Tickling is a complex phenomenon. Although tickling is clearly a physical sensation, the tickle reaction is inconsistent in a way that other stimuli are not. For example, if a hand is placed over an open flame, or into a bucket of icy water, the reflex reaction will be identical even among different people.
Such does not appear to be the case with tickling, which suggests that perception may play a role in how the brain processes the physiological tickling sensation.
Tickling is a Matter of Perception as Well as of Physiology
In 1872, Charles Darwin noted this anomaly in his book, On the Expression of the Emotions in Man and Animals. Darwin acknowledged how the body convulses when tickled, in a pattern “shown by the minute unstriped muscles, which serve to erect the separate hairs on the body, contracting near a tickled surface.” And yet the tickling reflex action seems to require at least three other elements to be successful.
- Familiarity – A child tickled by a family member will laugh uncontrollably, but will “scream from fear” if tickled by a stranger.
- Element of Surprise – If a person is aware of the spot to be tickled, then the ticklish sensation may be suppressed long enough until the tickler stops. Thus, the “precise point to be touched must not be known.”
- Degree and Place of Touch – According to Darwin, the tickling touch must not only “be light,” but also applied to an area of the body “not commonly touched, such as the armpits or between the toes, or...the soles of the feet.”
Science Cannot Explain the Mystery of Tickling
Although science can explain how the brain interprets the tickle sensation, it does not know why some people are ticklish and others are not. As a matter of physiology, the tickle response is “well established by four months of age” (Juan, 2006).
Darwin (1872) noted that “certain nerves are much more sensitive to tickling than others.” All things being equal, however, nerves alone do not explain differences in ticklishness. The palms of the hands contain almost as many nerve endings and sensory receptors as the soles of the feet. Yet, few people are ticklish on the hands, while many people are very ticklish on the feet.
If Darwin is correct in asserting that tickling is a form of social bonding, then it is possible that tickling may be a learned trait rather than an inherited attribute. In experiments with rats, Panksepp and Burghoff (2003) observed that the laughing response can be “successfully selected for and against within four generations of selective breeding.”
In the same way, Newman et al. (1993) showed how human “subjects will respond to a gesture that signals the onset of a tickle in the same way as to a tickle.” Using a classical conditioning technique associated with Ivan Pavlov, experimental subjects first were presented with neutral verbal stimuli, in which subjects were told that they were going to be tickled. Then, these short verbal phrases were followed by unconditioned stimuli (i.e., having their feet lightly stroked). Over time, subjects connected both stimuli and became conditioned to 'feel' tickled with words, even though they were never touched..
It is possible that some babies undergo Pavlovian conditioning, in which they learn to associate the smiles and laughter from a tickling parent with a tickling stimuli on body parts. Babies who are often touched and tickled as infants may be conditioned to become more ticklish as they grow older than babies who are seldom touched, or rarely tickled.
Can You Tickle Yourself?
The simple answer seems to be "no." The more complex answer is, "Maybe."
If tickling is a reflex reaction, then there should be little reason why self-tickling cannot be achieved in the same way as “a perfectly respectable knee-jerk reflex" can be produced "by tapping our own knees.” Yet, Harris (1999) also acknowledged that people cannot frighten themselves, so tickling may require sufficient force “that cannot be anticipated in advance” to allow self-tickling to occur.
In 2006, researchers at Queen's University Centre for Neuroscience Studies concluded that the brain was responsible for blocking the self-tickle phenomenon. According to Dr. Randy Flanagan, the human brain is bombarded daily with loads of sensory information. In order to allow normal function, the brain must “distinguish between what is caused by our own movements and what is due to changes in the outside world.” Self-movement is canceled to alert the senses to more important external movements, such as those associated with a predator stalking its prey.
The part of the brain that blocks self-tickling is the cerebellum (Blakemore et al., 2000). When self-tickling is attempted, those intentions are signaled to the brain. Darwin's element of surprise is gone, and so the self-tickle actions are canceled.
Neural impulses sent to the brain can be outsmarted by mechanical means. As Blakemore et al. (2000) demonstrated, subjects could induce a light tickling sensation by using a robot arm, a joystick, and remote control. Overall, though, the robot tickling was judged to be “less tickly, intense and pleasant” than being tickled by someone else.
While this may be reassuring to those who enjoy tickling, it provides small comfort to those who are afraid of being tickled.
References:
Blakemore SJ, Wolpert D, Frith C. 2000. Why Can't You Tickle Yourself? NeuroReport 11:11-16.
Harris CR. 1999. The Mystery of Ticklish Laughter. American Scientist. 87:344-51.
Juan S. 2006. What is the Purpose of Tickling? The Register.
Newman B, O'Grady MA, Ryan CS, Hemmes NS. 1993 (Dec). Pavlovian Conditioning of the Tickle Response of Human Subjects: Temporal and Delay Conditioning. Percept Mot Skills. 77(3 Pt 1):779-85.
Panksepp J, Burgdorf J. 2003. 'Laughing' Rats and the Evolutionary Antecedents of Human Joy? Physiology & Behavior. 79 533-47.
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