In 1897, two psychologists, Granville Stanley Hall and Arthur Allin, came up with the terms knismesis and gargalesis to refer to the two forms of tickling. Hall is an important figure in American psychology; he helped to found the American Psychological Association, and also started the first journal of psychology in America. Arthur Allin was a well known personality in psychology as well, especially in the Western states, where he trained many talented psychologists during his years at the University of Boulder.
Knismesis is light tickling, such as those sensations from a feather, gentle touch, insect, or mild electrical current. Heavy tickling to sensitive areas of the body such as the knees, feet, and ribs is referred to as gargalesis. While the terms are not widely used, they can make an interesting topic of conversation.
Most people associate knismesis with a sense of calm, and it is sometimes used to lull animals into a trance, because the gentle touch seems to settle and relax the body. It can also sometimes be perceived as an itchy sensation, which has led some psychologists to suggest that the difference between knismesis, which rarely produces laughter, and gargalesis, which produces laughter and violent physical reactions, may have something to do with an individual's personal itchiness threshold. The light touch of tickling is also used in erotic play in many cultures, because it heightens the sense of touch for the person being tickled.
Gargalesis, on the other hand, is a much firmer form of tickling, which sometimes feels almost painful. Some psychologists have theorized that humans respond to gargalesis with laughter and encouraging facial expressions to promote playful roughhousing, an important part of balanced social interaction and fitness. Unlike knismesis, gargalesis must be performed by someone else in order to be effective, and the reasons for this are unknown.
Most humans have experienced light tickling, although they may not have known the proper name. Try lightly running a hand across the opposite arm: notice that the sensation feels strange and almost itchy, but does not make you want to laugh or pull your arm away. Most people have more sensitive feet; sometimes the experience of knismesis on the foot becomes too intense, and you may want the sensation to stop after a few minutes. All areas of the body are sensitive to knismesis, unlike gargalesis, which needs to be performed on specific, sensitive areas of the body in order to evoke a response. Some individuals also simply do not respond to gargalesis.