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Tickle me,…Remy? Tickling rats reveals brain region responsible for tickling-induced laughter
Have you ever tickled a rat? No? Did you think a rat could be tickled? No? Well, prepare to be surprised! Amazingly, rats can be tickled (and have actually been used for tickling studies since 20001). New research using this technique, along with a technique that allows detection of ultrasonic vocalization (USVs), has now found that tickling promotes a mood-dependent firing of a group of neurons in the somatosensory cortex2. While this may initially seem like a strange scientific pursuit, tickling has been discussed for millennia by philosophers and scientists such as Socrates, Galileo and Darwin. Despite the long-standing curiosity surrounding tickling, the neurological mechanisms underpinning this physiological response heretofore remained elusive.

“Tickling the Baby” by Fritz Zuber-Buhler.
Tickling is an intriguing behavioral response for a number of reasons. First, normal human patients cannot tickle themselves, even if they tickle themselves in the same area and in the same way that would evoke ticklish laughter when performed by someone else. In fact, the ability to tickle oneself has been correlated with schizophrenia, presumably because schizophrenic patients are aware of their own intentions but cannot link the tickling sensation with the fact that they are the ones responsible3. This suggests that, in order for tickling to work in a normal subject, an element of tension and surprise is required. Additionally, Darwin proposed that tickling and humor are closely related, as they both evoke laughter, goosebumps, and convulsive muscle contractions. He and a German physiologist, Ewald Hecker, hypothesized that humor and tickling require a pleasant state of mind, meaning that in order to respond to tickling, prior neurological conditioning is required (the Darwin-Hecker hypothesis).
The evolutionary advantage for tickling response has also been discussed extensively. There are two types of tickling: the light gentle type that may feel itchy (knismesis), and armpit- or rib-poking type that induces laughter (gargalesis). Many animal species, including horses and even sharks, exhibit knismesis, likely as a way to shake off spiders and creepy-crawlies. However, relatively few species are capable of gargalesis; gargalesis is typically considered to be restricted to humans and our closest primate relatives. In these species, tickling is thought by some to have evolved as a mechanism for social bonding, promoting a light-hearted interaction between parents and offspring. Other researchers maintain, however, that tickling is an educational activity that teaches which parts of the body are vulnerable to attack, motivating individuals to protect these areas. It is thought that the positive response to tickling (laughter) encourages these play attacks in a safe environment. Whatever the evolutionary basis for tickling, the molecular basis for tickling has proven to be difficult to unravel.
Orangutans “laugh” both on the inhale and the exhale when they are tickled.
A human study had subjects either laugh forcefully (deemed humorous/voluntary laughter) or had a partner tickle their feet and measured the corresponding neuronal response using functional magnetic resonance imaging (fMRI). This study found that voluntary laughter, but not ticklish laughter, activated the “pleasure center” of the brain, as well as areas of the brain involved in “higher order function.” Voluntary laughter and ticklish laughter activated the primary sensory-motor cortex4. However, only ticklish laughter activated areas in the hypothalamus, a region responsible for regulating multiple functions, including visceral reactions. Additionally, in support of the hypothesis that tickling response is a defense response, only tickling activated areas thought to be involved in the anticipation of pain. The overlap between the subcortical centers that activate ticklish and humorous laughter suggested that ticklish laughter is a precursor to humorous laughter, supporting the Darwin-Hecker hypothesis.
To delve further into the molecular mechanisms driving ticklish laughter, scientists turned to a rat model of tickling. While higher primates had been thought to be the only mammals that exhibit gargalesis, it turns out that rats also emit a high-pitch, 50-kHz chirp in response to manual tickling by a human experimenter1! This chirp exists in stark contrast to a rat’s 22-kHz alarm call.
In the most recent study2 using a rat model of tickling, scientists monitored rats’ vocalizations and neuron firing in response to tickling, both in normal and anxiety-producing conditions. They made a few novel findings: first, rats rapidly approached the tickle-inducing hand and demonstrated unsolicited jumps, or “joy jumps” after tickling. These jumps can be observed in joyful creatures across multiple animal species, indicating that tickling was rewarding to the rats. Next, researchers found that neuron firing in the trunk region of the somatosensory cortex was elicited by tickling, and was greater during trunk tickling compared to light trunk touching. Surprisingly, neuron firing was also observed without tickling but during hand-chasing phases during breaks between tickles, suggesting a link between tickling and play behavior. Because tickling-induced vocalizations were known to be suppressed when rats were in anxiety-producing (anxiogenic) situations (alone on a stage, under a light), examiners also evaluated neuronal firing when rats were tickled in anxiogenic conditions. In these conditions, the study observed that neuron firing was also suppressed, indicating a correlation between the nerve firing and vocalization. Additionally, this data indicates that, much as Darwin and Hecker hypothesized, the mind must be in “a pleasurable condition” in order for ticklish laughter to occur. Given the similar behavior exhibited by rats and higher mammals during tickling, and the similarity in the regions of the brain that activate tickle responses, it appears that tickling is a very old, conserved form of social, physical interaction. What are your thoughts? Do you hate getting tickled? Let us know at tech@biolegend.com!
References:
  1. 50-kHz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables.
  2. Neural correlates of ticklishness in the rat somatosensory cortex.
  3. Individuals with pronounced schizotypal traits are particularly successful in tickling themselves.
  4. Exploration of the neural correlates of ticklish laughter by functional magnetic resonance imaging.
Contributed by Sarah Puhr, PhD.
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