Why You (and Your Cat) Like Being Stroked
Stroking has some surprising actions on the brain.
Posted April 11, 2016
Stroke your cat and he/she purrs. Dogs like being patted. Monkeys spend hours grooming each other: they are not looking for fleas (a common misconception: they are actually very clean animals), but it’s clearly a pleasant experience for them both. Techniques of massage vary, but some focus on the skin and are highly valued. If you walk through a door and bang your hand, you will rub it (that is, stroke yourself) to reduce the pain.
Mothers stroke their babies. Mother rats also stroke (lick) theirs, an essential action to allow them to urinate. But this has other effects. Rats whose mothers lick them less often grow up to be more sensitive to stress, and behave more anxiously in a strange environment. There are similar findings in humans: pregnant women showing high levels of anxiety tend to have babies that grow up to be more anxious and/or depressed than others. This is reduced if their mothers stroke them a lot during the first few months. Even lambs like to be stroked by humans. Adult humans also like being stroked. Rather counter-intuitively, the pleasure from being stroked increases with age. Synchronous stroking of the face increases that person’s trustworthiness. It also shortens the perception of passing time, whereas unpleasant touching (eg pinching) increases it.
The widespread significance of stroking is clearly an evolutionary adaptation. It’s a form of affiliative social contact. It is also, of course, an erotic experience in the right context. It’s therefore surprising that only recently has a special nervous pathway for stroking been identified. The skin has a wide variety of receptors that detect different kinds of touch. Broadly, they are divided into two categories: those that transmit their information to the brain through large nerve fibers (they are used for telling you about pressure on your skin, the position of your limbs, and how you recognize an object by its touch) and much smaller fibers, the so-called C fibers. The latter are interesting because they transmit the sensation of pain, but also the kind of touch that tells you, for example, that something has landed on your skin. Now comes the really interesting part: only a few years ago, it was discovered that there is a sub-category of C fibers that are specialized for transmitting the sensation of stroking. If you test them with a variety of stroking tempos, they respond best to moderate ones: the kind that most people feel the most pleasurable. Whether stroking has an erotic, affective or soothing action will depend on the context in which it occurs: the nerve fibers involved seem to be the same.
What does your brain do with this information: the stroking sensation? There is a particular part of the cerebral cortex, the outer mantle of the brain that gives it that wrinkled appearance, that responds to sensation. Stroking activates this area, whereas pinching the skin does not. So the brain is very alert to stroking. It doesn’t stop there. Near the front of the brain, there is an area called the anterior cingulate gyrus, also a part of the cerebral cortex. Stroking strongly activates this area as well. This gets more fascinating when I tell you that this area is known to be concerned with the sensation of pleasure, and even more intriguing when I tell you that it has also been implicated in depression. It shows altered activity in people with depression, and stimulating it has been used to relieve cases of treatment-resistant depression, with some success. We soothe each other in times of crisis by stroking.
Another area of the brain associated with pleasure, or liking things, is the amygdala. This is not part of the cerebral cortex, but has close connections with areas of cortex that are concerned with emotionality. Stroking increases the release of serotonin in the amygdala, whereas pinching (unpleasant) has the opposite effect. Various parts of the brain, therefore, seem tuned to the occurrence of stroking, which is why it is such a significant and widely-occurring behavior. Your brain, it seems, is waiting for you to be stroked.
Why can stroking the young have such long-lasting effects on children? Observations on mother rats revealed an astonishing fact. Like humans, rats vary in the amount of attention they give to their young. They can be divided in to those that care a lot–lick their young frequently–and those that seem to care less. As we have seen, the young of the former deal with stress better when they grow up than the latter. Deep in the brain is the hypothalamus, an area than controls many things, including the release of the stress hormone cortisol (corticosterone in rats).
There is a gene in the hypothalamus that is closely involved in this response: it regulates the action of cortisol (corticosterone) on the brain. The environment can alter the activity of this gene by a process called methylation. Baby rats with poor mothers have increased levels of methylation, and this may last a lifetime. It alters the activity of this gene and thus the way they cope with stress. It's an example of epigenetic control. There may be other advantages of being stroked. Stroking mice or cats increased the activity of their immune system, so they cope with infections better. Stroking, it seems, may be good for your health as well as your psyche.
Does this apply to humans? It looks as if it may. We can’t measure methylation in the brain of babies, or children who have experienced adversity in childhood, but we can measure it in the blood (we have to assume that this reflects what happens in the brain). Like baby rats, children of inadequate mothers show increased levels of this gene’s methylation in their blood. Of course, good motherhood is more than stroking, but affectionate physical contact reflects motherhood, so may be a proxy for it. The young brain seems particularly sensitive to skin stroking, as it does to many other pleasurable or adverse events. Epigenetic actions may be the reason these effects can be so long-lasting and difficult to eradicate.
Your brain is waiting for you to be stroked. And not only yours: the next time you stroke your cat, recall that there are good biological reasons why it likes it so much.