XiXinXing/Shutterstock
Source: XiXinXing/Shutterstock

What happens when we fall in love? It’s an age-old question for poets and
philosophers, and a newer one for neuroscientists. We have pictures of people’s brains after the fact, when certain areas light up in response to a photograph of a loved one. But researchers at Emory University’s Silvio O. Conte Center for Oxytocin and Social Cognition wanted to know what happens before that, in the “getting to know you” phase.

Now, in a first-of-its-kind study published in Nature this week, they reveal what they found — a connection in which one part of a female’s brain tells another part of her brain that maybe that guy over there is the one. Beyond giving us insight into our romantic responses, the results might eventually be used to help those with impaired social abilities, such as people with autism spectrum disorder.

A caveat: This study was done in prairie voles. But don’t scoff; the rodents are well-known in scientific circles for their long-term, devoted, monogamous relationships, a.k.a. “pair bonds.” They provide a useful — or “beautiful,” in the words of one of the paper’s authors — model for looking at what happens in the brain when two individuals connect. And this kind of study isn’t possible in people. “Technically and logistically, it would be hard to design an experiment in humans to track how somebody falls in love,” says Robert Liu, a computational neuroscientist in Emory’s Biology Department and a senior author of the paper. The study also tells us nothing about the process of choosing one mate over another; these prairie voles had arranged marriages.

What the scientists were able to do was watch what happened inside a female’s brain in real time after they put her in a cage with a male — “cohabitation,” or the rodent version of living together — and they interacted and began to bond. Prairie voles interested in bonding move closer and closer to each other and eventually engage in a behavior called “side by side huddling," which Liu calls "an expression of the bond that’s being formed ... it’s something that emerges over time.”

Liu and his colleagues were watching two brain areas — the medial prefrontal cortex, which is involved in executive function and decision making, and the nucleus accumbens, which is associated with reward and addiction. They took electrophysiological recordings of those two areas as well as video of the voles’ behavior for the six hours of the experiment. They found that the medial prefrontal cortex exerted a certain amount of control over the nucleus accumbens. The connectivity between these areas suggests one is telling the other how to respond to social cues — to find them attractive. “We learn to appreciate the smells, or the voice, or how our partner looks,” Liu says. “Presumably that’s all through some way of making those cues more rewarding to us. Perhaps that’s what we’re getting a glimpse of.” Since humans have a similar circuit, albeit with a lot “more territory,” Liu says the same thing may well be happening in us.

Intriguingly, a baseline conversation between these two brain areas was going on before the females met the males — and there were noticeable differences in how active the circuit was across individuals. “What was really surprising was that the level of activation was predictive of how quickly the animal would become affiliative,” Liu says. This would seem to hint at why some individuals fall in love so easily, and others are more resistant.

What about sex? That was also surprising. In prairie voles, mating does not necessarily lead to a pair bond; it’s the huddling that really matters. But when the animals have their first mating bout, sex can accelerate bonding. Some animals showed more change in the level of functional connectivity between the two brain areas than others did. The degree of change was also predictive. “If you get a bigger boost coming out of that mating bout, you’ll start to huddle more quickly,” says Liu. Enough said.

Finally, the scientists used the innovative technique of optogenetics, in which they stimulate specific brain areas with light to doublecheck their results. They put voles together in a way that would not normally lead to bonding (shorter time, no sex). In animals who’d had this circuit stimulated, even that hint of a possible relationship was enough to trigger bonding behavior. In unstimulated animals, it didn’t.

Emory University
A pair of prairie voles bonding.
Source: Emory University

This last result is what leads to the speculation that someday this circuit could be stimulated in people who have difficulty developing social bonds. Optogenetics is most likely too invasive for humans for the foreseeable future. Liu suggests, however, that some brain stimulation techniques that are already in use in people, like transcranial magnetic stimulation, might work eventually, if they could be refined to target a more specific area of the brain. “That might be one way to enhance social function by making social cues more rewarding,” Liu says.

Meanwhile, perhaps the rest of us should be a little more appreciative of huddling.

References

Amadei, E.A., et al. "Dynamic corticostriatal activity biases social bonding in monogamous female prairie voles." Nature. 2017 May 31. doi: 10.1038/nature22381.https://www.ncbi.nlm.nih.gov/pubmed/28562592

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