People
laugh when tickled. It sounds natural, but its mechanism remains to be
determined. Shimpei Ishiyama from the Bernstein Center for Computational
Neuroscience investigates the neural correlate of ticklishness in rats.
CB: What is your project about?
SI: Nobody knows how ticklishness is encoded. We speculated that ticklishness might be represented in the brain, most likely in the somatosensory cortex. For my project, I tickle rats which are implanted with electrodes in this brain region.
Why do you use rats to study ticklishness?
Tickling is considered to mimic play behavior. Therefore, we needed a very social animal. Mice are not that social compared to rats. We use young rats because they are more ticklish than older rats. Correspondingly, young rats play a lot, while old ones do not. Jack Panksepp was the first to discover that rats vocalize intensively when tickled. He proposed a very bold theory that rat vocalization might be an evolutionary ancestor of human laughter.
How do rats laugh?
Rats evoke ultrasonic vocalizations that are divided into 22 kHz, indicating negative emotions, and 50 kHz, indicating positive emotions. They emit 50 kHz vocalizations during social interactions with conspecifics or mating, while consuming addictive drugs, and during ‘play fight’. We found that rats have specific patterns of vocalization depending on the stimulus. There is a pattern which looks really complicated and appears particularly when the rats are touched or tickled. There are other patterns called “Trill” and “Modulated”; these are continuously pronounced during a break between bouts of tickling. And there are others that are pronounced during touching or tickling. However, I would hesitate to call this the same as human ticklishness or human laughter, but at least rat ticklishness and laughter.
How do you tickle the rats?
Young rats target the nape of the neck during play fight. Professor Panksepp discovered that tickling the nape of rats induces more 50 kHz vocalizations than tickling other parts, which indicates a close relationship between ‘tickling’ and play fight in rats. For the experiment, I put a rat in a big box which is placed underneath a camera. I then put on gloves and tickle the rat with my hand. If I tickle the rats' trunk, they laugh heavily; if I tickle them on other body parts such as the tail, they are less excited.
How do you define ticklishness?
There are two types of ticklishness. One is called knismesis – a very light, itchy, annoying feeling of ticklishness, like the touch of a feather. This does not induce laughter. You can even do it by yourself anywhere on your body. The other is called gargalesis. This ticklishness induces heavy laughter. Obviously that is the one I am studying. Rats show positive emotions and a lot of vocalizations when exposed to ticklish stimuli. It can even make them addicted.
How do you record the rats' brain activity?
We implant 8 tetrodes, which consist of 32 electrodes, within a space of approximately 2.5 mm in the somatosensory area representing the trunk region, because we think this is the most ticklish body part. The electrodes cover nearly the whole area. We use tetrodes that extracellularly detect spikes. These represent action potentials from single cells. In the recorded traces you see different spike amplitudes. Each amplitude size corresponds to a single cell corresponding to the position of the electrode. Therefore, we can separate the signals by analyzing the amplitudes and many other features – this is called clustering – and measure the frequency of action potentials of single units.
Do you see specific activity in the somatosensory cortex while tickling the rats?
Yes indeed! We detected specific firing during the tickling phase when I overlay the electrophysiology data with the video information of the tickling session. The high firing rate was phase-locked to the tickling. There is always baseline firing, but I found cells that fire more during tickling phases.
How do you know that the neuronal activity corresponds to tickling and not just touch?
We just started our investigations and we are not sure yet. Some cells in the somatosensory cortex fire intensively when I tickle the rat, but less when I gently touch the rat. You can assume that this higher activity during tickling compared to gentle touch might be the neural correlate of ticklishness.
Some people are not as ticklish as others. Do you also observe this in rats?
Yes, I do. Rats also have characters. Some are really jumpy, some are very shy. Typically the jumpy rats are more ticklish. Ticklishness is also dependent on mood. If I give the same physical stimuli but the rat's mood is less excited, it vocalizes less. I have bright lights in my setup; rats are scared of bright light as they are nocturnal animals. Under bright light, they don’t move much and laugh less, even if the tickling stimuli are the same.
The findings have since been published in Science
Claudia Willmes, PhD Alumna AG Eickholt / AG Schmitz
This article originally appeared March 2015, Volume 08 Issue 01, Humor
CB: What is your project about?
SI: Nobody knows how ticklishness is encoded. We speculated that ticklishness might be represented in the brain, most likely in the somatosensory cortex. For my project, I tickle rats which are implanted with electrodes in this brain region.
Why do you use rats to study ticklishness?
Tickling is considered to mimic play behavior. Therefore, we needed a very social animal. Mice are not that social compared to rats. We use young rats because they are more ticklish than older rats. Correspondingly, young rats play a lot, while old ones do not. Jack Panksepp was the first to discover that rats vocalize intensively when tickled. He proposed a very bold theory that rat vocalization might be an evolutionary ancestor of human laughter.
How do rats laugh?
Rats evoke ultrasonic vocalizations that are divided into 22 kHz, indicating negative emotions, and 50 kHz, indicating positive emotions. They emit 50 kHz vocalizations during social interactions with conspecifics or mating, while consuming addictive drugs, and during ‘play fight’. We found that rats have specific patterns of vocalization depending on the stimulus. There is a pattern which looks really complicated and appears particularly when the rats are touched or tickled. There are other patterns called “Trill” and “Modulated”; these are continuously pronounced during a break between bouts of tickling. And there are others that are pronounced during touching or tickling. However, I would hesitate to call this the same as human ticklishness or human laughter, but at least rat ticklishness and laughter.
How do you tickle the rats?
Young rats target the nape of the neck during play fight. Professor Panksepp discovered that tickling the nape of rats induces more 50 kHz vocalizations than tickling other parts, which indicates a close relationship between ‘tickling’ and play fight in rats. For the experiment, I put a rat in a big box which is placed underneath a camera. I then put on gloves and tickle the rat with my hand. If I tickle the rats' trunk, they laugh heavily; if I tickle them on other body parts such as the tail, they are less excited.
How do you define ticklishness?
There are two types of ticklishness. One is called knismesis – a very light, itchy, annoying feeling of ticklishness, like the touch of a feather. This does not induce laughter. You can even do it by yourself anywhere on your body. The other is called gargalesis. This ticklishness induces heavy laughter. Obviously that is the one I am studying. Rats show positive emotions and a lot of vocalizations when exposed to ticklish stimuli. It can even make them addicted.
How do you record the rats' brain activity?
We implant 8 tetrodes, which consist of 32 electrodes, within a space of approximately 2.5 mm in the somatosensory area representing the trunk region, because we think this is the most ticklish body part. The electrodes cover nearly the whole area. We use tetrodes that extracellularly detect spikes. These represent action potentials from single cells. In the recorded traces you see different spike amplitudes. Each amplitude size corresponds to a single cell corresponding to the position of the electrode. Therefore, we can separate the signals by analyzing the amplitudes and many other features – this is called clustering – and measure the frequency of action potentials of single units.
Do you see specific activity in the somatosensory cortex while tickling the rats?
Yes indeed! We detected specific firing during the tickling phase when I overlay the electrophysiology data with the video information of the tickling session. The high firing rate was phase-locked to the tickling. There is always baseline firing, but I found cells that fire more during tickling phases.
How do you know that the neuronal activity corresponds to tickling and not just touch?
We just started our investigations and we are not sure yet. Some cells in the somatosensory cortex fire intensively when I tickle the rat, but less when I gently touch the rat. You can assume that this higher activity during tickling compared to gentle touch might be the neural correlate of ticklishness.
Some people are not as ticklish as others. Do you also observe this in rats?
Yes, I do. Rats also have characters. Some are really jumpy, some are very shy. Typically the jumpy rats are more ticklish. Ticklishness is also dependent on mood. If I give the same physical stimuli but the rat's mood is less excited, it vocalizes less. I have bright lights in my setup; rats are scared of bright light as they are nocturnal animals. Under bright light, they don’t move much and laugh less, even if the tickling stimuli are the same.
The findings have since been published in Science
Claudia Willmes, PhD Alumna AG Eickholt / AG Schmitz
This article originally appeared March 2015, Volume 08 Issue 01, Humor
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