- Mar 2018
A time interval between the stimulation and response.
I. Fried, C. L. Wilson, K. A. MacDonald, E. J. Behnke, Nature 391, 650 (1998).
Doctors performing brain surgery were able to cause the patient to laugh by electrically stimulating the anterior supplementary motor area. This area of the brain is also involved in human speech.
S. J. Blakemore, D. M. Wolpert, C. D. Frith, Nat. Neurosci. 1, 635–640 (1998).
This paper describes the use of fMRI brain scans of humans experiencing externally produced tickling or self-produced tickling.
The authors conclude that the somatosensory cortex is active in response to tickling and that the cerebellum plays a role in muting this response during self-produced tickling.
C. R. Harris, N. Christenfeld, Cogn. Emotion 11, 103–110 (1997).
Harris and Christenfeld showed that comedy-induced laughter does not increase subsequent tickle-induced laughter and vice versa.
(ii) We observed “mood-dependent” alteration of activity in the trunk somatosensory cortex, specifically an activity increase after tickling phases (Fig. 2, B and C), anxiogenic suppression of responses (Fig. 3, D and E), and a reduction of microstimulation thresholds for evoking calls after tickling. Such “mood-dependent” modulation of the somatosensory cortex is unexpected, as it is nontactile and there is little evidence to date for mood effects in other cortical areas.
Ticklishness is mood dependent: It is increased by a playful state and suppressed by an anxious state.
This result is surprising, because the somatosensory cortex has previously been shown to a have a role in detecting touch, but not in incorporating the emotional value of that touch.
Microstimulation in the deep layers, but not in the superficial layers, evoked USVs (Fig. 4I).
USVs are triggered by stimulation of neurons in the deep layers of the somatosensory cortex, but not by stimulation of neurons in the surface layers of the somatosensory cortex.
Tickling-evoked USVs were significantly suppressed in the anxiogenic condition and recovered in control conditions (Fig. 3, B and C). Similarly, anxiogenic conditions suppressed neuronal response to tickling (Fig. 3D) and inverted the sign of response index to tickling (Fig. 3E).
When rats were anxious, tickling no longer caused an increase in USV production or the neuron firing rate.
To test whether neuronal responses to tickling are also modulated by such conditions, we tickled rats in both control (Fig. 3A, left) and anxiogenic settings, such as under bright illumination and on an elevated platform (Fig. 3A, right).
The authors wanted to test whether ticklishness in rats is mood dependent, as it is in humans.
To do this, they used bright lights and height to trigger anxiety in the rats. They then recorded USVs and the neuron firing rate while tickling the rats under these conditions.
As in the cells shown in fig. S2B, responses to tickling predicted play responses (chasing hand) across the population (Fig. 2E), which suggests a neural link between tickling and play behavior.
The data suggest a possible connection between hand chasing and tickling as both response indices are similar.
Most cells increased their firing rate during trunk tickling, trunk gentle touch, and chasing hand (~77%, ~67%, and ~80% of the cells showed higher firing rates during interaction than during break, respectively; fig. S2B, top, and C to E), whereas a minority of cells were suppressed during interaction phases (fig. S2B, bottom, and C to E).
Most of the neurons showed an increased rate of firing during tickling, gentle touch, and hand-chasing. However, there were some neurons that had a decreased rate of firing during these behaviors.
The rate of firing during breaks was greater than it was during pretickling, but lower than during tickling.
Similar to USVs, activity in the trunk cortex was lower before initial tickling (Fig. 2, B and C, Pre) than during the short breaks between interactions (Fig. 2, B and C, Break).
The rate of neural activity showed a pattern similar to the rate of USV production: Activity increased during tickling and remained elevated (relative to pretickling) during breaks.
Rats seemed to warm up to tickling and vocalized less before the initial interaction than during breaks between interaction episodes (Pre versus Break; Fig. 1, C and D).
Rats produced more USVs during the breaks between tickling than they did in the time before being tickled.
(ventral tickling, 4.45 ± 0.28 Hz; ventral gentle touch, 2.58 ± 0.21 Hz; n = 16, P < 0.001; mean ± SEM, paired t test).
SEM is the standard error of the mean. It is related to standard deviation and provides information about the likelihood that the studied sample represents the whole population.
A paired t-test is used to compare two measurements (in this case ventral gentle touch vs. ventral tickling) to see if they are significantly different. A P-value of 0.05 or less is an indicator of statistical significance.
rats rapidly approached the tickling hand, and tickling induced unsolicited jumps accompanied by 50-kHz USVs (Freudensprünge, “joy jumps”; movie S2), which can be seen in joyful subjects in various mammalian species (14–16). We visually categorized spectrograms of an extensive set of USVs (34,140 calls) into modulated, trill, combined, and miscellaneous call types (Fig. 1Band fig. S1).
The authors tickled rats while recording their behaviors and vocalizations.
A spectrogram is a visual representation of sound waves. It provides quantitative information about the pitch and volume of the rat calls.
By looking at spectrograms the researchers sorted the vocalizations into three categories.
Rat 50-kHz vocalizations indicate positive emotional valence
Scientists characterized rat USVs and demonstrated that some (22 kHz) are associated with negative reactions and others (50 kHZ) are associated with positive reactions to their environment.
"Valence" is a psychological way of categorizing an emotional state as good (positive) or bad (negative).
To address such questions, we need a better understanding of the neural correlates of ticklishness.
The authors set out to determine which regions of the brain are involved in rat ticklishness and how ticklishness is impacted by mood. The articles below provide short summaries of this work.
Read more in Science: http://www.sciencemag.org/news/2016/11/watch-these-ticklish-rats-laugh-and-jump-joy
Read more in Smithsonian: http://www.smithsonianmag.com/science-nature/ticklish-rats-get-giggles-when-theyre-good-mood-180961061/
Read more in Popular Science: http://www.popsci.com/rats-are-ticklish-when-they-are-in-good-mood?dom=rss-default&src=syn
Tactile means relating to touch.
Nontactile neural responses are caused by triggers other than direct touch, such as behaviors or emotional state.
Neurons that are excited by tickling are also excited by play, and neurons that are suppressed by neurons are also suppressed by play. This suggests a link between tickling and play at the neuronal level.
Activity in the brain that corresponds to an external, physical, or behavioral event.
- Feb 2018
S. Vrontou, A. M. Wong, K. K. Rau, H. R. Koerber, D. J. Anderson, Nature 493, 669–673 (2013)
The authors identify subgroups of sensory neurons in hairy skin that detect either pleasant sensations (stroking or massage) or unpleasant sensations.
Central mechanisms of tickling were investigated by functional magnetic resonance imaging (fMRI) in human brains (9); that study, which used tickling stimuli evoking knismesis and observed somatosensory cortex activation, suggested that self-tickle suppression might be mediated by the cerebellum.
fMRI was used to detect brain activity in healthy volunteers who were being tickled. The somatosensory cortex was one region that was activated by tickling.
When patients were asked to tickle themselves, there was also activity in the cerebellum, a region of the brain associated with coordinating movement. The researches suggest that the cerebellum may have a role in detecting self-tickling and suppressing the tickle response.
we restricted this analysis to break periods.
The authors wanted to look at the relationship between neural activity and USV production. However, they needed to remove the possibility that both of these things are independent responses to tickling.
For this reason, they carried out the experiments during breaks, when USVs and increased neuronal activity were occurring, but tickling was not a factor.
Our recordings revealed that USVs and neuronal activity in the trunk cortex are modulated in a similar way by tickling and anxiogenic conditions. We wondered whether tickling-evoked USVs and neuronal responses to tickling are causally linked. We therefore aligned neuronal firing to the onsets of USVs (Fig. 4, A and B).
The authors observed that USV production and neuron firing change in the same ways in response to tickling.
In this set of experiments they wanted to see if there was also a causal relationship. In other words, is the neuronal activity triggering the production of sounds or do they just happen to occur at the same time?
Anxiogenic conditions suppress tickling-evoked USVs in rats (7)
This idea may have been originally proposed by Charles Darwin. He wrote that if a child is tickled by a stranger, the child would scream from fear rather than laugh.
- Jun 2017
(iv) Microstimulation-evoked vocalizations suggest that deep-layer but not superficial-layer cortical activity is sufficient to trigger vocalizations.
Microstimulation of deep layers of the somatosensory cortex is sufficient to trigger tickling-associated vocalizations.
C-fibers, unmyelinated afferents, are putatively involved in pleasurable touch in rodents (22)
C-fibers are a subgroup of touch receptors that respond to gentle pleasant touch.
A physical interaction with social meaning.
Tickling-evoked calls are not simple reflexes in response to touch.
The response to tickling is different than the general response to being touched. It is dependent on the type of touch, the location of touch, and the emotional state of the rat.
The increase of vocalizations after initial tickling (Fig. 1, C and D) and anxiogenic suppression of tickling-evoked calls (Fig. 3, B and C) support Darwin’s idea that “the mind must be in a pleasurable condition” for ticklish laughter (4).
There are two pieces of evidence that show the link between mood and ticklishness in rats:
First, the initial round of tickling seems to predispose rats to both play behaviors and to an increased tickling response in subsequent interactions.
Second, anxiety suppresses the rats' responses to tickling.
The lowest level stimulation current that causes USVs.
Similar to USVs, neuronal firing rates increased more during tickling than during gentle touch on the trunk (Fig. 2D).
Neural activity in the trunk cortex was greater during tickling than during gentle touch.
22-kHz alarm calls
A rat vocalization in response to fear or the presence of a threat.
Dopamine is a neurotransmitter that is involved in reward and reinforcement pathways in the brain.
Dopaminergic mechanisms use dopamine to promote repetition of and re-exposure to these positive things
Read more in Science News for Students: https://www.sciencenewsforstudents.org/article/explainer-what-dopamine
because human imaging studies suggested this candidate region (9, 10)
Scientists have previously used fMRI brain scans to determine which parts of the human brain showed increased activity during tickling.
kHz is an abbreviation for kilohertz, which measures frequency in cycles per second. In this context it is measuring the frequency of a sound wave.
Humans can typically detect sound waves with frequencies up to 20 kHz, but that range declines with age.
(iii) The strong call-related activation of the trunk somatosensory cortex points to an involvement in tickling-evoked vocalizations. Call-related firing in the somatosensory cortex is much stronger than call-evoked activity in the auditory cortex (18).
There is a strong correlation between the production of tickle-related USVs and activity in the somatosensory cortex.
(i) We found that tickling can evoke intense neuronal activity in the somatosensory cortex (Fig. 2). Moreover, play behavior, which induces anticipatory vocalizations in rats (Fig. 1, F to H) (17) and humans (23), evoked neuronal activity similar to the activity evoked by tickling (fig. S2E).
Tickling and the anticipation of tickling are both associated with activity in the somatosensory cortex.
When microstimulation was directly preceded by tickling, more USVs were evoked
"Priming" the rats by tickling them prior to microstimulation increases the number of USVs produced.
Although rats had no interaction with the experimenter, they emitted USVs (Fig. 4H, top).
Stimulation of somatosensory neurons (in the absence of tickling) is sufficient to cause the production of USVs.
This shows a direct causal link between the increased neural activity and the increased production of USVs.
To test whether firing of somatosensory neurons causes USVs, we microstimulated the trunk cortex (Fig. 4G).
The authors used a small electrical current to trigger neurons in the somatosensory cortex to fire in the absence of tickling. They then determined whether this neural stimulation caused the production of USVs.
Furthermore, the effect was more prominent in layers 4 and 5a than in the superficial layers (Fig. 4F).
Neural firing rates in the deeper layers of the cortex have a stronger relationship to USVs than rates in the surface layers of the cortex.
The activity of trunk somatosensory neurons was correlated with USV emissions: Neurons increased their firing rate before and during USV emissions (Fig. 4, C and D, Before versus On).
The firing rate of neurons is highest just before and during USV production. This shows that these two events are correlated.
Activation by multiple stimuli.
An outside factor that can also impact the dependent variable.
Remarkably, neuronal responses were also observed during hand-chasing phases, when rats were not touched by the experimenter (Fig. 2B and fig. S2E).
There was also increased activity in the somatosensory cortex during hand-chasing, despite the fact that the rats were not being touched.
This was interesting, because the primary function of the somatosensory cortex is to detect touch.
We simultaneously performed single-unit recordings in the trunk region of the somatosensory cortex (Fig. 2A). We obtained high-quality recordings of neuronal responses elicited by tickling and gentle touch (Fig. 2B and fig. S2A).
The authors implanted a microelectrode into the somatosensory cortex of the rat's brain and recorded neural activity (rate of neuron firing) in this region during tickling, touch, breaks, and play.
Play behavior (rat chasing experimenter’s hand; Fig. 1F) also evoked USVs (Fig. 1, G and H, and movie S3) (17).
Rats produce USVs during play behavior, even if they are not currently being tickled.
Tickling the ventral trunk evoked the largest number of USVs (Fig. 1D) and the largest fraction of combined USVs (Fig. 1E).
The ventral (belly) region of the rat is more ticklish than the dorsal (back) or tail regions.
tactile neural representation
Neural representation indicates the region of the brain that is activated in response to some stimulus.
Tactile neural representation refers to the region (the somatosensory cortex) that is activated in response to some sort of touch.
Afferents are the neurons that send signals inward, back to the central nervous system and the brain.
Somatosensory afferents are the neurons that send signals to the brain in response to touch.
Animals of the same species.
dedicated peripheral mechanisms of tickle
Sensory neurons that specifically detect tickling.
Electrical stimulation in various brain areas is known to evoke laughter
Doctors can directly stimulate the brains of conscious patients in order to identify and avoid critical regions of the brain during surgery. This technique is also allowing scientists to learn more about the functions of the human brain.
Tickling sensations can be differentiated into laughter-inducing “gargalesis” and non–laughter-inducing light touch, “knismesis” (1)
There are a range of animals that produce sounds that may be evolutionarily linked to human laughter. However, it can be difficult to interpret the emotions of animals.
Read more from the BBC: http://www.bbc.com/earth/story/20170518-why-humans-chimpanzees-and-rats-enjoy-being-tickled
J. Burgdorf, P. L. Wood, R. A. Kroes, J. R. Moskal, J. Panksepp, Behav. Brain Res. 182, 274–283 (2007).
The authors show that microstimulation of regions of the brain associated with reward pathways can trigger 50 kHz USVs in rats.
They also use drug treatments to demonstrate the involvement of dopamine signaling pathways in the prodution of 50 kHz USVs.
E. Bobrov, J. Wolfe, R. P. Rao, M. Brecht, Curr. Biol. 24, 109–115 (2014).
Activity in the barrel cortex (a region of the somatosensory cortex) of rats is higher during social touch than it is when the rat is touched by an object. This suggests that the somatosensory cortex plays a role in both detecting touch and integrating information about the social context of the touch.
V. Gazzola et al., Proc. Natl. Acad. Sci. U.S.A. 109, E1657–E1666 (2012).
fMRI imaging shows that the somatosensory cortex plays a role in both the sensory detection of touch and the integration of emotional responses to that touch.
B. Newman, M. A. O’Grady, C. S. Ryan, N. S. Hemmes, Percept. Mot. Skills 77, 779–785 (1993).
This article describes the use of Pavlovian conditioning in humans to create an "expectation" of tickling. This is offered as an explanation for the tendency of people to respond to tickling gestures in the same way they would respond to actual tickling.
J. Panksepp, J. Burgdorf, Physiol. Behav. 79, 533–547 (2003).
Panksepp and Burgdorf describe the initial discovery and characterization of 50 kHz calls in the rat and present the case for interpreting these calls as laughter.
C. R. Harris, Am. Sci. 87, 344–351 (1999).
Harris reviews our current understanding of ticklishness in humans.
This paper includes descriptions of the two types of tickle, different types of laughter, the physiology of human ticklishness, and the possible evolutionary basis of ticklishness.