Motivation and emotion/Book/2016/Tickling and emotion

Overview


What is tickle? What is the act of tickling and what set of behaviours are involved? What emotions does tickling evoke? Why can't a response be evoked from tickling ourselves?

This chapter will discuss some of the existing theories and empirical research on tickling, with a particular focus on the physiology and neurology of tickle, the emotions and responses tickling evokes, tickling as a social behaviour, and the association between tickling and laughter.

What is Tickle?
Tickle has been defined as a fundamental sensation of the skin, along with touch, pain, temperature, and priuritus (Selden, 2004). However, unlike pain and itch, tickle only has extrinsic triggers and no intrinsic stimulation (Selden, 2004). The sensation of tickle can only be elicited by an external stimulus, which can vary from a light movement across the skin, to a rougher, deeper pressure that is applied repeatedly in certain regions of the body (Harris, 2012). The responses that are associated with tickle are dependent on the level of stimulation, and can be both pleasant and aversive. These responses include smiling, laughing, feeling annoyed or agitated, and also withdrawal from the stimulus (Selden, 2004). What makes tickle a particularly interesting sensation of the human experience is the evidence that demonstrate two simultaneous, contradictory responses that are elicited from tickling or feeling the tickle sensation: laughter and withdrawal from stimulus.

Aside from being a sensation, tickle is also a social behaviour (Provine, 2004). Playful tickle induces positive emotions in infants, young children, and even adults (Provine, 2004). Tickling is a favourite method used by mothers to make their infants laugh in attempt to bond with their little ones during the preverbal stage of their infant's life (Selden, 2004). The laughter response that is elicited through tickling enforces social play behaviour and social bonding between both human primates and non-human primates (Wildgruber et al., 2013). Thus, tickle serves an important function in the social setting by building rapport with children as well as maintaining intimate bond between two people (Schreibman et al., 2015).

Physiology of Tickle
The sensation of tickle is a result of tactile stimulation on the body and seems to have two distinct components (Harris, 1999). Hall and Allin (1897) have coined the scientific terms knismesis and gargalesis to describe the two different types of tickle human beings experience. These two components differ in the level of stimulation as well as the emotions and responses it induces.

Knismesis
Knismesis is a light or feather-type sensation, resulting from a light movement across the skin with the added component of motion (Hall & Allin, 1897). This type of tickle is associated with the more aversive response (e.g., negative emotions such as feeling annoyed) as it resembles the sensation of a crawling insect on one's surface of the skin (Selden, 2004). It has been suggested that the aversive response knismesis induces is due to an evolutionary function. Harris (1999) described the annoying itch-like sensation from knismesis as a prompt for an individual to scratch or rub the tickled spot to remove the foreign stimuli from their skin. Thus, knismesis evokes more of a reflex-like response rather than a voluntary reaction.

Gargalesis
Gargalesis, on the other hand, is a higher stimulation of tickle that is stroked across the skin in certain regions of the body (Hall & Allin, 1897). This rougher, deeper-pressure movement induces a pleasurable feeling and is associated with symptoms of positive emotions such as smile and laughter (Selden, 2004). Unlike knismesis, this laughter-inducing tickle is best considered as a social behaviour rather than a reflex (Harris, 1999). Gargalesis is also the sensation provoked in playful tickle (Selden, 2004).

Neurology of Tickle


Research studies using functional magnetic resonance imaging (or fMRI) have found the increased activity of certain brain parts when one experiences the sensation of tickle. An increased cerebral blood flow in the somatosensory cortex - or S1 - is shown in response to tickle induced by an external stimulus (McGlone et al., 2002). This particular area of the cortex is the primary receiver of sensory inputs from the body relayed via the thalamus (Hine & Martin, 2015). The S1 occupies the postcentral gyrus and is devoted to process information from the somatic receptors (Colman, 2008). When one receives an externally administered tactile stimulus that produces the tickle sensation, the S1 shows an increased activity, suggesting that tickle is indeed a fundamental sensation of the skin.

Researchers have also revealed an increased dopamine release in the nucleus accumbens. An experimental study with adolescent rats elucidated whether playful tickle can trigger dopamine release in the nucleus accumbens and whether the release of dopamine mediates positive emotions (Hori et al., 2013). In this study, 35- to 40-day-old rats received tickling stimulation for five minutes and light-touch stimulation for the same length of time. The marker for positive emotion was measured in ultrasonic vocalisations (USVs) as adolescent rats emit 50 kHz USVs as a symptom of positive emotion. The effect of tickling stimulation was then compared with light-touch stimulation, and the results revealed that tickling stimulation for five minutes significantly increased dopamine concentration in the dialysate of the nucleus accumbens (Hori et al., 2013). It also revealed that the 50 kHz USVs emitted during tickling are mediated by increased dopamine release in the nucleus accumbens (Hori et al., 2013).



The results of the study by Hori et al. (2013) have great implications for understanding the emotional effects that tickling evokes. The nucleus accumbens is a crucial part of the mesolimbic pathway, also known as the reward centre of the brain (Murrin, 2007). The neurons in the nucleus accumbens receive input from the dopaminergic neurons from the ventral tegmental area, and the mesolimbic pathway is driven by the dopamine release in this particular structure (Ikemoto, 2010). Dopamine plays an important role in regulating mood and behaviour and is a significant part of the brain's reward and pleasure centres (Sharot, Shiner, Brown, Fan, & Dolan, 2009). Dopamine is also associated with motivation, addiction, attention, and lust (Sharot et al., 2009). The evidence showing that tickling stimulation triggers dopamine release in the nucleus accumbens then suggests, despite engaging in a defensive reaction, tickling evokes positive emotions and is perceived by the brain as a rewarding and pleasurable experience. Thus, using tickling as an interactive play in the social setting may be useful in forming interpersonal bonds with others, both humans and animals, as the dopamine release that tickling triggers would result in positive emotions.

Why Can't One Tickle Oneself?
One of the unique characteristics of tickling is that one cannot make oneself laugh via self-tickling. The sensation of tickle is perceived by a part of the brain called the somatosensory cortex, or S1. However, when tickle is felt as a sensation resulting from a self-produced stimulation, the S1 does not show the same increase in activity as it does during an externally administered stimulation (Blakemore, Wolpert, & Firth, 1998). This suggests that a self-produced tactile stimulus is perceived by the brain as less ticklish than the same stimulus generated externally. It is important to note, however, that most research on the subject relies heavily on self-report, and subjective experiences of the sensation of tickle vary among individuals (Simpson, 2001). Thus, further research using a different method is required to reduce the current strong degree of subjectivity.

An experimental research study that confirmed the ineffectiveness of self-administered tickles was conducted by Weiskrantz, Elliott, and Darlington (1971). In this experiment, 30 undergraduate students were tickled using an apparatus that consisted of a plastic pointer that moved on the sole of the participants' bare feet. The subjects were tickled in two out of three conditions: by the experimenter with sole control over the handle, the subject tickled himself with control over the handle, or in which the experimenter controlled the handle, but subject held it with his arm passively following the movements. The apparatus allowed the assessment of external feedback to the skin and muscles associated with the movement, which was interpreted as cancellation signals. The experimenters then compared the cancellation signals between self- and externally-administered stimulation, and the results revealed a highly significance difference in score between the two different stimulation, where all subjects reported that being tickled by the experimenter was more ticklish than self-tickling (Weiskrantz et al., 1971). This study suggests that it is indeed possible to further examine this topic under an experimental setting, although a more thorough and effective procedure is needed to achieve a final answer as to why laughter cannot be induced via self-tickling.

Tickling as a Social Behaviour


Tickling is also a form of non-verbal communication with high relevance for social interactions (Provine, 2004). Tickling is an effective and a favourite method used by parents to make their babies laugh, and even between friends, family, and lovers (Provine, 2004). The type of playful tickle that two human (and non-human) primates engage in is usually the laughter-inducing gargalesis tickle (Selden, 2004). This type of tickle provokes positive emotion, providing a useful means of communication in developing interpersonal relationships and reinforces play behaviour and social bonding (Wildgruber et al., 2013). The elicited response that manifest the feeling of pleasure from tickling is laughter.

Laughter is a signal of social communication among humans and non-human primates that is often a result of positive experiences or interaction with another being (Wildgruber et al., 2013). The type of laughter induced by tickling has been suggested to have distinct underlying mechanisms compared to the other types of laughter (e.g., complex social laughter as defined by Wilgruber et al. (2013)). This type of laughter has been termed "ticklish laughter" (Provine, 2004).

Neural Correlates of Ticklish Laughter
The neural root of ticklish laughter in humans has been found to be within the limbic system. A study using an fMRI revealed a significant role of hypothalamic activity in evoking ticklish laughter (Wattendorf et al., 2013). It was found that tickling followed by involuntary laughter activated parts within the limbic system, which are the lateral hypothalamus and amygdala, as well as the parietal operculum and the right side of the cerebellum (Wattendorf et al., 2013). Tickling was also found to activate the higher-order sensory regions which has been associated with the conscious perception of both skin contact and pain (Watterndorf et al., 2013), which may explain the defensive reaction people engage in when being tickled.

Darwin-Hecker Hypothesis of Laughter/Humour
In attempt to understand the nature of ticklish laughter, many theories emphasise the relationship between humour and laughter. The integration of two theories that has been most researched in regards to ticklish laughter is the Darwin-Hecker hypothesis of laughter/humour (Wattendorf et al., 2013). Darwin (1872) and Hecker (1873) both suggested that there are commonalities that underlie humour and tickle, and advanced the idea that there is an existing link between tickling and humour (as cited in Harris & Christenfeld, 2010). These commonalities include both humour and tickle trigger laughter and the qualities as stimuli (humour has been considered as a "tickling of the mind") (Watterndorf et al., 2013).

Researchers have attempted to explore the relationship between tickle and humour as proposed by this particular hypothesis. One research examined the relationship between the responses elicited by tickling and by humour using behavioural and emotional responsivity measures (Harris & Christenfeld, 2010). The results found no evidence that ticklish laughter increased laughter to comedy nor that comedy-induced laughter increased subsequent laughter to tickle (Harris & Christenfeld, 2010). This finding implies that the only commonality between humour and tickle is only in the behavioural response they elicit, which are smiling and laughter.

Why Are Some People More Ticklish Than Others?
Throughout human history, the controversy as to why certain regions of the body is more ticklish than others has been argued and discussed. However, the answer to this particular topic remains a mystery until the present day. It has, thus, been a challenge to discover why some people are more ticklish than others (Selden, 2004). Researchers have attempted to explain this phenomenon by identifying the correlation of body parts and sensitivity to the sensation of tickles. Harris (1999) found that college students are most ticklish in the underarms, waist, ribs, and soles of the feet. Another investigation into this topic also discovered that children are most ticklish on their soles, underarms, neck, and under the chin (Hall & Allin, 1897). These findings, however, have yet to explain why these regions of the body seem to be a more sensitive spot for tactile stimulus than others. The question as to why some people are more resistant to ticklishness then, thus, remains unanswered.

Conclusion
Tickle has been defined as both a sensation and a social behaviour. While some questions in regards to this peculiar phenomenon remain unanswered, many researchers have attempted to explain the effects and mechanisms underlying the sensation of tickle and the act of tickling. The sensation of tickle evokes responses that are both pleasurable and aversive, such as smiling, laughter, and even defensive actions such as withdrawal from the stimulus. These responses are dependent on the two distinct component of tickles: knismesis and gargalesis.

While many researchers have identified body parts that are more ticklish than others, none was able to explain why these regions are more ticklish than others to this present day. However, current theories and research studies have demonstrate the significance of tickle as an evolutionary and social function. It will be interesting to see the direction in which future research choose to pursue to explore further into the topic.

Test Your Knowledge!
What have you learnt from this chapter? Choose the correct answers and click "Submit": {Tickle has been defined as - a sensation - a social behaviour - a type of stimulus that can vary in levels of pressure + both a sensation and a social behaviour
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{Gargalesis is characterised by a _____ movement across the skin. - light - moderate-pressure - deep-pressure
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{Results from Hori et al.'s (2013) study revealed the increase of ____ release in the ____ as a result of tickling stimulation. - serotonin; amygdala + dopamine; nucleus accumbens - epinephrine; mesolimbic pathway - dopamine; ventral tegmental area
 * type=""}

{Children are said to be most ticklish in which parts of the body? + soles of feet, under the chin, underarms, and neck - palms of the hands and top of head - toes, fingers, and nail beds - none of the above
 * type=""}