Knismesis and gargalesis are the scientific terms, coined in 1897 by psychologists G. Stanley Hall and Arthur Allin, used to describe the two types of tickling.
Knismesis refers to the light, feather-like type of tickling. This type of tickling generally does not induce laughter and is often accompanied by an itching sensation. The knismesis phenomenon requires low levels of stimulation to sensitive parts of the body, and can be triggered by a light touch or by a light electrical current. Knismesis can also be triggered by crawling insects or parasites, prompting scratching or rubbing at the ticklish spot, thereby removing the pest. It is possible that this function explains why knismesis produces a similar response in many different kinds of animals.
Gargalesis refers to harder, laughter-inducing tickling, and involves the repeated application of high pressure to sensitive areas. This so-called heavy tickle is often associated with play and laughter. The gargalesis type of tickle works on humans and primates, and possibly on other species. Because the nerves involved in transmitting light touch and itch differ from those nerves that transmit heavy touch, pressure and vibration, it is possible that the difference in sensations produced by the two types of tickle are due to the relative proportion of itch sensation versus touch sensation.
While it is possible to trigger a knismesis response in oneself, it is usually impossible to produce gargalesthesia, the gargalesis tickle response, in oneself.
The condition of extreme sensitivity to tickling is called Hypergargalesthesia.