Pinches, punches, tickles and reading Braille: touch plays many roles in our lives . But our ability to detect different types of touch varies greatly across different parts of our body, from the ticklish soles of our feet to the relaxing sensation of a shoulder or head massage. Want to know exactly what effect you're going to have when you touch someone? Read on for our user's guide to touch.
Fingers
Each of your fingertips has more than 3000 touch receptors, making them extremely sensitive and capable of detecting subtle variations in texture. Two types of touch receptor are found in particularly high numbers - Merkel's discs, which detect prolonged pressure, and Meissner's corpuscles, which respond to initial contact and motion. As well as helping you appreciate the texture of different objects, Merkel's discs also convey information about an object's weight and how hard you are gripping it. Meissner's corpuscles, meanwhile, will detect if that object is slipping from your grasp, enabling you to tighten your grip.
Shoulders
Need to unwind? A relaxing shoulder-rub often does the trick. Studies suggest that moderate pressure massage can lower our heart rate and reduce levels of the stress hormone cortisol, as well as boosting alertness. It seems to do this by stimulating the vagus nerve, which conveys sensory information about how the rest of the body is functioning to the brain and central nervous system. To get these benefits, you have to rub hard enough to move the skin, suggesting pressure receptors are involved.
Feet
For most people, having their feet tickled results in uncontrollable laughter, squirming and kicking. But why do certain body parts elicit this response and not others? Just like the fingers and palms, the soles of the feet lack the CT fibres that convey so-called emotional touch, which might explain why we don't find having them stroked particularly pleasurable. Both are also covered in Meissner's corpuscles – although the fingers and hands contain even more and tend to be less ticklish. One suggestion is that we are ticklish in areas of the body that are touched less often. Though the feet experience constant pressure, they don't experience light touch very often. Another possibility is that which body areas we find ticklish is influenced by tickling games we played with our parents during childhood.
Genitals
Both the clitoris and the base of the head of the penis are rich in Meissner's corpuscles. However, they have relatively few Merkel's discs, which respond to prolonged pressure. As a result, although the clitoris and penis are extremely sensitive to touch, they lack the fine discriminative abilities of the fingertips. As David Linden at Johns Hopkins University in Baltimore, Maryland, points out, one upshot of this is that you cannot read Braille with your genitals.
The bad news is that touch receptors in these areas may become less responsive to touch as we age, possibly due to falling levels of sex hormones. Post-menopausal women and those with low levels of oestrogen are less sensitive to sexual touch, for example.
Nipples
The nipples are profoundly responsive to touch, which can trigger the release of both oxytocin and prolactin, hormones that promote feelings of relaxation. In lactating women, touch stimulates milk production. But many people also find nipple-touching sexually arousing. One recent study showed that it triggered the same brain response as stimulating the genitals.
Torso
The skin of the chest, sides and back contains a hundred times fewer touch receptors per centimetre than the fingertips. This makes it far harder to pinpoint precisely where you are being touched on your torso, or in how many places. Try this experiment: close your eyes and ask a friend to simultaneously (and gently) prod you with the sharp ends of two pencils, first held close together and then further apart. At what distance can you distinguish the two points? Try it on your fingertips, the back of your hand, your torso and elsewhere.
Most people can feel the two points 2 to 4 millimetres apart on the fingertips, but on the back they have to be at least 3 to 4 centimetres apart.
It may not be good at counting pencil pricks, but the skin on the back of the neck, back and forehead is rich in CT-fibres, which convey emotional touch. This may be one reason why we find the stroking of these areas pleasurable.
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