If you give a rat a tickle

researcher tickling rat

Editor's Introduction

Neural correlates of ticklishness in the rat somatosensory cortex

Why are some people very ticklish and others not? Have you ever tried to tickle yourself? Scientists are just beginning to understand how and why people respond to tickling. The authors of this paper use rats as a model system to study ticklishness. Like humans, rats produce specific sounds when they are tickled. By quantifying these sounds and looking at activity in the brain, scientists characterized the factors that trigger or suppress ticklishness in rats. These findings may eventually help us to understand the strange phenomenon of human ticklishness.

Paper Details

Original title
Neural correlates of ticklishness in the rat somatosensory cortex
Original publication date
Vol. 354, Issue 6313, pp.757-760
Issue name


Rats emit ultrasonic vocalizations in response to tickling by humans. Tickling is rewarding through dopaminergic mechanisms, but the function and neural correlates of ticklishness are unknown. We confirmed that tickling of rats evoked vocalizations, approach, and unsolicited jumps (Freudensprünge). Recordings in the trunk region of the rat somatosensory cortex showed intense tickling-evoked activity in most neurons, whereas a minority of cells were suppressed by tickling. Tickling responses predicted nontactile neural responses to play behaviors, which suggests a neuronal link between tickling and play. Anxiogenic conditions suppressed tickling-evoked vocalizations and trunk cortex activity. Deep-layer trunk cortex neurons discharged during vocalizations, and deep-layer microstimulation evoked vocalizations. Our findings provide evidence for deep-layer trunk cortex activity as a neural correlate of ticklishness.


Tickling sensations can be differentiated into laughter-inducing “gargalesis” and non–laughter-inducing light touch, “knismesis” (1). The former is a peculiar, often funny form of social touch, which has been discussed for more than two millennia (23). Still, major questions remain unanswered: Why does tickling induce laughter? Why are tickling effects so mood-dependent (4)? Why do body parts differ in ticklishness (1)? Why can’t we tickle ourselves (2)? Is ticklish laughter different from humorous laughter (46)? To address such questions, we need a better understanding of the neural correlates of ticklishness. We took advantage of groundbreaking advances that provided evidence for tickling-evoked 50-kHz vocalizations and primitive forms of joy in rats (78). On the basis of this work, we measured “rat ticklishness” in our work as the propensity to “laugh” (emit 50-kHz calls) upon being tickled. We focused on the somatosensory cortex because it is the largest tactile neural representation in mammals, because human imaging studies suggested this candidate region (910), and because work on somatosensory afferents provided no conclusive evidence for dedicated peripheral mechanisms of tickle.

Movie S1.Tickling and touch-evoked vocalizations Ultrasonic vocalizations during tickling and gentle touch on different body parts. The audio signal provides ultrasonic vocalizations that have been transposed to the human audible range.

We tickled and gently touched rats on different body parts (Fig. 1A, ventral tickling) and observed a variety of ultrasonic vocalizations (USVs; Fig. 1B), in particular during tickling (movie S1). Rat 50-kHz vocalizations indicate positive emotional valence (11). Consistent with earlier claims that tickling is rewarding through the dopaminergic system (71213), 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 (1416). 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). Both tickling and gentle touch evoked USVs (Fig. 1C), but tickling induced more USVs than did gentle touch (ventral tickling, 4.45 ± 0.28 Hz; ventral gentle touch, 2.58 ± 0.21 Hz; n = 16, < 0.001; mean ± SEM, paired t test). 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). Tickling the ventral trunk evoked the largest number of USVs (Fig. 1D) and the largest fraction of combined USVs (Fig. 1E). Play behavior (rat chasing experimenter’s hand; Fig. 1F) also evoked USVs (Fig. 1, G and H, and movie S3) (17). Consistent with our sense that rats experienced the experimental setting as emotionally positive, we did not observe 22-kHz alarm calls.

figure 1

Fig. 1. Tickling and play behavior (chasing the experimenter’s hand) evoke ultrasonic vocalizations. (A) Tickling of the ventral trunk of a rat. (B) Spectrograms of ultrasonic vocalizations (USVs). USVs were categorized into modulated (left), trill (middle), and combined (right) call types. (C) USVs during tickling and touch. Raster plots and beige boxes indicate USV onsets and interaction phases, respectively. (D) USV rate during different phases (n = 16 recordings from four animals). Data are means ± SEM. P value refers to analysis of variance (ANOVA); pairwise comparisons are denoted as ***< 0.001 and *= 0.014 (paired t test). (E) Fraction of USV types falling into the different categories [the same data from (D)] during tail tickling (1869 USVs) and ventral tickling (3380 USVs; combined, ***< 0.001, Fisher exact test). (F) Play behavior: a rat chasing the experimenter’s hand. (G) USVs during chasing hand. (H) Fraction of USV types during chasing hand [3181 USVs; 15 recordings from three animals; colors as in (E)].


How do rats react to different types of touch and tickling stimuli?

Panel A

A photo of a researcher tickling the rat on its ventral trunk (belly).

Panel B

A visual representation of the sound waves of the rat’s ultrasonic vocalizations (USVs). They are separated into categories by call type: modulated, trill, and combined.

Panel C

This panel shows the pattern of different USVs emitted from the rat before tickling (pre), during tickling (beige boxes), and during breaks. Compared to the “pre” condition, all types of USVs increase during tickling and breaks.

Panel D

This panel shows the rate of USVs before tickling, during tickling on different body areas, and during breaks. The largest increase in USV rate occurs during ventral tickling.

Panel E

This panel is a visual comparison of the percentage of different categories of USVs during tail and ventral tickling.    

Panel F

This panel shows an example of play behavior. The rat is chasing the experimenter’s hand.

Panel G

This panel is similar to Panel C, and shows the USVs emitted before, during (beige box), and after hand chasing.

Panel H

This panel shows a visual percentage of the percentage of different categories of USVs during play behavior (hand chasing).

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). 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). Remarkably, neuronal responses were also observed during hand-chasing phases, when rats were not touched by the experimenter (Fig. 2B and fig. S2E). 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). Similar to USVs, neuronal firing rates increased more during tickling than during gentle touch on the trunk (Fig. 2D). 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.

Figure 2

Fig. 2. Tickling and play behavior (chasing hand) modulate firing rate in trunk somatosensory cortex neurons. (A) Left: Cytochrome oxidase stain of a coronal trunk somatosensory cortex section (scale bar, 200 μm; D, dorsal; L, lateral; WM, white matter). Right: Histology [black curves, layers; red ovals, lesions; dashed line, tetrode track; cross, recording site of (B)]. (B) Histogram of firing rate in a layer 5a cell during tickling, touch, and play (beige boxes; Pre, pre-interaction). Data were binned into 3-s intervals. (C) Firing rates in pre-interaction (Pre) versus in break (Wilcoxon signed-rank test). (D) Firing rates during trunk gentle touch versus trunk tickling (Wilcoxon signed-rank test). (E) Population data indicating a correlation between trunk tickling and chasing hand response indices (ρ, Pearson linear correlation coefficient; data fitted with line; n denotes number of cells).


How do different types of tickling affect neural activity in the somatosensory cortex?

This figure shows where brain activity was recorded in response to tickling and play behavior.

Panel A

A coronal section is made by cutting along a plane that divides the body into front and back sections. The cortex of the brain can be divided into layers (L1-L6) based on the morphology and function of the neurons. This is visualized using a cytochrome oxidase stain, which stains mitochondria in the cell bodies of the neurons.

A device measuring the firing rate of neurons was placed in section 5a of the trunk cortex (marked with an X). The path of insertion is shown with a dotted line and ovals mark small lesions created at the end of the experiment to mark which electrode was inserted where.

Panel B

This panel shows levels of neural activity (measured in firing rate of neurons) before and after certain touches are applied. Data before the first beige box (at 40s) represents brain activity before tickling (pre). The beige boxes represent individual tickling or instances of play behavior (interaction), and data in-between the boxes represents activity after the rat has just been touched (break).

Neural activity in the somatosensory cortex increases the most during tickling.

Panel C

This panel compares neural activity before the rat is being touched (pre; below the line), and after it has been touched (break; above the line). Each circle represents a neuron (75 neurons in total). The line in the center of the graph represents neurons that fire at the same rate before and after tickling. Because most data points fall above the line, it is concluded that the firing rate during the break period is higher than the pre-interaction period.

Panel D

This panel compares how the type of touch affects neural activity. Most data points fall above the line, indicating that trunk tickling caused more of a response than trunk gentle touch.

Panel E

The response index measures the increase or decrease of brain activity during tickling. In this graph most data points are in the positive quadrant for both hand chasing and trunk tickling. Further, some tickling-suppressed neurons are also suppressed by hand chasing. From this graph, it can be concluded that there is a positive correlation between the tickling and hand chasing responses (i.e., they have similar responses).

Anxiogenic conditions suppress tickling-evoked USVs in rats (7). 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). 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).

Figure 3

Fig. 3. Anxiogenic suppression of USVs and neuronal responses to tickling in trunk somatosensory cortex. (A) Control (left) and anxiogenic condition (right). (B) USVs in control and anxiogenic conditions. Black boxes, interaction phases; raster plots, USVs; D, dorsal tickling; V, ventral tickling; Dg, dorsal gentle touch; Vg, ventral gentle touch; T, tail tickling; Ch, chasing hand; Misc., miscellaneous. (C) USV rate during trunk tickling under control versus anxiogenic conditions; n = 6 recordings from four animals. Data are means ± SEM (paired t test). (D) Peristimulus time histograms (PSTHs) of firing in a cell during dorsal tickling (beige boxes) under control and anxiogenic conditions. (E) Response indices of cells for trunk tickling under control versus anxiogenic conditions (n = 27 cells, 6 recordings from four animals). Data are means ± SEM (paired test).


How does the mood affect a rat’s response to being tickled?

Panel A

Diagrams of the two conditions that were tested. The control environment on the left is a dimly lit cage. The anxiogenic (anxiety-causing) condition on the right involves placing the rat on an elevated stand under bright lights.

Panel B

Plot showing the frequency of modulated, trill, combined, and miscellaneous noises in response to tickling. Rats made all four types of noises frequently when they were tickled in the control environment. When the rats were tickled in the anxiogenic environment, they only produced modulated and miscellaneous sounds, but at a much lower rate. This diagram shows that rats produce fewer USVs when they are anxious.

Panel C

Similar to Panel B, this graph shows the rate at which rats produce USVs. The rats released USVs at a higher rate in the control environment than the anxiogenic environment.

Panel D

This image shows the activity in the neurons. The beige box shows the time period during which the rats were being tickled. The periods before and after this box are when the rats remained untouched.

The periods when the rats were untouched show similar neural activity in both the control and anxiogenic environments. The period when the rats were being tickled varied significantly. Neurons fired more frequently when the rats were tickled in the control environment than in the anxiogenic environment.

Panel E

This panel shows the response index that corresponds to Figure 3D. A positive response index means that neural activity increased in frequency when the rats were being tickled. A negative response means neural activity decreased when the rats were being tickled.

The figure shows that when the rats were in their control environment, neural activity increased during tickling. When the rats were in their anxiogenic environment, neural activity decreased during tickling.

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). To avoid a confounding effect of the coactivation of the trunk cortex and USVs by tickling and touch, we restricted this analysis to break periods. 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). This effect was strongest for combined USVs (Fig. 4E). Furthermore, the effect was more prominent in layers 4 and 5a than in the superficial layers (Fig. 4F). To test whether firing of somatosensory neurons causes USVs, we microstimulated the trunk cortex (Fig. 4G). Although rats had no interaction with the experimenter, they emitted USVs (Fig. 4H, top). Threshold amplitudes to trigger USVs varied between 50 and 300 μA. USVs were locked to microstimulation onset and were evoked after short latencies of 50 to 100 ms (Fig. 4H, bottom). When microstimulation was directly preceded by tickling, more USVs were evoked and current thresholds were lower. Microstimulation in the deep layers, but not in the superficial layers, evoked USVs (Fig. 4I).

Figure 4

Fig. 4. Neuronal activity in deep layers of trunk somatosensory cortex is associated with USVs, and deep-layer microstimulation evokes USVs. (A) Left: Cytochrome oxidase stain of a coronal trunk somatosensory cortex section (scale bar, 200 μm; D, dorsal; L, lateral; WM, white matter). Right: Histology [black curves, layers; red ovals, lesions; dashed line, tetrode track; cross, recording site of (B)]. (B) PSTH of firing rate in a layer 5a cell aligned to USV onsets (in break phases with no previous USV onset within 500 ms). (C) PSTH of firing rate aligned to the onset of USV (n denotes number of cells). (D) Firing rate before USV [–300 to –200 ms from USV onset, Before in (C)] versus firing rate upon USV [0 to 100 ms from USV onset, On in (C)]. The same population data from (C) are plotted for each cell (Wilcoxon signed-rank test). (E) Response indices for each USV type, using the same data from (C) and (D). Mod., modulated; Comb., combined. Data are means ± SEM. P value refers to one-way ANOVA; pairwise comparisons are denoted as **= 0.007 and *= 0.010 (paired t test). (F) Average response indices in different cortical layers. Numbers of cells are shown in parentheses. Data are means ± SEM. P value refers to one-way ANOVA on ranks; pairwise comparisons are denoted as ***< 0.001 and **= 0.003 (t test). (G) Microstimulation in trunk somatosensory cortex. (H) Top: USVs during microstimulation (beige boxes; 200 μA, 100 Hz, 2 s). Raster plots indicate USV onsets. Bottom: Average PSTH of USV rate aligned to stimulation onsets (averaged over 278 stimulations that evoked USVs at 18 sites from three animals; data were binned into 50-ms intervals). (I) Number of sites in different layers of trunk somatosensory cortex, where microstimulation did (“success,” black) or did not (“failure,” gray) evoke USVs. P value refers to χ2 statistics.


Is the neural activity in the somatosensory cortex causing the USVs?

Panel A

This image shows a cross section of the rat’s somatosensory cortex and the site of the electrode (see Figure 2A for additional details).

Panel B

This panel shows a histogram of representative neural activity (firing rate) before, during, and after vocalization. The neural activity peaks when the rat produces vocalizations (t=0).

Panel C

This panel shows a summary histogram of neural activity (firing rate) compared to the time of vocalization. Neural activity increases just prior to vocalizations (labeled “on”).

Panel D

This graph compares the neural firing rate before USV to the firing rate during USV (on). The cluster of points above the line indicates increased neural activity during the USV.

Panel E

This panel shows the relative increase in neural firing during each of the three classes of USVs (modulated, trill, and combined). The largest increase in activity occurs during combined USVs.

Panel F

This panel shows the relative increase in neural firing during USVs for each layer of the somatosensory cortex. The strongest correlation between USVs and neural firing occurs in L5a.

Panel G

In order to test whether firing of somatosensory neurons triggers USVs, experimenters sent a specified amount of electrical current through the electrodes connected to the rats’ cortices to stimulate the neurons.

Panel H

Relative timing of the microstimulation and the onset of the resulting USV.

USVs begin shortly after stimulation occurs, indicating that stimulation of the somatosensory cortex is correlated with USVs.

Panel I

This panel shows the success (USVs produced) or failure of microstimulation of different areas of the somatosensory cortex. Stimulation of L5a had the highest success rate of USV production.

Our findings confirm key conclusions of Panksepp and Burgdorf (8): Rats vocalize during tickling in a mood-dependent fashion. 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). Tickling-evoked calls are not simple reflexes in response to touch. Rats rarely emit combined calls during social facial touch with conspecifics (1819). Rats emitted combined calls preferentially during tickling on the belly, the most ticklish body part (as assessed from calling rate). Combined calls might be a relatively tickle-specific vocalization in rats. Remarkably, similar call types have been described during conspecific play behavior (20). The numerous similarities between rat and human ticklishness, such as tickling-evoked vocalizations and anxiogenic modulation, suggest that tickling is a very old and conserved form of social physicality.

Peripheral mechanisms of pleasurable touch were first studied by Zotterman in cats and suggested that knismesis is carried in part by pain fibers (21). C-fibers, unmyelinated afferents, are putatively involved in pleasurable touch in rodents (22). 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. Recent human fMRI identified activation of the lateral hypothalamus, parietal operculum, amygdala, cerebellum, and somatosensory cortex by ticklish laughter (10).

Four of our results localize tickle processing to 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). (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. (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). (iv) Microstimulation-evoked vocalizations suggest that deep-layer but not superficial-layer cortical activity is sufficient to trigger vocalizations. The short latencies of evoked calls indicate few intervening processing steps between trunk activity and calls. Electrical stimulation in various brain areas is known to evoke laughter with or without mirth (2427), but stimulation-evoked ticklish laughter has not been reported so far, and our results might be different from previously reported laughter evoked by brain stimulation.

In line with lesion evidence, our observations suggest a neural link among tickling, play, and the somatosensory cortex (28). Other findings have implicated the somatosensory cortex in social information processing (2931). The observation that the somatosensory cortex is involved in the generation of tickling responses suggests that this area might be more closely involved in emotional processing than previously thought. Identification of the neural correlates of ticklishness will allow us to frame questions about tickling in neural terms and thus help us to understand this mysterious sensation.

Supplementary Materials


Materials and Methods

Figs. S1 and S2

Movies S1 to S3

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  32. Acknowledgements: Supported by BCCN Berlin, Humboldt-Universität zu Berlin, SFB665, and the Deutsche Forschungsgemeinschaft Leibniz Prize. We thank V. Bahr, T. Balmer, A. Clemens, R. de Filippo, J. Diederichs, C. Ebbesen, K. Hartmann, M. Kunert, C. Lenschow, F. Mielke, W. Muñoz-Miranda, A. Neukirchner, C. Posey, R. Rao, U. Schneeweiß, and A. Stern. All of the data are archived at the BCCN Berlin server and will be available for download upon request.