How did it happen that their lips came together? How does it happen that birds sing, that snow melts, that the rose unfolds, that the dawn whitens behind the stark shapes of trees on the quivering summit of the hill? A kiss, and all was said.
- Victor Hugo -
I was browsing through the MSN news site the other day, and an item caught my eye: “Scientists uncover the chemistry of kissing”. A psychologist by the name of Wendy Hill from Lafayette College in Pennsylvania conducted a simple, but revealing, experiment. She recruited 15 couples, and measured levels of different hormones both prior to, and after, 15 minutes of kissing and 15 minutes of holding hands. Her main findings were that 15 minutes of serious smooching, but not 15 minutes of hand-holding, decrease the levels of a hormone called cortisol, and increased the levels of another hormone called oxytocin.
Cortisol might be familiar to some of you, this is the main hormone that is released in response to stress, so the finding that kissing decreases cortisol means that we scientists have just proven something that every one else already knows: Kissing is a very effective means of stress reduction (assuming both parties are agreeable to participating, of course).
Some of you (in particular, women who have given birth and the few men who have paid attention to the trials that their mates endure in giving birth) may also recognize oxytocin. Oxytocin has multiple functions; perhaps the best advertised is that it induces uterine contractions during labor. In fact obstetricians use oxytocin (under the brand name of Pitocin) to induce labor when induction is deemed appropriate. Another well-known function of oxytocin is its involvement in the “let down” reflex during breast feeding. When little Johnny or Mary suckles, oxytocin is released, and serves as the key that opens the door allowing breast milk to migrate from the storage area to the delivery platform.
Given these two “primary” functions of oxytocin, you might find it strange that this hormone has anything to do with kissing. Hormones can induce their effects in two ways, by acting on the brain and spinal cord, which as you will recall, constitutes the central nervous system. So effects mediated through the brain or spinal cord are referred to by us ” experts” as “central” effects. But hormones can also induce effects by acting on other organs or systems, these effects are referred to, collectively, as “peripheral”. Oxytocin’s ability to induce labor and to stimulate lactation are both peripheral effects; for instance, give oxytocin to an unconscious pregnant woman, and voila, instant contractions!
The association between kissing and oxytocin becomes apparent when you start looking at what this hormons does centrally. Over the past 15 or 20 years, we have accumulated a fair bit of evidence implicating oxytocin’s central effects in “monogamous pair bonding” or mating for life. For instance, administer oxytocin into the brain of a female prairie vole just prior to introducing her to a male, and you’ve created a marriage made in heaven. So it’s not so surprising that oxytocin gets released during one of the principle acts that humans use to express affection.
But this raises another question: Why is kissing a principle act of affection? Look across human cultures and you’ll find that expressing affection by exchanging “the big wet one” is almost universal. Why is the lip-lock so powerful? Why don’t we get the same thrill out of holding hands, or rubbing bums, or any kind of tactile contact? One answer, it seems to me, comes from the finding that our brain has been built to prioritize tactile sensation from the oral area. And we have known this for quite some time…
In 1870, two German physiologists by the names of Eduard Hitzig and Gustav Fritsch discovered what is now known as the motor cortex. The motor cortex is a specialized area of the outer layer of the brain (known as the cortex) that controls movement. Take your index finger, place it just in front of your ear, and then run it over your skull across to your other ear, and you have traced the motor cortex. Using dogs, Fritz and Hitzig found that you can induce different movements in different parts of the body by stimulating different parts of the motor cortex. Four years later, the famed Scottish neurologist David Ferrier found the same thing in monkeys. Equally, if not more important, Ferrier also discovered what has become known as the “sensory cortex”. This strip of the cortex, located right behind the motor cortex, is where the brain monitors the state of the entire body. This is the area that receives input about touch, pain and temperature from all the different parts of the body.
And now, for the “piece de résistance” that brings the story to its end: Let’s jump ahead to the 1930s, when Wilder Penfield of the Montreal Neurological Institute was developing a safer and more reliable surgical intervention for the treatment of epilepsy. Epilepsy is just an electrical storm that spreads throughout your brain (for reasons that remain unclear to this day). The storm always has a starting point (known as the focus) and the storm just spreads out from there, affecting the entire brain. Neurologists felt that if they could surgically remove the area where the storm originates, then you have solved the problem. But there is a catch: You can’t remove a part of your brain that is responsible for a very important function. For example, you don’t want to remove the area of the brain that is responsible for language, because if you do you may have a patient that is free of seizures but can’t communicate anymore.
But how do you know what area of the brain serves what purpose? Here is where Penfield’s genius shines through: The brain itself does not have pain receptors, so all you need to do is to anesthetize the scalp and skull (to block the pain receptors located there) and you can operate on a fully conscious, and more or less very comfortable, patient. If you then stimulate different parts of the brain, the patient is able to tell you what he is feeling, thinking and experiencing, and you can also view whether or not different parts of the body move in response to stimulation. If there does not seem to be much going on following stimulation, then that part of the brain is relatively safe to remove. If it stimulates speech, then you know this is an area that is involved in language, and you better leave it alone.
By performing this procedure on hundreds of epileptic patients over the years, Penfield was able to “map” both the sensory and the motor cortices. And in so doing, Penfield found the reason why a kiss is such a powerful means of non verbal communication.
Have a look at the picture below, which is the sensory cortex (or, in the language of us neuroscientists, the somatosensory homunculus). Note that the picture portrays, graphically, how much of the sensory cortex is devoted to receiving sensory input from different parts of our bodies. Take a gander at the amount of cortex that is devoted to sensation from the lips (which I have highlighted in red). I’ve also taken the liberty of highlighting, in yellow, the sensory representation for the tongue (for those of you who prefer to express your intimacy in French). Take each one individually, and its easy to see that the lip and tongue are two of the biggest areas; put the two of them together and… well, let’s just say it is easy to see why we express affection orally. The brain is built precisely for this reason.
Also take a moment to contrast the lip and tongue representation with the amount of cortex that is devoted to the genitalia (which I have highlighted in green). I think most reasonable observers would conclude there is no neurological support for the claim (made mostly by men, of course) that the genitals are THE MOST SENSITIVE part of their bodies. And whereas I am definitely over-interpreting, I also like to think that this is one way in which the brain prioritizes the importance of affection and sex. In the long haul, we feel more human by sharing a big wet one, then engaging in the horizontal mambo.
04 Mar 2009