By Luciano Marinelli
Happiness is a term that can be equated to well-being, meaning, life satisfaction and welfare. It is an emotional state that plays a crucial part in our lives, and drives many of our life decisions and actions. According to Charles Darwin, affective emotions drive evolution of species, being adaptive responses to the environment. These allow us to find ways to survive and reproduce, attributing meaning to things in the world (Kováč L., 2012; Kringelbach M.L. et al., 2011). The discovery of the brain’s pleasure centre by psychologists Olds and Milner in 1954 (Olds J. et al., 1954) led to scientists investigating the links between the brain and happiness, which will be outlined in this review.
Happiness can be divided into two different conceptualizations: hedonia and eudaimonia. Hedonia is a state of subjective well-being. It is short-lasting and is caused by sensory stimuli, and can be, for example, the positive sensation from the taste of good food. On the other hand, eudaimonia is an inner feeling of satisfaction and psychological harmony, not depending on external stimuli, and refers to the realization of one’s own potential. Achieving a goal is an example of this (successconsciousness.com; Fave A.D. et al., 2010). While once thought to be completely unrelated to each other, research has shown that pleasure (hedonia), combined with engagement and meaning, is needed in order to achieve happiness (eudaimonia), hence life enjoyment (Fave A.D. et al., 2010).
Because of this connection, a hedonic approach is used to understand happiness, meaning the brain activity behind hedonic states are used to study the neurobiology of happiness. However, one of the limitations of investigating pleasure is its subjective nature. Progress has been made by identifying objective aspects of pleasure which are beyond its subjective ones, and brain activity is investigated using these (Kringelbach M.L. et al., 2011).
Studies have identified several hedonic generators in the brain, including subcortical brain regions (called hedonic hotspots) and several in the cortex (Kringelbach M.L. et al., 2011), which are mostly part of the dopaminergic mesocorticolimbic pathway (Smith K.S. et al., 2009; Berridge K.C. et al., 2015). A hedonic hotspot is a limbic brain site where “pleasure mechanisms are sufficiently concentrated together in one anatomical locus to cause pleasure enhancement when neurally activated” (Smith K.S. et al., 2010). These sites are embedded in a larger hedonic circuit, which includes the mesocorticolimbic pathway, and as a whole increases hedonic experience when stimulated (Smith K.S. et al., 2010). These hotspots include the nucleus accumbens and the ventral pallidum.
The nucleus accumbens, one of the main hotspots, interacts with other distributed components like the ventral pallidum and the lateral hypothalamus to cause pleasure from food rewards in particular. Opioid neurotransmitter is the main responsible for this (Smith K.S. et al., 2005). Because it contains opioid receptors, opioids can be used to investigate its role in pleasure. Opioid microinjection increased “liking” reactions from sucrose taste up to two times the original number, but only when injected into the nucleus accumbens’ medial shell, meaning only this region of the nucleus accumbens is responsible for “liking” reactions. This hotspot is also involved in the “wanting” of these sensory rewards, even though this is not specific to any nucleus accumbens region (Smith K.S. et al., 2010; Castro D.C. et al., 2015). The results from opioid microinjection highlight the role of this structure in sensory pleasure from food as well as in seeking it.
The nucleus accumbens projects onto the ventral pallidum, which exchanges information with the former to amplify the hedonic impact from sensory rewards. Because it is the structure to which pleasure-related neural pathways converge, it is often referred to as the “final common pathway”. Like the nucleus accumbens, it enhances both the motivation to eat and the hedonic impact and is also stimulated by opioids, as opioid stimulation in its caudal half elicited hedonic reactions as a result of sucrose taste (Smith K.S. et al., 2010). Furthermore, Cromwell et al. showed that local lesions in this area not only eliminated hedonic impact from sucrose, but also caused active “disliking” reactions in rats to sucrose (Cromwell H.C. et al., 1993), the only brain region in which this has been observed (Smith K.S. et al., 2005).
The nucleus accumbens and the ventral pallidum are both linked to the orbitofrontal cortex, which codes for hedonic evaluations, including anticipations, appraisal and memory of pleasurable stimuli (Kringelbach M.L. et al., 2010). Unlike the previously mentioned subcortical structures, the orbitofrontal cortex is responsible for the subjective pleasure experience. It has also been shown to be responsible for subjective ratings of pleasantness and unpleasantness for a particular stimulus (Funahashi S., 2011). Other cortical structures, such as the insula, medial prefrontal and cingulate cortices, are involved in subjective pleasure, even though it is unclear whether they code for pleasure or emotions in general (Kringelbach M.L. et al., 2010).
The orbitofrontal cortex is also part of the brain’s frontal default network. The default network is a neural circuit which is active during passive moments and when remembering past events or imagining future events (Buckner R.L., 2013). Literature has proposed that it carries internal modes of cognition and states of consciousness, which are essential components for eudaimonic happiness (Kringelbach M.L. et al., 2010); lesions in the brain’s default network have been in fact linked to depression (Wise T. et al., 2017). Because the default network, responsible for eudaimonic happiness, is connected to the previously mentioned hedonic hotspots, responsible for hedonic pleasure, the link between pleasure and happiness can be observed. However, even though pleasure may contribute to achieving happiness, it will never cause happiness and life satisfaction by itself.
The major limitations of investigating happiness remain its subjective nature and the relationship between pleasure and happiness. Even though a lot of research has been done on these psychological states, the mechanisms that different brain systems use are still unclear, and this topic remains a grey area. Hopefully, future science advancements will allow this research to be carried out more accurately in order to have a greater understanding of happiness, allowing us to find more and better ways in which we can achieve a meaningful and well-lived life.
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