By Sabino Pastor Mendez
Pair bonding and affection are not exclusive to humans. Rodents establish mother-offspring bonds although these are nonselective and short lived. Rodent mothers care for any infant in their surroundings and this attachment is limited to the nest and dependent on pregnancy hormones. Still, monogamous prairie voles display mating-based recognition and form long-lasting couples. Primate’s enlarged neocortex allows them to form selective attachments. They can create exclusive maternal bonds which are hormone primed but not hormone dependent. However, most primates are not monogamous and those who are show proximity-based partner preference. This suggests that primates form none or nonselective pair bonding (Feldman, 2017).
The ability shared by most mammals to form affective bonds indicates the presence of an evolutionary ancient system underpinning maternal care and pair bonding. This mechanism involves the crosstalk of dopamine (DA) and oxytocin (OT) circuits. DA acts in the nucleus accumbens (NA) to induce feelings of reward and therefore goal-oriented behaviours. DA blocks the inhibitory output of NA GABAergic (inhibitory) medium spiny neurons which project into the ventral pallidum (VP) in the basal ganglia (Scofield et al., 2016). Disinhibition of the accumbent-pallidal pathway makes the VP accessible to the inputs of glutamate (excitatory) neurons. This leads to increased motor neuron activity which translates in an energetic behaviour that enhances reward seeking. This process is intensified by the binding of OT to its corresponding receptor in NA neurons, as it increases the affinity of D2-type receptors to DA (Feldman, 2017). In addition, the activation of neurons specifically suited to identify sensory-motor reward patterns in the NA by OT enables them to remember the temporal patterns of social reward. This allows the brain to internalize the social partner and its preferences, encode relationship-specific patterns of social exchange and draw reward from the matching of self and partner’s actions (social synchrony) which lead to consolidation of the bond (Báez-Mendoza and Schultz, 2013). Research in rats indicated that OT also causes long-term cellular depression in the amygdala. This attenuates the amygdalar response to aversive social stimuli, reducing fear and facilitating bonding (Maroun & Wagner, 2016). Therefore, DA produces vitality and motivation, and OT provides the soothing and tranquillity necessaries for bond formation via its regulatory effects on the hypothalamic-pituitary-adrenal axis and its anxiolytic properties. This combination creates the perfect neurobiological state for the formation of new attachments.
DA and OT systems also underpin the formation of attachment bonds in humans. Human studies involving OT administration or allelic variability and methylation of the OT receptor gene have implicated the OT system in human maternal, paternal, romantic and friendship attachments. However, unlike other mammals, humans can form exclusive, long-term, mating-independent pair bonds built on romantic love. Romantic love is based on representations and memory, adapts to cultural norms, incorporates socio-cognitive abilities of empathy and trust and extends from the here-and-now so it can be felt in its absence (e.g., deceased partner). This requires complex high-order processes that involve learning, memory, planning and predictions. Therefore, the OT and DA circuits of the limbic system represent a mammalian general mechanism and only in humans they connect with multiple cortical sites via ascending and descending projections that support human’s long-term exclusive attachments. These connections allow humans to combine reward, passion, proximity seeking, vitality and unconscious motivation with trust, empathy, and commitment to create and maintain long-lasting attachments (Feldman, 2017).
For example, communication between the reward pathway and the orbitofrontal cortex (part of the prefrontal cortex) is implicated in the representation of pleasantness and in effortful, goal-directed actions to tend long-term relationships. The orbitofrontal cortex selects among rewards and enables the avoidance of intermediate rewards toward long-term attachment goals, shaping affiliations in the context of cultural norms and personal preferences (Parsons et al., 2013). In addition, several frontotemporal-parietal structures have a vital role in social cognition, evaluations of other’s state, social goal interpretation and prediction making (Frith and Frith, 2006). Human bonds rely on these as they build on the individual’s ability to appreciate multiple perspectives, understand the partner’s goals and motives, and keep in mind his/her values and concerns.
In conclusion, love exists specifically in humans as the result of a complex interaction between dopamine and oxytocin reward circuits of the limbic system and higher cerebral structures of the human brain cortex. Yet, love goes beyond any biological insights and must be considered in the context of the individuals experiences, expressions and aspirations.
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Frith, C. D. and Frith, U. (2006) ‘The Neural Basis of Mentalizing’, Neuron. Cell Press, pp. 531–534. doi: 10.1016/j.neuron.2006.05.001.
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