Laughter, from being Tickled
Tickling is the act of touching a part of a body in a way that causes involuntary twitching movements or laughter.[1] The word "tickle"ⓘ evolved from the Middle English tikelen, perhaps frequentative of ticken, to touch lightly.[1]
In 1897, psychologists G. Stanley Hall and Arthur Allin described a "tickle" as two different types of phenomena.[2] One type is caused by very light movement across the skin. This type of tickle, called a knismesis, generally does not produce laughter and is sometimes accompanied by an itching sensation.
Physiology
[
Tickling results from a mild stimulation moving across the skin, and is associated with behaviors such as smiling, laughter, twitching, withdrawal and goose bumps.
The tickle can be divided into two separate categories of sensation, knismesis and gargalesis.
Knismesis, also known as a "moving itch", is a mildly annoying sensation caused by a light movement on the skin, such as from a crawling insect.
This may explain why it has evolved in many animals.[3]
For example, a dog exhibiting the scratch reflex is an example of knismesis.
When stimulated in the saddle region, most dogs will exhibit a reflexive rhythmic twitching of their hind legs.
This reflex can be brought on by actions such as scratching, brushing, stroking, or even tapping the sensitive area.
Horses also exhibit a response to knismesis, as they can be observed twitching the panniculus carnosus muscle in response to insects landing on their sides.
Gargalesis reactions refer to a laughter-provoking feeling caused by a harsher, deeper pressure, stroked across the skin in various regions of the body.[3]
These reactions are thought to be limited to humans and other primates, although some research has indicated that rats can also be tickled in this way.[4]
A German study also indicates that the gargalesis type of tickle triggers a defense mechanism for humans in the hypothalamus conveying submissiveness or fleeing from danger.[5]
It appears that the tickle sensation involves signals from nerve fibres associated with both pain and touch.
In 1939, Yngve Zotterman of the Karolinska Institute studied the knismesis type of tickle in cats, by measuring the action potentials generated in the nerve fibres while lightly stroking the skin with a piece of cotton wool.
Zotterman found that the "tickling" sensation depended, in part, on the nerves that generate pain.[6]
Further studies have discovered that when the pain nerves are severed by surgeons, in an effort to reduce intractable pain, the tickle response is also diminished.[7]
However, in some patients that have lost pain sensation due to spinal cord injury, some aspects of the tickle response do remain.[8]
Tickle may also depend on nerve fibres associated with the sense of touch.
When circulation is severed in a limb, the response to touch and tickle are lost prior to the loss of pain sensation.[9]
It might be tempting to speculate that areas of the skin that are the most sensitive to touch would also be the most ticklish, but this does not seem to be the case.
While the palm of the hand is far more sensitive to touch, most people find that the soles of their feet are the most ticklish.[9]
Other commonly ticklish areas include the belly, sides of the torso, underarms, ribs, midriff, neck, back of the knee, thighs, buttocks, feet and perineum.[10]
Some evidence suggests that laughing associated with tickling is a nervous reaction that can be triggered; indeed, very ticklish people often start laughing before actually being tickled.[11]