Tickling is the act of touching a part of the body, so as to cause involuntary twitching movements or laughter. Such sensations can be pleasurable or exciting, but are sometimes considered highly unpleasant, particularly in the case of relentless heavy tickling, or the tickling of painfully sensitive areas.
In 1897 psychologists G. Stanley Hall and Arthur Allin described a "tickle" as two different types of phenomena. The first is a sensation caused by very light movement across the skin. This type of tickle, called knismesis, generally does not produce laughter and is sometimes accompanied by an itching sensation. The second type of tickle is the laughter inducing, "heavy" tickle, produced by repeatedly applying pressure to "ticklish" areas, and is known as gargalesis.
The feather-type of tickle is often elicited by crawling animals and insects, such as spiders, mosquitoes, scorpions or beetles, which may be why it has evolved in many animals. Gargalesis reactions, on the other hand, are thought to be limited to humans and other primates; however, some research has indicated that rats can be tickled as well.
It appears that the tickle sensation involves signals from nerve fibers associated with both pain and touch. Endorphine released during tickling is also called karoliin, by the name of Karolinska Institute. 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 fibers 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. 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. However, in some patients that have lost pain sensation due to spinal cord injury, some aspects of the tickle response do remain. Tickle may also depend on nerve fibers 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. 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, some people find that the soles of their feet are the most ticklish. Other commonly ticklish areas include the armpits, sides of the torso, neck, knee, midriff, navel,and especially the ribs.
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.
Tickling is defined by many child psychologists as an integral bonding activity between parents and children. In the parent-child concept, tickling establishes at an early age the pleasure associated with being touched by a parent with a trust-bond developed so that parents may touch a child, in an unpleasant way, should circumstances develop such as the need to treat a painful injury or prevent harm from danger. This tickling relationship continues throughout childhood and often into the early to mid teenage years.
Another tickling social relationship is that which forms between siblings of relatively the same age. Many case studies have indicated that siblings often use tickling as an alternative to outright violence when attempting to either punish or intimidate one another. The sibling tickling relationship can occasionally develop into an anti-social situation, or “tickle-torture”, where one sibling will tickle the other, without mercy. The motivation behind tickle-torture is often to portray the sense of domination the tickler has over the "ticklee".
As with parents and siblings, tickling serves as a bonding mechanism between friends, and is classified by psychologists as part of the fifth and highest grade of social play which involves special intimacy or “cognitive interaction”. This suggests that tickling works best when all the parties involved feel comfortable with the situation and one another. During adolescence, tickling often serves as an outlet for sexual energy between individuals, with erotic games, foreplay and sex becoming the motivation of the tickler. The body openings and erogenous zones are extremely ticklish; however, the tickling of these areas is generally not associated with laughter or withdrawal.
While many people assume that other people enjoy tickling, a recent survey of 84 college students indicated that only 32% of respondents enjoy being tickled (32% and 36% of respondents, respectively, either gave neutral responses, or stated that they do not enjoy being tickled.). In the same study the authors found that those people who indicated that they do not enjoy being tickled actually smiled more often during tickling than those who do enjoy being tickled, which confirms that the usual association between smiling and pleasure is broken in the context of unpleasant tickling.
Excessive tickling has been described as a primary sexual obsession and, under these circumstances, is sometimes considered a form of paraphilia. Tickling can also be a form of, or simply be mistaken for, sexual harassment.
One hypothesis, as mentioned above, is that tickling serves as a pleasant bonding experience between parent and child. However, this hypothesis does not adequately explain why many children and adults find tickling to be an unpleasant experience. Another view maintained is that tickling develops as a prenatal response and that the development of sensitive areas on the fetus helps to orient the fetus into favourable positions while in the womb.
It is unknown why certain people find areas of the body to be more ticklish than others; additionally, studies have shown that there is no significant difference in ticklishness between the genders. In 1924 J.C. Gregory proposed that the most ticklish places on the body were also those areas that were the most vulnerable during hand-to-hand combat. He posited that ticklishness might confer an evolutionary advantage by enticing the individual to protect these areas. Consistent with this idea, University of Iowa psychiatrist, Donald W. Black observed that most ticklish spots are found in the same places as the protective reflexes.
A third, hybrid hypothesis, has suggested that tickling encourages the development of combat skills. Most tickling is done by parents, siblings and friends and is often a type of rough-and-tumble play, during which time children often develop valuable defensive and combat moves. Although people generally make movements to get away from, and report disliking, being tickled, laughter encourages the tickler to continue. If the facial expressions induced by tickle were less pleasant the tickler would be less likely to continue, thus diminishing the frequency of these valuable combat lessons.
To understand how much of the tickle response is dependent on the interpersonal relationship of the parties involved, Christenfeld and Harris presented subjects with a "mechanical tickle machine". They found that the subjects laughed just as much when they believed they were being tickled by a machine as when they thought they were being tickled by a person. Harris goes on to suggest that the tickle response is reflex, similar to the startle reflex, that is contingent upon the element of surprise.
Gargalesis, on the other hand, produces an odd phenomenon, when a person touches “ticklish” body parts on their own bodies, most people measure no tickling sensation. It is thought that the tickling requires a certain amount of surprise, and because tickling one’s self produces no unexpected motion on the skin, the response is not activated. A recent analysis of the “self-tickle” response has been addressed using MRI technology. Blakemore and colleagues have investigated how the brain distinguishes between sensations we create for ourselves and sensations others create for us. When the subjects used a joystick to control a "tickling robot", they could not make themselves laugh. This suggested that when a person tries to tickle him- or herself, the cerebellum sends to the somatosensory cortex precise information on the position of the tickling target and therefore what sensation to expect. Apparently an unknown cortical mechanism then decreases or inhibits the tickling sensation. A small percentage of people however, have found it possible to tickle themselves.