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Scientists uncover neurobiological basis for romantic love, trust, and self


In new studies, scientists are discovering the neurobiological underpinnings of romantic love, trust, and even of self. New research also shows that a specific brain area - the amygdala - is involved in the process of understanding the intentions of others, in particular when lying is involved.

Using brain imaging, researchers Helen Fisher, Arthur Aron, Lucy Brown and colleagues find that feelings of intense romantic love are associated with specific activity in dopamine-rich brain regions associated with reward and motivation. Those study participants who expressed more romantic passion on a questionnaire showed more brain activity in these regions. Those in longer relationships showed more activation in emotion-related areas as well. And men and women tended to show some different brain responses. The researchers conclude that romantic love may be best classified as a motivation system or drive associated with a range of emotions. Further studies of intense, early stage romantic love may help to define how the brain encodes reward and memory.

In this experiment, 17 young men and women who had "just fallen madly in love" were tested with functional magnetic resonance imaging (fMRI) to identify the brain circuitry of romantic love.

“We believe romantic love is a developed form of one of three primary brain networks that evolved to direct mammalian reproduction,” says researcher Helen Fisher, PhD, of Rutgers University in New Brunswick, NJ. “The sex drive evolved to motivate individuals to seek sex with any appropriate partner. Attraction, the mammalian precursor of romantic love, evolved to enable individuals to pursue preferred mating partners, thereby conserving courtship time and energy. The brain circuitry for male-female attachment evolved to enable individuals to remain with a mate long enough to complete species-specific parenting duties.”

In the study, participants alternately viewed a photo of a beloved and a photo of a familiar, emotionally neutral individual, interspersed with a distraction task. The researchers hypothesized that intense early stage romantic love is: (1) primarily associated with dopamine pathways in the reward system in the brain; and (2) primarily a motivation system (as opposed to an emotion) oriented around planning and pursuit of a pleasurable reward - an intimate relationship with a preferred mating partner.

“Our evidence suggests that both hypotheses are correct,” says Lucy Brown, PhD, of the Albert Einstein College of Medicine in New York. “We found specific activity in regions of the right caudate nucleus and right ventral tegmental area. These brain areas are rich in dopamine and are part of the brain’s motivation and reward system. Elevated levels of central dopamine produce energy, focused attention on novel stimuli, motivation to win a reward and feelings of elation - some of the core feelings of romantic love. Activity in other regions changed also, including one that another imaging study has shown to became active when people eat chocolate.”

The researchers also found that those who scored higher on the "Passionate Love Scale," a questionnaire administered prior to scanning, also showed more activity in the caudate. Arthur Aron, PhD, of SUNY Stony Brook, NY, says, “This result is among the first to show a direct link between responses to a survey questionnaire and a specific pattern of brain activation."

Fisher, Aron, and Brown also found a tendency toward gender differences. Among them, most of the women in this study showed more activity in the body of the caudate, the septum, and posterior parietal cortex, regions associated with reward, emotion and attention; most of the men in this study showed more activity in visual processing areas, including one associated with sexual arousal.

Aron, Fisher and Brown have embarked on a follow-up fMRI study of men and women who have recently been rejected in love. They wish to understand the full range of brain systems associated with this primordial, powerful and universal human phenomenon.

In another study, Paul Zak, PhD, and his colleagues at Claremont Graduate University investigated trust - something that pervades nearly every aspect of our daily lives. Even so, the neurobiological mechanisms that permit human beings to trust each are not understood.

In the new research, Zak and his colleagues find that when someone observes that another person trusts them, oxytocin - a hormone that circulates in the brain and the body - rises. The stronger the signal of trust, the more oxytocin increases. In addition, the more oxytocin increases, the more trustworthy (reciprocating trust) people are.

“Interestingly, participants in this experiment were unable to articulate why they behaved they way they did, but nonetheless their brains guided them to behave in ‘socially desirable ways,’ that is, to be trustworthy,” says Zak. “This tells us that human beings are exquisitely attuned to interpreting and responding to social signals.

The findings are even more surprising because monetary transfers were used to gauge trust and trustworthiness, and the entire interaction took place by computer without any face to face communication. Signals of trust are sent by sending money that participants earned to another person in a laboratory, without knowing who that person is or what they will do. That, is, there is a real cost to signaling that you trust someone.

In the experiment, people were recruited and paid $10 for showing up. Then they took seats in a large computer lab and were matched up in pairs, but this was done completely anonymously so that no one knew (or would know) the other person in his or her pair. One-half of the participants (decision-maker 1s) then had the opportunity to send none, some, or all of their $10 show-up fee to the other person in their pair. Whatever is sent is tripled. So, if $4 was sent, the other person would have $22 ($4 tripled, plus the $10 show-up fee the second person receives). The second decision-maker could then send some amount of this money back to decision-maker 1, but need not. This is how the researchers produced a social signal of trust: decision-maker 1’s only reason to transfer money to the other person is because he or she trusts that that person will understand why the money is being sent to them, and in turn will return some to them (be trustworthy). All subjects are told that the initial monetary transfer is tripled, and there is no deception of any kind.

After each person makes his or her decision, they were taken to another room and four tablespoons of blood were taken from an arm vein. Animal studies have shown that oxytocin, a hormone little studied in humans, facilitates social recognition and social bonding, for example, bonding of mothers to their offspring, and in some monogamous species the bonding of males and females in a family unit.

Based on the animal studies, the scientists hypothesized that what is happening in the trust experiment is that people are forming temporary social bonds with the other person in their pair. “This is just what we found. The stronger the signal of trust, the more oxytocin increases, and the more trustworthy people are. This is surprising given the sterile laboratory environment of the interaction so that the effect of oxytocin on face-to-face interactions must be quite strong,” says Zak.

He also found that women in the experiment who are ovulating were significantly less likely to be trustworthy (for the same signal of trust). This effect is caused by the physiologic interactions between progesterone and oxytocin, and it makes sense behaviorally: women who are, or are about to be, pregnant, need to be much more selective in their interpretation of social signals, and also need more resources than at other times.

Zak’s lab is now studying brain activation patterns when people receive signals of trust, as well as in the physiologic responses to trust signals in patients who have neurological damage. Trust is an essential part of our daily lives, from walking down the street to driving to countless other daily activities, so that discovering the neurobiology of trust tells us something important about human nature: that we are so highly social that we pick up social signals of trust and act on them even when we are not consciously aware of these signals. Our brain acts as an internal compass that guides us towards the “right” thing to do.

In another imaging study, scientists at Stanford University located brain areas associated with self and self relevance. The new work helps answer questions such as why people hear their own names in the din of a cocktail party or the fog of sleep.

In the study, Wemara Lichty, PhD, and her colleagues used rapid event-related fMRI to dissociate brain activations related to names. Sixteen females heard five different auditory stimuli: 1) a tone; 2) a low frequency name (not self-relevant), 3) a high frequency name (not self-relevant), 4) a self-relevant name (e.g., sister or best friend), and 5) their own name. To ensure that participants were attending, they performed a simple task of pushing a button for each sound; specifically one button was pressed if a sound was the same as the preceding one, and a different button was pressed if the sound was the same. They listened to a total of 250 sounds over a period of 12 minutes.

The study was designed to answer the question: Is there something special about our own name and the names of those we are close to; i.e., is there a hint of that relationship in brain activations? The researchers identified areas special for personally relevant names compared to non-personally relevant names: The left medial prefrontal cortex, an area that has been associated with self, was active. “Interestingly, this putative self-related area was also active in a study for names of close associates. This suggests that the medial prefrontal cortex may be involved in processing a personal network related to the self,” says Lichty. Although imaging studies have not evaluated this, behavioral studies have shown that on many cognitive tasks, the performance of self and close others is often similar and quite different from that of persons not known. Also activated was the left posterior cingulate, an area involved in autobiographical memory.

The study also addressed whether there is something completely unique about a person hearing his or her name. Are there areas activated only by one’s own name and not the name of others we know? Results showed that the right middle temporal gyrus was active. “This may suggest that the special status of one’s own name is related to altered cortical perceptual representations. Enhanced hearing of one’s own name may be associated with decreased thresholds for auditory cortical activation. However, the activation may also be related to self as separate from close others as suggested by the similarity of our area of activation with the findings of an fMRI study of activations related to faces of oneself and one’s partner,” Lichty says.

She notes that understanding how we differ from each other and how we are related to each other can offer insight, both into the essential aspects of our individual and communal identity. Clinically, this could be of import regarding understanding of individuals who may have weak (underdeveloped, undifferentiated) self. In addition, it may provide greater insight into relationships.

Another new study explores the brain mechanisms involved in deception. What happens when you spot deception in a human movement? The sort of thing a hitter tries to do every time a pitcher prepares to throw a ball.

Working out whether there is deception results in activation of the amygdala, a structure in the brain involved in perceiving fear and in learning about fearful or threatening stimuli. Our new finding is that the amygdala is also involved in understanding the intentions of others, in particular when lying is involved and when actions are being scrutinized.

“Our study finds a link between emotional brain systems and the complex brain network used to read intention in the movements of others,” says Richard Frackowiak, MD, of University College London. “So, emotional responses can be driven by factors other than empathy with someone else’s emotions. The emotional brain responds to an intention to deceive even when the deception involves a trivial action.”

The clinical importance of this work is for patients with amygdala damage. For example, there are abnormalities reported in the amygdalae of adults with autism. Such patients tend to be excessively trusting. This may not be to do with character judgment as such, but with failing to recognize potential social threats in a stream of observed actions or gestures.

In a scanning experiment Frackowiak and his colleagues approached this issue by getting actors to lift boxes with weights. Sometimes the experimenter lied to them about the weight in the box so their movements were likely subtly modified. Subjects were shown these films while brain activity was recorded in a scanner and were asked to rate whether the actor had been deceived on each occasion.

The researchers specifically compared activity in subjects’ brains when they judged that an actor was deceived with that when they thought all was above board in order to isolate brain regions associated with perceived deception. In future work these scientists plan to compare situations in which an actor is trying to deceive a third party with those in which the actor is trying to deceive the subjects themselves. This will indicate whether it is simply the perception of deception that is important or whether the object of that deception matters.

Many experiments have been performed using imaging to study cognitive processes such as attention, memory or action. But, Frackowiak says we do not simply base our judgments on reason. “The amygdala can be regarded as part of our emotional core system and our results show that we are deeply affected when we think someone is trying to deceive us, over even so simple a matter as the weight of a box. The interaction between emotion and cognition is thus becoming clarified.

Dawn McCoy | Society for Neuroscience
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