In a new study published recently in Nature Neuroscience, a group of researchers say they have discovered a new class of nerve fibers responsible for sending pleasure signals to the brain. They believe that the study could shed light on the role that physical contact plays in sustaining long-term emotional bonds between humans.

Researchers say that the patients' skin had to be stimulated at a certain speed and in certain locations in order for these nerves to discharge their pleasure-inducing messages.

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The study examined the neurological responses of twenty patients as their arms were lightly stroked at various speeds. They found that when the subjects identified a stimulus as pleasant, a class of neurological tissue known as "C-tactile" nerve fibers had been stimulated.

They also discovered that these fibers were only activated within a certain range of speeds - between 4-5 cm per second - and that slower and faster stimuli were not able to produce the same pleasurable sensation.

In addition, they observed that "C-tactile" nerve fibers were only located on hairy areas of the arm, and that similar stimulation of the hands did not have the same effect.

Professor McGlone of Unilever, who transitioned to the private sector after years of university research in the field of neurology, referred to this phenomenon as a sort of socio-biological "design".

"We believe this could be Mother Nature's way of ensuring that mixed messages are not sent to the brain when it is in use as a functional tool," he said, adding that the speed at which arm-stroking is considered pleasurable corresponds to the same speed with which mothers caress their infants and couples express affection.

So how does our culturally constructed life experience interact with all this neural circuitry? Let's do a thought experiment: Imagine the sensation that would result if your sweetheart caressed your arm during a loving, connected time. Now imagine that very same caress delivered in the middle of an unresolved argument. Both of these caresses produce the same pattern of electrical activity in the C-tactile fibers, yet they feel profoundly different, one comforting and the other irritating. This comes from the fact that the posterior insula also integrates information from other senses and emotional centers. These other streams of information are combined with the C-tactile caress signals to produce the ultimate experience. When you're in midargument (or any other situation where touching is unwanted), the caress-induced activation of the posterior insula is strongly blunted, and it won't feel pleasant.

A caress feels best when it is delivered with a small amount of force and a speed of about 1 inch per second. Stroke slower, and it feels like an unwelcome crawling bug; faster, and it feels perfunctory rather than loving. If we were to insert an electrode into a sensory nerve serving the forearm and record electrical signals from a single caress-sensing fiber, we would find that it responds strongly (that is, it fires the largest number of electrical impulses) to this optimal caress speed and much less to faster or slower speeds.

The caress sensing fiber is also tuned to respond most vigorously to caresses delivered by an object (or hand) with the surface temperature of human skin, about 90 degrees Fahrenheit, which is also the temperature that feels the best to most people. And when people are placed in a brain scanner, this same tuning for an ideal caress is reflected in the activation of the posterior insula, the positive emotional touch region of the brain: The greatest insular activation is found in response to a caress delivered with moderate force and speed at human skin temperature.

Neurobiology recommends warm skin and moderate pressure, moving at 1 inch per second.

...The hormone also plays an important role in human bonding, including the bonding that occurs in maternal and romantic relationships.

After a baby is born, skin-to-skin contact helps promote the production and release of oxytocin. This helps increase bonding between a mother and baby. In adult romantic relationships, physical contact - including hugging, massaging, kissing, and holding hands - contributes to the release of this bonding hormone.