Kissing, hand holding, and somatotopic organization in the brain

Have you ever wondered why humans express affection by kissing and hand holding?  As with most things in life, the more you think about it, the weirder that it seems.  Here’s an interesting insight from neuroscience that is fun to think about next time you decide that you want to over-analyze your love life even more than already happens.  

The more we learn about the brain, the more we realize how incredibly complex it is in its wiring, organization, and functionality.  As humans, we like to organize things into chunks that make sense to us.  Brain region A does this, and brain region B does this.  In the early part of the 20th century, we were at the early stages of understanding what parts of the brain did what, and neurosurgeon Wilder Penfield played an important role in figuring out some really interesting things about how the brain is organized. While performing brain surgeries on 126 epileptic patients (who were awake and under local anesthesia only), he would stimulate certain parts of the brain’s outermost part, the cortex, with an electrode and essentially ask the patient, “Where did you feel that?”  They would respond the foot or the elbow or the ear or whatever area corresponded to that specific brain region.

What he and other contemporaries found was that in area S1, the somatosensory cortex, a part of the outermost brain layer that processes sensory information, the cortex, was somatotopically organized.  In Greek, soma = body, and topos = place.  Basically, it means that the somatosensory cortex is very predictably ordered in the information that it processed from different body parts.  He was able to map sensations from various parts of the body to specific places within the somatosensory cortex and quantified the amount of area dedicated to different regions.  For example, we know that sensory input to the face is processed by the lower, more lateral (side) regions, whereas inputs from the rest of the body are processed in the regions closer to the top of the skull.  Take a look at this map — sometimes called a “sensory homunculus” — to make things easier:

Ok ok.  Big deal.  What’s that got to do with kissing?  Take a look again at the image: how much space is dedicated to information coming from the mouth areas and hands?  Compare that with the space dedicated to the entire rest of the arm.  It’s huge!  We have a ton of neurons whose job it is to process information from our fingers and mouth (lips, tongue, throat).  The paper in which Penfield and co-author Boldrey published their findings and original drawings of the homunculus noted that a vastly disproportionate amount of the somatosensory cortex was dedicated to the hands/fingers and face, especially the lips, tongue and teeth.   

From an evolutionary standpoint, it makes sense that we would want to have a lot of information dedicated to those things, which would allow us to make finer adjustments with our fingers and allow for better assessment of food, which are both advantageous to survival.  Several studies have shown that holding hands and kissing decrease cortisol (stress) levels and increase levels of oxytocin, our favorite trendy “love” chemical.  This means that information from the fingers and lips, which undergoes some processing in the somatosensory cortex, is able to create biological and chemical changes in other regions.  It is also interesting to know that this brain region has dopamine receptors, which means that it is likely to communicate with the brain’s pleasure and reward systems.

Keep in mind that this article is not the end-all explanation for why we like hand holding, but a fun idea to think about next time you and a partner are kissing or holding hands.  If you really want to impress, mention that they’re stimulating a significant portion of your somatosensory cortex and they’ll think you’re super smart.  And attractive!

There you go —  relationship advice from a neuroscientist!

Edited by Jennifer Sieben and Lana Ruck