by Catherine Nahm.
For group page, The Social Brain.

Romantic Love

Romantic love produces a set of complex emotions that are distinctly different from other types of love, such as maternal love, or compassionate love[1] . It does not only affect behaviours, but also alter the brain's neuronal activities of the affected individual significantly. Recent advances in brain-imaging techniques have enabled close examination of the phenomenon from the neurobiological perspective. fMRI studies have shown active blood oxygenation level-dependent (BOLD) response observed in subcritical mesolimbic areas during the course of romantic love[2] [3] [4] , which are critical for dopaminergic reward system. The unusual physiological and behavioural changes observed during the early stage of romantic love, are implicated with transient changes in the neuronal activity. Love-induced changes in the brain show rapid onset, yet these are sufficient to alter the individual's cognitive processing significantly. In addition, the changes in neural signalling pattern and synapses are strongly associated with the longevity of romantic love[5] . Together with social psychological studies, the neurobiological approach to romantic love views the critical mechanism to be the system of mammalian mate choice, and that human brain has developed implicit mechanisms in order to maximize the beneficial effect for the chance of survival and reproductive success.[6] [7]




(I) Early-Stage Romantic Love

Up to date, there is no strong experimental evidence that clearly defines the duration of each “stage” of romantic love. Yet in many studies, the duration of “early, intense stage” of romantic love is from 1 month and less than 1 year in most cases
[8] [9] [10] . During the period, a number of physiological and psychological changes can be observed, for example “intense feelings of euphoria, well-being and preoccupation with the romantic partner[11] ” the affected individual may also exhibit behavioural changes. fMRI studies on brains of those who are deeply in love suggest that these changes are correlated with modification in neural connections and activity patterns over time.

fMRI scans taken from individuals who are in the early, intense stage of romantic love, show that the dopaminergic reward and motivation centre and the associated structure are implicated in romantic love. In particular, the right ventral tegmental area (rVTA), the caudate, the anterior cingulate cortex and the insula show a marked increase in BOLD response when a photograph of the significant other was presented as the love stimuli, compared to neutral stimuli (photographs of strangers) or moderate stimuli (photographs of friends or of close relation) (Aron 2005, Fisher 2005, Ortigue 2007, Xu 2011, Kim 2009). Similar activation pattern was also observed with name of the romantic partner was used as love stimuli[12] . The degree of activation is associated with the intensity of love (Ortigue 2011) and less activated in people who are not satisfied with the current relationship [13] .



nfig001.jpeg
Xu et al (2010): Positive-Versus-Neutral Stimulus Contrast


1-1. Ventral Tegmental Area

The ventral tegmental area (VTA) is an area found in the midbrain[14] , and constitutes the dopaminergic system including mesocortical, mesolimbic and mesoaccumbens systems together with associated structures such as ACC and NAcc [15] . Pharmacological intervention studies show that the VTA is implicated in learning the association of rewarding stimuli[16] , including maladaptive reward-directed behaviours, such as cravings and drug abuse[17] . Based on pharmachological studies, it was also suggested that the mechanism of addiction is shared also in romantic love, and that love can also be an ‘addictive behaviour[18] .’


Increased BOLD response in VTA evoked by association of a romantic partner[19] [20] [21] [22] suggests that the DA reward and motivation system mediates goal-directed reward seeking behaviour in romantic love[23] . This may explain obsessiveness and impulsiveness found in early stage romantic love (Fisher 2005). The elevated activation of VTA in response to love stimuli was specific to romantic love, for no significant differences were found between basal state and when neutral or non-romantic stimuli were presented (Xu 2011, Fisher 2005, Acevedo 2011, Aron 2005).

In addition, neuronal activities in mesoaccumbens pathway involving VTA to Nucleus accumbens (NAcc) are implicated in arousal state during sexual activities[24] . Significant increase in EEG was observed in VTA and NAcc was observed during the sexual intercourse of male rats, in which the response peaked during post-intromission period [25] .


Non-dopaminergic neurons such as cholinergic and subcortical glutamic neurons projecting to VTA are also suggested to play an important role in shaping reward-mediated behaviour, through their interaction with DA neurons. For example, input from orexin neurons from the lateral hypothalamus (LH) to VTA is involved in learning stimuli-reward association through facilitation of glutamate release[26] , and has been implicated in reward processing [27] . Orexin neurons are known to mediate wakefulness, feeding and arousal [28] , and although no strong experimental evidence is available yet, changes in hyperactivity, feeding behaviour and/or sleeping (Fisher 2005, Reynaud et al 2010) are commonly exhibited in those who are engaged in romantic relationship.

Furthermore, VTA exhibits lateralization with different types of ‘love’. Right VTA (rVTA) activation was correlated with romantic love stimuli, and left VTA (lVTA) with compassionate, friendly-type love stimuli (Xu 2011, Acevado 2011, Aron 2005).

The reward mediated dopaminergic system is designed to facilitate behaviours that are beneficial to our survival (Xu 2010, Fisher 2005) and romantic love contributes to success in reproductive success. Fisher et al (2005) propose that the mechanism underlies romantic love is the system of mammalian mate choice, in which attractiveness of potential partner is primarily determined by the reproductive success.


1-2. ACC and insula: Sexual Desire, Attraction and Arousal

Wu_2009_ACC_GLUT.jpg
Wu et al (2009): Fig 2 Sexual attraction enhances glutamate transmission in mammalian anterior cingulate cortex from

ACC and insular constitute the core component of the circuitry of sexual arousal in humans[29] , and highly activated upon p
resentation of erotic stimuli [30] . In romantic love, ACC is believed to be involved in processing of sexual pleasure and arousal[31] (Wu et al 2009, Reynaud et al 2010), along with reward expectation. In particular, the neuronal activity in ACC is correlated with attraction to opposite sex (Wu et al 2009). Wu et al (2009) found that in male rats, c-fos expression within layer II/III of ACC was elevated upon exposure to female rats, accompanying increase in ACC glutamatergic excitatory synaptic transmission. In addition, in the same study it was found that the male rat’s interest to female rats was reduced when the neuronal activities in ACC were inhibited by pharmacological means.

The insula is implicated in many emotional components of romantic love including sexual desire, and studies suggest erotic stimuli elicit high response in the region, often together with ACC and the associated structures[32] .




1-3. Nociception and Romantic Love

ACC and insular cortex are also involved in nociceptive circuitry. In a study, images depicting painful stimuli elicited increased response in ACC and the insula, and this response was significantly lower when unrelated others were perceived to receive the stimuli. But when the romantic partner was imagined, the increase in activity pattern was similar to those of the participant’s[33] .

Interestingly, when it comes to real pain experience, experimental data suggest that romantic love may have analgesic effects. Individuals who are engaged in love perceive less pain than those who are not, showing reduced BOLD activity in ACC, insula and related structures
[34] [35] , as well as reduction in Pain Rating Scale score values[36] . All above studies suggest that romantic love also involves modification of neural network involved in pain perception.


(II) Long-Term Relationships

General (including non-scientific) beliefs of romantic love are that, although romantic love is highly rewarding, over time all those love-induced changes will eventually fade away, or transform into some other softened types of love, and that romantic love in long-term relationships such as marriage is virtually non-existent. There are many reasons hypothesized for the phenomenon including a view that romantic love is metabolically demanding (Fisher 2005), yet no strong evidence is available due to the difficulty of experimental set-up. But through Passionate Love Score (PLS) and fMRI scans, the intensity of romantic love in long-term relationships can be indirectly measured and studied.

2-1. Activation and Deactivation

Some fMRI studies that had examined changes in neuronal activities induced by early, intense romantic love also conducted follow-up studies on the participants. In the follow-up studies, reduced activities were observed in many of the brain regions that had shown significant increase earlier. Kim et al (2009)[37] reports deactivation of the right caudate nucleus as well as significant reduction in PSL values even after 6 months. Xu et al (2011)’s follow-up data 18 months later after the initial study also reported decreased activity in the caudate tail, fusiform and the cerebellum. However, some regions also exhibited increase in activation, such as superior frontal gyrus, subgenual area (Xu et al 2011), cingulate gyri, inferior frontal gyri, supramarginal gyri (Kim et al 2009). These data suggest that the vigorous nature of romantic love and its impact on the neural network of the brain.



2-2. Romantic Love in Long-Term Relationships


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Fig 1 from Acevedo et al (2011) Neural correlates of long-term intense romantic love

Although changes in neural network are rapid and dynamic in romantic love, long-term relationships are not necessarily devoid of romance. And high level of activity in DA rich regions elicited by love stimuli is also important in long-term romantic love. In couples married for more than 20 years, high PLS scores showed correlation with high neuronal activities in the mesolimbic regions, the right VTA in particular (Acevedo et al 2011). Other regions that showed an increase in activation level include the caudate, SN, insula cortex, cingulate cortex etc (Acevedo et al 2011), and most of these activation pattern overlaps with the regions activated in early-romantic love (Fisher 2005, Xu 2011, Aron 2005). Greater sexual frequency level seems to be correlated with the activity in the left posterior hippocampus (Acevedo et al 2011).




3. Neural Mechanisms of Maintenance of Romantic Relationships

The onset of romantic love is relatively rapid, but upon the onset, a number of neural changes are made in a way that stimulates the DA rich area to seek for further rewards. Although reward-seeking behaviours are not restricted to a single romantic relationship, the brain has developed mechanisms to maintain the current relationship, which will help to increase the reproductive success and to avoid negative social outcomes. Regions of the prefrontal cortex are highly implicated in the maintenance of romantic love.

3-1. VLPFC, RVLPFC and pDMPFC: Implicit Control of Emotions

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Meyer (2011) et al - Fig 3. Activation in the bilateral ventrolateral prefrontal cortex (VLPFC) and posterior dorsomedial prefrontal cortex (pDMPFC) showed significantly increased activation when participants rejected compared to accepted attractive alternatives in the no time pressure compared to time pressure conditions.

Meyer et al. (2011) suggested that the active recruitment of neurons in the prefrontal cortex is implicated in maintenance of romantic love. The VLPFC, RVLPFC and pDMPFC showed marked increase in BOLD response when the subjects were to assess the physical attractiveness of their opposite sex. Subjects who were involved in romantic relationship tended to give lower score to the attractive figures than those who were not, and this tendency was made unconsciously, suggesting that there exists an implicit control mechanism that reduces the chance of becoming attracted to potential partners other than the current lover[38] . PFC is an important region for carrying out executive function, through selective facilitation and inhibition of neurons projected to the area. Thus high activation level shown in VLPFC, RVLPFC and pDMPFC suggest that these areas may be involved in governing impulsive emotions that may threat the stability of current romantic relationship. For more information on PFC, visit Neurobiology of Morality.

3-2. Mirror Neurons

Ortigue et al (2008) suggested a critical involvement of the mirror neuron system (MNS) in maintenance of romantic relationship, that people who were involved in romantic relationships use MNS to understand their partners. The partner’s intention was understood much better and more effectively than the control group[39] . Understanding intentions of partner is seen important in resolving conflict in a romantic relationship, yet understanding the partner and negotiating one’s need are not well practiced during the early stage of romantic love. And the duration of romantic relationship was influenced by the partner’s ability to catch conflicts and make attempts to resolve them [40] . It is possible that MNS plays a role in these types of tasks, contributing the longevity of the relationship.




4. References


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