrast, the ventral cingulate region is active when subjects experience pain or during autonomic arousal. Importantly, both ventral and anterior dorsal cingulate regions are active during emotion and social interaction. Anterior cingulate activation was observed when subjects judged their own affective responses. Thus, the anterior cingulate area could participate in the self-knowledge of the subject’s subjective feelings. Cingulate participation in the self-knowledge of one’s own subjective feeling could be important for us to experience happy feelings.[18] CONCLUSIONS Happiness is a kind of subjective feeling of positive emotions. Since the psychological process of reward is closely linked to pleasure, studies to elucidate the neural mechanisms of happiness have focused on the neural circuitry for processing reward information. Although the neuroscience of happiness is still in the primitive stage compared with the neuroscience of perception and motor control, key brain areas related to happiness have been clarified using functional brain imaging techniques. These brain areas include the orbitofrontal cortex, the cingulate cortex, the medial prefrontal cortex, the insula, the nucleus accumbens, the ventral pallidum, the substantia nigra, and the ventral tegmental area. When we consider neural mechanisms of happiness, we need to distinguish between neural mechanisms for liking and those for wanting, and between those that encode and those that cause pleasant feelings. A reward activates hedonic hotspots and generates pleasant feelings or positive affections (liking). At the same time, the reward also activates motivational systems and produces incentive behavior (wanting). On the other hand, cortical areas, including the orbitofrontal cortex, the insula, and the medial prefrontal cortex, are considered to participate in “liking” mechanisms. Happiness has been incorporated into many programs that involve psychosocial rehabilitation with a fair degree of success. There are views that happiness is a state of mind and may also be a personality trait that is linked to resilience and well-being. There is a need for an understanding of the neurobiological basis for happiness as it will provide insight into the various neural correlates of a happiness both as a state of mind and a personality trait. Based on findings seen across structural and functional neuroimaging, the various brain structures involved in the generation of happiness can be elucidated and an attempt to neuromodulate these circuits involved in the generation and maintenance of happiness may be attempted. There is some evidence that orbitofrontal activity encodes the subjective experience of pleasure or happiness. However, it is not yet clear whether orbitofrontal activity actually causes positive emotion or pleasure. Further experiments are needed to identify the brain area that causes the subjective experience of happiness. Further neuroimaging and neurobiological models need to be created to elucidate the true neural correlates of happiness.