The Neuroscience of Happiness

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Reward circuit: Three key areas implicated in the dopaminergic reward circuit in the brain: NAcc, ACC, OFC

Can neuroscience inform us about the nature of happiness? With so many people interested in how to be happy, great utility would exist in being able to measure this emotion objectively with neuroscientific methods. While considerable progress has been made measuring happiness through subjective self-report, it historically been difficult to connect to a comprehensive underlying neurobiology. Yet, recent meta-analyses and developments in neuroimaging suggest progress in our ability to objectively measure the emotional experience of happiness. These recent findings are not without controversy. Much debate still exists around whether happiness can be examined as a discrete emotion or a part of a dimensional continuum. Researchers also contest whether happiness can be mapped onto a specific region of the brain or only examined as part of a dynamic neural network. Below is an overview of the recent findings and debate. The article concludes by integrating these findings, discussing future directions, and taking the view that, unfortunately, we still have a ways to go before neuroscience can consistently identify states of happiness and meaningfully inform better paths toward its obtainment.


Categorical vs. Dimensional View

Basic emotions: Activation likelihood maps from Vytal and Hamann (2010) representing regional activity consistently associated with each discrete emotion. Horiontal lines overlaid on the sagittal image (far right) show locations of axial slices. Left side corresponds to left side of image. Color gradient represents degree of activation consistency.[5]

Scientists have primarily measured two aspects of happiness up to now through subjective self-report: hedonia (pleasure) and eudaimonia (meaning and a life well-lived).[1] Researchers also examine happiness through measuring positive affect, analyzing both the state (including physiology and neural substrates) and the subjective experience of emotion (involving cognition).[2] Yet, debate exists among affective researchers on the biological nature of emotions and how they should map onto the brain.

Basic Emotions View

Some argue for a categorical approach, stipulating that happiness is one of a number of basic emotions, including anger, sadness, happiness, fear, and disgust.[3] Basic emotion theorists take these discrete emotions to be innate, universal, and to have unique physiological patterns, facial expressions, and neural correlates.[4] In other words, happiness should exist and be measurable in the brain distinct from the other basic emotions.

A recent meta-analysis by Vytal and Hamann (2010) provide some support for this approach to emotion research. In their analysis of 83 PET and fMRI neuroimaging studies from 1993 to 2008 with activation likelihood estimation, the researchers found that each of the five basic emotions was associated with a consistent and discrete set of neural correlates with regional brain activations.[5]

Dimensional view

Dimensional approach: Graphical representation of the affective circumplex, with the valence dimension represented horizontally on the x-axis and arousal vertically on the y-axis.

In contrast to the basic emotions view, a dimensional approach claims these discrete emotions are instead categories or elaborated responses arising from more fundamental biological properties such as valence (degree of pleasantness) and arousal (level of intensity), often combined with cognitive processes such as appraisal.[6] From this perspective, happiness would not be a biologically ingrained, innate emotion, but, the product of an emotional experience of high valence and, depending on cultural context, some arousal. Dimensional theorists would then be more interested in looking for the neural correlates of the dimensional axes (e.g., positive valence) than the specific emotions.

Past research on lateralization has suggested that positive and negative emotions may be associated with greater activation in the left and right hemispheres, respectively. For instance, one study of 84 older adults found greater left than right superior frontal activation with electroencephalography was associated higher levels of self-reported hedonic and eudaimonic well-being[7] However, a meta-analysis by Murphy et al. (2003) found that neural activity associated with positive and negative emotions was found to be relatively symmetrical,[8] and another meta analysis by Wager et al. (2003) similarly found limited support for valence-specific lateralization of emotional activity in frontal cortex.[9]

A different dimensionalist hypothesis for identifying neural correlates of happiness is that subregions of orbitofrontal cortex process positive and negative valence and that the amygdala processes level of arousal.[10] Indeed, past EEG and lesion studies have pointed to at least partially separable neural systems for positive and negative emotions.[8] More specifically, some researchers have found evidence that the medial edge of the orbitofrontal cortex has been connected to positive and negative valence.[11] Others have found that activation of the Nucleus Accumbens, part of the Basal Ganglia, correlates with positive arousal, but not with negative arousal.[18] However, the same meta-analyses above found that the spatial distributions of positive and negative emotions were not significantly different,[8] and additional studies suggest a more complex process by which multiple regions represent arousal, valence, and their interaction. [10][12]

Locationist vs. Networks Approach

Locationist approach

Originating with William James, psychologists have long debated whether emotions map one-to-one with specific regions in the brain or not.[13] Locationists hypothesize that, indeed, basic emotions such as happiness consistently and specifically correspond to distinct brain regions.[14] This theory has traditionally been predicated on the involvement of the amygdala in emotions of fear, although the amygdala has at times also been connected to other emotions as well.[15] When most people ask about the neuroscience of happiness, they likely wonder if a particular part of the brain is solely responsible for happiness. If so, we could objectively measure well-being more easily than relying on self-report. However, recent meta-analyses do not lend support to the locationist approach to happiness.[8] Instead, in one analysis, for instance, each region that was activated for one discrete emotion was also activated for at least one other as well.[14]

Networks approach

In light of individual brain regions often contributing to multiple emotions, a one-to-one mapping approach for hedonic pleasure or life satisfaction gives way to a more complex, network-based model of emotion.[16] As Kringelbach and Berridge (2009, p. 482) write, “analogous to scattered islands that form a single archipelago, hedonic hotspots are anatomically distributed but interact to form a functional integrated circuit.” [11] In rhesus monkeys, for instance, the rostral cingulate gyrus has been found to project to rostral superior temporal gyrus, midorbitofrontal cortex, and lateral prefrontal cortex and ventral anterior cingulate sends projections to the anterior insula cortex, premotor cortex, and orbitofrontal cortex, among others (most of which are implicated in pleasure or happiness, as discussed below).[17] Corticial and sub-cortical networks involved in pleasure and happiness have anatomical links to many regions across these orbitofrontal, medial prefrontal, and cingulate cortices. While a consistent and discrete network of happiness per se has not yet been fully uncovered, we will now review the main candidates for such a network that have been most implicated in neuroimaging studies of happiness.

Brain Areas

Basal ganglia situated in the brain
Ventral pallidum and basal ganglia situated in the brain

To ascertain which brain regions are implicated in a neural network of happiness, individual imaging studies alone are insufficient due to low statistical power, heterogeneity in task design, method, and imaging technique.[15] Instead, meta-analyses are relied upon to determine consistent and disciminable neural network patterns since they pool effects across multiple studies. Below are key findings.

Basal Ganglia

The principal components of the basal ganglia include the nucleus accumbens (NAcc), striatum (caudate nucleus and putamen), substantia nigra and subthalamic nucleus.[19] Within these structures, a mini-limbic dopaminergic system including the nucleuss accumbens, ventral palladum and ventral tagmental area (VTA) is purportedly involved in the experience of pleasant emotion. Indeed, in one major meta-analysis,[15] almost 70% of happiness induction studies reported activation in the basal ganglia, across responses to happy faces,[20] pleasant pictures,[21] pleasant sexual and successful competitive arousal.[22] In another meta-analysis by Wager et al. (2003), positive emotions were found to be more likely to activate the basal ganglia than negative emotions.[23] The NAcc in particular appears to play a important role in pleasure. One study found that as NAcc activation increased when viewing faces, happiness tended to increase, possibly driven by the NAcc’s enhanced dopaminergic activity.[24] Further, whereas anticipation of increasing rewards elicited both increasing self-reported happiness and NAcc activation, anticipation of increasing punishment elicited neither.[25]

The ventral pallidum also appears to be a key component to any neural network of happiness. Of all the hedonic brain regions, only damage to the ventral pallidum fully abolished the capacity for pleasure in rodent studies[2] In humans, a case study also found anhedonia following bilateral lesion to the ventral pallidum.[28] However, while the NAcc, ventral palladum and other limbic components of the basal ganglia appear to be consistently implicated in reward circuitry, a one-to-one matching of the basal ganglia to happiness would be oversimplified. For one, while basal ganglia activation was reported in 70% of happiness studies (as mentioned above), it was also reported in 60% of studies evoking disgust.[15] In Murphy et al. (2003), similarly, a reliable and specific association existed between disgust and the basal ganglia[8]. However, it appears that the globus pallidus is the primary component of the basal ganglia implicated in disgust (in over 70% of the studies), leaving the NAcc, VTA, substantia nigra, and ventral pallidum appearing in tact in their involvement in pleasure and happiness.[8]

Orbitofrontal cortex integrated with the nucleus accumbens and VTA of the basal ganglia, situated in the brain.

Orbitofrontal Cortex

Neuroanatomically connected to the nucleus accumbens is the orbitofrontal cortex (OFC), another brain region highly involved in the processing of pleasure and thus a strong candidate for any hedonic brain network. The OFC may further be split into a mid-anterior and anterior region. The mid-anterior region is purported to be highly involved in the coding of the subjective experience of pleasures of the senses - neuroimaging studies have shown strong correlations to pleasantness ratings for food, sexual orgasms, drugs, chocolate, and music.[2] The more anterior region is supposedly involved in more complex positive reinforcers such as monetary gain.[2] A meta-analysis by Kringelbach and Rolls (2004) of 87 publications from 1994 to 2003 confirmed the involvement of different sub-regions of the OFC in both reward and punishment. Whereas the lateral OFC is related to the evaluation of punishment, the medial OFC was found to be consistently involved in the processing of reward. [26]

Anterior cingulate cortex (ACC): Sagittal MRI slice with highlighting location of the ACC

Anterior Cingulate Cortex

The ventral part of the anterior cingulate cortex (ACC), or Brodmann area 24, is also connected to the nucleus accumbens (and many other regions implicated in emotion) and appears highly important for any neural network associated with happiness. Indeed, studies on therapeutic interventions involving either cognitive behavioral therapy or deep brain stimulation found that reduced depressive symptoms were associated with increased activity in ventral ACC, along with increased hippocampus volume and a number of other metabolic changes.[27] Meta-analyses also implicate parts of the ACC in happiness. Murphy et al. (2003) for instance found that the rostral supracallosal ACC (as well as dorsomedial PFC) was the most consistently activated region found in happiness. But the same study found that the rostral part of the ACC was also activated by sadness.[8] In contrast, a more recent meta-analysis by Vytal and Hamann (2010) that included new data from an additional 30 studies since Murphy et al. and used a more spatially sensitive activation likelihood estimation (ALE) method found that activation of both left ventral and rostral ACC were highly associated with happiness but not sadness, nor any other negative emotions.[5]

Other candidates

In Vytal and Hamann's 2010 meta-analysis, the largest activation foci associated with happiness was located primarily in the right superior temporal gyrus (STG), or Brodmann's area 22.[5] STG is normally implicated in auditory or language processing, but it has also been found to be important for social cognition,[29] particularly in perceiving emotions in facial stimuli. While many of the studies in Vytal and Hamann's meta-analysis involved viewing facial expressions, there did not appear to be a significantly higher number of happiness studies involving facial recognition. Perhaps given the importance of social connection to well-being, certain sub-components of STG may be particularly sensitive to positive facial expressions. This is an area worth further exploration.

Conclusion

Based on recent meta-analyses in affective neuroscience, it appears that pleasure may be objectively measured through a combination of "hedonic" interconnected cortical and sub-cortical regions, including, but not limited to, dopaminergic components of the basal ganglia (NAcc, VTA, ventral palladum), the midanterior subregion of the orbitofrontal cortex, the right rostral and ventral anterior cingulate cortex, and, possibly, the right superior temporal gyrus. However, a unified neuroanatomy of happiness remains elusive. Given the wide variance among the meta-analytic studies and lack of clear consistency and specificity, we cannot place too much confidence in our ability to both predict subjective well-being nor discriminate levels of happiness between subjects based on their neuroimaging feedback.

The above meta-anlayses relate largely to hedonic pleasure elicited in laboratory settings through film, facial expression, music, food or image mood inductions. While some studies looked at changes in ACC activation outside the lab following behavioral therapy,[27] the studies herein for the most part investigated only the hedonic component of happiness, or the momentary experience of pleasure. The eudaimonic component - i.e., a broader sense of meaning, contentment, engagement, or well-being, has been explored to a lesser degree. Given emotion and cognition are functionally integrated systems,[30] neural correlates of eudaimonic happiness likely implicate a broader scope of cortical structures, but may be just as important to measure as hedonic pleasure to ascertain an objective state of happiness. However, individual differences in age, sex, and personality likely make the waters murkier. Past meta-analyses have found substantial differences in certain brain regions for positive and negative emotions.[31] It is also possible that most studies done in the lab do not reach the level of emotion intensity (in terms of valence) that people experience outside. While it may be reasonable to assume that the same brain regions are implicated across multiple levels of intensity for the same emotion, we cannot rule out the possibility that some key brain regions for measuring happiness are only activated above certain thresholds, which have not yet been empirically explored in the lab given natural constraints on stimulus intensity.

Future Directions

The bulk of the neuroimaging studies discussed herein examined the correlation between emotions and brain regions, but did not assess whether these brain regions are essential for happiness. To arrive at a more comprehensive emotional model of happiness, other methods such as studies with nonhuman animals (for reward circuitry) and human neuropsychological studies with focal brain lesions and deep brain stimulation are likely needed. Using more ecologically valid emotional experiences in the laboratory will also help fill out a more full range of neural correlates of happiness. For instance, meaningful news items such as end-of-semester or quarter grades, test results, or sports game scores (for serious fans) can voluntarily be withheld till experimentation to try to measure the neuroanatomy of happiness involved in achievement, satisfaction, or excitement from real-world events.

References

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Further reading

External links

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