We analyzed the subjects’ brain activity with 128-electrode, high-density EEG [electroencephalography] and compared their brain activity while they were feeling desire with activity when they were thinking, “No, not for me.” When the subjects had no feeling of desire, we saw activity in an area involved in body recognition called the extrastriate body area, indicating, as you might expect, that they were evaluating the body they saw in the picture. But when they were feeling desire, we saw activation not only in the extrastriate body area but in a brain region called the temporo-parietal junction, which is known to be very important for the understanding of the self and one’s own body image. This suggests that sexual desire isn’t just a matter of evaluating a body. The brain is also doing some more surprising cognitive work.

With EEG, we know not only where desire occurs in the brain but when it activates each region. And we found this interesting dynamic: First we found activation in the visual area, and then following that—very fast, so it’s still unconscious—we found lots of activity in this region [the temporo-parietal junction] that’s involved in body image, and then back to the visual.

Why do you think we are able to find someone unattractive more quickly than we’re able to judge them desirable?
It’s as if you have an unconscious checklist in your mind: Does this guy fit my checklist or not? If there is one point that doesn’t match, the guy is rejected. But if he passes the first point, then you have to check the next one and the next one. The thing that’s fantastic about the desiring mind is that it all happens so quickly. Within 200 milliseconds our brain knows whether we desire someone—before we know it consciously!

If the temporo-parietal junction is involved in our own body image and sense of self, why does it engage when evaluating other people?
Ah, that is a very interesting question. We believe everything that happens to “self” is stored and processed in this temporo-parietal junction because it is important for integrating information and abstract concepts. So all the experiences you’ve had with desire, including positive experiences, are stored somewhere there—not just in the hippocampus [the primary memory-storing area of the brain]. When you meet a new person, your past experiences influence your evaluation of the person in the present moment.

You have also looked at the cognitive process of love. What did you find?
Again we were interested in how the unconscious influences behavior. We presented each participant with the name of his or her beloved on a monitor, subliminally [faster than the threshold of awareness], and contrasted that with the name of a friend and the name of a stranger.

Then we asked the participants to do a demanding cognitive task that required focused attention. Love is supposed to activate the dopamine system, which is involved in reward and pleasure but also plays a role in cognition and movement. Our hypothesis was that if love activates the dopamine system, it should boost functions related to that system, like cognition and motor functions. So we should see a faster reaction time.

We found a huge and significant effect when we presented the name of the beloved: The participants were much faster at the cognitive tasks that we were asking them to do, but only when we asked them to do challenging tasks. When we asked them to do less demanding tasks, we didn’t see the same effect. We believe that passionate love activates a specific brain network that helps people make faster cognitive connections and mental associations. It isn’t just speeding up their motor skills.

Do the patterns of brain activation associated with love overlap with those for lust?
We see two different networks for love and desire, but we’re still trying to understand how they differ. They have common areas in the parietal lobe and the temporo-parietal junction. The desire network is more complex, more distributed than the love network. It’s the opposite of phrenology: There’s not just one part of the brain that does it all.

Are there practical applications for this research?
Yes, absolutely. If love and desire involve regions of the brain that govern the sense of self, then by understanding self-image we can not only help people who have body-image disorders, such as anorexia, but also help people who have desire and sex disorders. There is a famous saying in French, “Love yourself before you love others.” It’s a cliché, but maybe there is some truth to it.

This article is a sample from DISCOVER's special Brain issue, available only on newsstands through December 28.