The Neuroscience of Intimacy

Why our brains compel us to kiss and hold hands

Posted Mar 03, 2015

Ever wonder why, when we want to feel closer to our lover, we kiss them on the lips, interlink tongues (French kissing)  or hold their hand? Why not touch elbows, press ears together or bump foreheads?

Kissing and holding hands seem so…well…natural that we never stop to wonder what’s going on in our brains that compels us to use certain body parts to express affection and not others.

A plausible answer can be found by examining this figure, which shows how much sensory brain tissue is devoted to different body parts ( also called the “Homunculus”).  The somatosensory cortex of your brain, which processes touch information, devotes disproportionately large numbers of neurons to  your hands, lips and tongues, as shown by the amount of cortex devoted to different body parts (the larger the body part in the figure, the more volume in the brain occupied).

Notice, for example, that just one lip occupies more space in your brain than an entire arm! Your tongue is a small fraction of the size of your foot, but look how much larger the brain map of a tongue is than that of a foot!

To get an intuitive sense of what these means for sensation, take two toothpicks, place their tips apart and touch the two tips first to your arm, then to your lips. As you do this, see how far apart you have to spread the tips of the two toothpicks in order to notice that there are two points of touch instead of one.

You will find that the spacing of toothpick tips that still gives you a feeling of two tips vs. one is much smaller on your lips than on your arm. This so called “two point” threshold means that you lips have greater spatial acuity, or resolving power, than your arms, just as the center of your vision is much sharper than your peripheral vision.

An even simpler test is to run the tip of your tongue across the cutting edge of a front tooth. You will feel small ridges on the tooth. But if you place that same tooth on your arm, you will not discern the small ridges. Why? Because your lips and your tongue map onto many more neurons in your brain than your arms.

This inequality in brain real estate allows us to make subtle distinctions in the textures of foods (and other things we put in our mouths) with our lips and mouth, whereas our arms—whose main function is to move our hands around—do not need to make such fine distinctions.

Similarly, our fingers, which must sense small differences in size and shape so that we can make fine motor movements, also have disproportionate representation in the brain.

With this in mind, it is clear that when we touch lips (as shown in the figure), tongues or fingers, we are literally maximizing the amount of contact between our two brains. This yields the greatest sensory richness and “brain intimacy” while enabling us to communicate subtle nuances of affection (or lust) with fine movments of our lips and tongue.

The figure of the homunculus also explains why some cultures—such as Inuits—touch noses as greetings or signs of affection. Notice how much somatosensory real estate is devoted to noses!

The figure of two “homunculi” kissing may not be very romantic, but the neurological result—maximum mutual brain exposure and intimacy—certainly is!

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