Neural basis of contagious itch and why some people
are more prone to it
Henning Hollea,1, Kimberley Warneb,c, Anil K. Sethc,d, Hugo D. Critchleyc,e, and Jamie Wardb,c
aDepartment of Psychology, University of Hull, Hull HU6 7RX, United Kingdom; andbSchool of Psychology,cSackler Centre for Consciousness Science,dSchool
of Informatics and Engineering, andeBrighton and Sussex Medical School, University of Sussex, Brighton BN1 9QU, United Kingdom
Edited by Dale Purves, Duke–National University of Singapore Graduate Medical School, Singapore, and approved October 12, 2012 (received for review
September 19, 2012)
Watching someone scratch himself can induce feelings of itchiness
in the perceiver. This provides a unique opportunity to character-
ize the neural basis of subjective experiences of itch, independent
of changes in peripheral inputs. In this study, we first established
that the social contagion of itch is essentially a normative response
(experienced by most people), and that the degree of contagion is
related to trait differences in neuroticism (i.e., the tendency to
experience negative emotions), but not to empathy. Watching
video clips of someone scratching (relative to control videos of
tapping) activated, as indicated by functional neuroimaging, many
of the neural regions linked to the physical perception of itch,
including anterior insular, primary somatosensory, and prefrontal
(BA44) and premotor cortices. Moreover, activity in the left BA44,
BA6, and primary somatosensory cortex was correlated with sub-
jective ratings of itchiness, and the responsivity of the left BA44
reflected individual differences in neuroticism. Our findings high-
light the central neural generation of the subjective experience of
somatosensory perception in the absence of somatosensory stim-
ulation. We speculate that the habitual activation of this central
“itch matrix” may give rise to psychogenic itch disorders.
functional MRI|pruritus|visual induction|insula|touch
evoked by watching someone scratch himself or by listening to
a lecture on dermatologic conditions (1, 2). Although many
aspects of the neurobiology of itch are now appreciated (3, 4),
the standard definition of itch (“an unpleasant sensation asso-
ciated with an urge to scratch”) and its description as a symptom
within clinical disorders remain essentially subjective and based
on self-report. The study of the neural basis of contagious itch
presents a unique opportunity to explore the neural basis of
subjective itch experience that is dissociated from the normal
Functional neuroimaging investigations of itch [predominantly
functional MRI (fMRI)] typically use an invasive, localized ad-
ministration of histamine to induce itch (5–8). This approach has
revealed the engagement of a network of regions (the so-called
“itch matrix”) that includes the anterior insula, cingulate cortex,
primary somatosensory cortex, premotor cortex, prefrontal cor-
tex, thalamus, and cerebellum. Within this network, there is
functional specialization that reflects the multifaceted nature of
itch (i.e., its sensory, motor, and affective attributes), with the
proposal that anterior insula and cingulate cortex may code the
affective components of itch (4). Of note, these regions are also
linked to the processing of (and awareness of) interoceptive
bodily signals, including pain, cardiovascular activity, and hunger
(9, 10). These internal signals are motivationally salient, and thus
their representation may correspondingly engender an urge for
action (11)—that is, scratching in the case of itch. The planning
of scratching movements is linked to premotor activity, whereas
the intention to scratch (or not) is linked to engagement of the
prefrontal cortex (12), consistent with this area’s recognized role
in willed actions (13). Primary and secondary somatosensory
tch is—to some degree—socially contagious. Subjective feel-
ings of itchiness and observable increases in scratching can be
cortices have been proposed to support the sensory (i.e., spatial,
temporal, and intensity) aspects of the experience (4); however,
activity within almost all parts of the itch matrix is correlated with
subjective ratings of itch intensity (5, 6, 14), suggesting in-
terdependence of the sensory, motor, and affective components
of itch. Previous fMRI studies were constrained by the meth-
odological limitation that the experience of histamine-induced
itch emerges rather slowly, taking approximately 1 min to reach
peak intensity after onset of infusion (5), followed by a slow
decay. This time course means that little of the moment-to-mo-
ment fluctuation in subjective itchiness can be related to evoked
changes in brain activity, constraining analytic power. We show
that visual induction of itch does not suffer from this limitation.
Although no previous study has examined the neural correlates
of visually induced itch, several researchers have suggested that the
“mirror neuron system” may be essential for contagious itching (2,
4). Mirror neurons, first reported in the macaque brain, respond to
both a self-executed action and the sight of an action performed by
another person (15). In macaques, mirror neuron-containing
regions include the premotor and inferior frontal cortices and in-
ferior parietal lobe (16). Neurons with similar properties have
been observed in the human brain as well (17). In humans, this
system may extend beyond action perception to perception of
feeling states. For instance, Carr et al. (18) suggested that viewing
a facial expression activates emotion-related parts of the brain via
the motor-based mirror system, and that this could be the neural
basis of empathy (19). There is compelling evidence linking em-
pathy with some forms of emotional or behavioral “contagion” (20,
21), although contagious itch has not been considered in any
previous studies. However, some studies did not implicate action-
based mirror systems as the interface between perception and
feeling (22, 23), but suggested instead that feeling states can be
shared without obligatory motor simulation.
In the present study, we used fMRI to examine the neural
basis of contagious itch. Before conducting the neuroimaging
study, it was important to verify that itch sensations could be re-
liably induced from the visual observation of scratching actions
(Fig. 1). In this behavioral study, video clips showing images of
one of two people (male, female) scratching one of five body
parts (upper/lower × left/right arm, and midline chest) were
shown to participants. Still images from these videos are shown
in Fig. 2. An equal number of control videos showed people
tapping the same body part. After each video was viewed, the
participants (n = 33) were asked to rate how itchy they felt on
a scale of 0 to 7. Participants were filmed during the behavioral
study, allowing analysis of the degree to which observing the stimuli
elicited spontaneous scratching. Another group of participants
Author contributions: H.H., A.K.S., and J.W. designed research; K.W. performed research;
H.H. and K.W. analyzed data; and H.H., A.K.S., H.D.C., and J.W. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1To whom correspondence should be addressed. E-mail: email@example.com.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
| November 27, 2012
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| no. 48 www.pnas.org/cgi/doi/10.1073/pnas.1216160109
(n = 18) completed the task during fMRI scanning (providing
both behavioral and neuroimaging data); however, this group
was explicitly instructed to refrain from scratching while in
Behavioral Results: Itch Ratings. A repeated-measures ANOVA on
the rating data with the factors Condition (scratch vs. control),
Sex (of person shown in the video) and Body Part (five locations)
was conducted. The results are summarized in Fig. 1. There was
a significant main effect of Condition [F(1,50) = 56.45; P <
0.001]. That is, the video clips depicting scratching elicited
greater feelings of itchiness than the control videos, and the ef-
fect size was large (Cohen’s d = 0.85). In addition, there was a
main effect of Body Part [F(4,200) = 2.77; P < 0.05, Greenhouse–
Geisser corrected], with post hoc t tests confirming that scratching
of the left upper arm site elicited greater itchiness than the other
sites that did not differ from one another (a small effect size of
d = 0.14 comparing the left upper arm with the mean of other
sites). No other main effects or interactions were significant. The
itch ratings on trial N were uncorrelated to the ratings on trial
N + 1, suggesting that induced itchiness did not carry over sig-
nificantly from trial to trial (mean correlation, 0.05 ± 0.24). As
such, we confirmed that the stimuli are appropriate inducers of
itch, and, moreover, that experiencing itch from observing itch is
essentially a normative response. However, the magnitude of this
response differed across individuals; for example, the range of
mean scores in response to the scratching videos was 0.00–6.03.
To further verify the validity of our approach, we identified
and counted occurrences of spontaneous rubbing or scratching
movements made by participants during the behavioral experi-
ment by analyzing the video recordings. Overall, 64% of partic-
ipants (21 of 33) produced at least one such movement during
the experiment (mean, 6.7 movements). A total of 132 scratches
(59.5%) were produced during or immediately after observing a
scratch video, compared with 90 scratches (40.5%) for the control
condition. There was a significant association between the type of
video watched and whether or not participants would scratch
themselves [χ2(1) = 11.09; P < 0.001]. This seems to reflect the
fact that, based on the odds ratio, the odds of participants
scratching themselves were 1.64-fold greater if they were currently
watching a scratch video compared with watching a control video.
Behavioral Results: Individual Differences in Itch Contagion. Itch
contagion was calculated as the tendency to report itchiness in
response to videos of scratching relative to the control stimuli
(difference score; scratch − control ratings). We first established
that participant sex does not significantly affect itch contagion.
There was no difference in level of itch contagion between male
and female participants [difference scores of 1.02 ± 1.33 and
1.53 ± 1.33, respectively; t (49) = 1.27; both Ps = not significant].
We then correlated the difference scores with the various scales
on the personality and empathy questionnaires (24–26). In terms
of correlations with trait variables, the sole significant predictor
was neuroticism, the tendency to experience negative emotions
(r = 0.34; P < 0.05). Higher neuroticism was linked to greater
itch contagion. The full pattern of correlations is summarized in
Fig. S1. Of note, there was no tendency toward a link between
empathy and itch contagion, with many of the empathy scales
showing negative trends in such areas as “perspective taking”
(the tendency to take someone else’s viewpoint) and “empathic
concern” (the tendency to respond compassionately).
Finally, we found that the intensity of itch ratings was linked
to the number of spontaneous scratches produced during the
experiment (r = 0.35; P < 0.05). In other words, participants
who assigned higher overall itch ratings in response to the video
stimuli also tended to scratch themselves more often while
watching these videos.
fMRI Results. We found that observing scratching movements
relative to observing control (tapping) movements activated the
major areas of the itch matrix, including the thalamus, primary
somatosensory cortex, premotor cortex (BA6), and insula. We
also noted activation in the left BA44, extending into BA6, as
well as bilateral activations in the lateral-occipital complex and
cerebellum (Fig. 2A and Table 1). The reverse contrast (tapping–
scratch) revealed no significant activations.
We next used the itchiness ratings that participants assigned
after each video to characterize the degree to which responses
within the activated brain regions correlated with the subjective
experience of itch. This parametric analysis indicated that only
activity in the left BA44, primary somatosensory cortex, and BA6
was significantly related to itch intensity (r = 0.69, 0.90, and 0.71,
respectively; Fig. 2B). The correlation did not meet the criterion
for significance in the right insula [r(13) = 0.47; P = 0.067]. Results
for the whole-brain analysis of the parametric itch effect are
shown in Fig. S2.
Given the behavioral finding of an association between neu-
roticism and itch contagion, we entered the individual factor
score for this particular trait as a covariate into the group-level
statistical model. This allowed us to identify brain areas in
which the magnitude of the brain-based categorical itch effect
(scratch − control) was significantly correlated with neuroticism.
Left BA44 was the only region to show a significant correlation,
and the direction of the correlation was positive (Fig. 2A).
To further characterize how activation levels change over the
course of the 20-s clips, we assessed the strength of the categorical
itch effect (scratch − control) separately for the first half (1–10 s)
and the second half (11–20 s) of each block. This analysis revealed
left BA2, BA44, and BA6 activation only during the first 10 s of a
video, suggesting a more stimulus-driven role for these areas. In
contrast, activation in the right anterior insula was much more sus-
tained, suggesting a more continuous process occurring in this area.
The first important finding of the present study is that on a be-
havioral level, social contagion of itch is a normative response
(i.e., experienced by most people). When participants were free
to scratch, most (64%) did so at least once. This puts itch on
a par with other types of socially contagious behavior, including
laughter (47%; ref. 27) and yawning (40–60%; refs. 21, 28).
Furthermore, participants who experienced stronger feelings of
itchiness during the experiment also tended to spontaneously
scratch themselves more often when free to do so, indicating
a correspondence between self-report and observable behavior.
chest L forearmR forearm L upperarmR upperarm
Part of Body
Itchiness Ra?ng (0-7)
participants, as indicated by ratings. The scale ranges from 0 (not at all) to 7
(extremely), with 4 as moderate. n = 51. Error bars indicate 1 SEM.
Degree to which watching videos induced feelings of itchiness in the
Holle et al.PNAS
| November 27, 2012
| vol. 109
| no. 48
Our findings characterize the central neural substrates medi-
ating the social contagion of itch by identifying regions that
support the subjective experience of itch. Importantly, observing
itch activated the same set of brain regions associated with
feelings of itch induced by an irritant, such as histamine (5–7).
This shared network includes the anterior insula, premotor
cortex, primary somatosensory cortex, and prefrontal cortex.
One region not activated in our study but typically activated by
chemical induction of itch is the midcingulate cortex, although
not all studies of itch have reported activity here (14, 29). The
magnitude of activation across this “itch matrix” reflects the
main effect of viewing itch-related videos (relative to non-itch
control stimuli), and tends to correlate with the subjective in-
tensity of itchiness reported for these stimuli.
There is good evidence that the anterior insula is a core node
in the network for shared pain (reviewed in ref. 23), and our
results demonstrate that itch may be shared in the anterior insula
as well. Furthermore, the response in the right anterior insula
was sustained throughout the duration of the stimulus, in con-
trast to most other regions, which displayed a strong response in
the early phase only (Fig. 2C). The (right) anterior insula is part
of a tightly connected neural network engaged in interoceptive
awareness (30), that is, representation of motivationally salient
subjective feelings related to the body’s internal state, including
C-fiber–mediated sensations such as itch, tickle, and visceral pain
(31). These insular bodily representations may subserve at least
two functions relevant to contagious itch. First, the anterior
insula may act as a comparator in a predictive coding model of
interoception, according to which subjective feeling states arise
from top-down predictions of interoceptive signals (32). Second,
these predictive representations may allow simulation of how
a specific stimulus feels to others (33). Combining these views,
slices is indicated by the number above (x coordinate in the MNI system) and is also shown in the coronal section on the right. The scattergram shows a
significant correlation between the magnitude of the categorical itch effect in left BA44 and neuroticism, as measured by the BFI (50). (B) Magnitude of the
parametric itch effect in key regions. n = 15. The bold black line indicates the significance threshold (all r > 0.52 are significant at P < 0.05, two-tailed). LOC,
lateral occipital complex. (C) t values of the categorical effect in key regions for the first half (1–10 s) and second half (11–20 s) of the 20-s video clips. n = 18.
The critical t value (P < 0.05, two-tailed) is in bold black.
(A) Cross-sections showing regions significantly activated (P < 0.05, corrected) in the comparison of scratch and control. n = 18. The position of sagittal
| www.pnas.org/cgi/doi/10.1073/pnas.1216160109Holle et al.
anterior insula activity thus may be related to sharing the un-
pleasant bodily sensations that accompany itch.
Several of the brain regions linked to contagious itch are im-
plicated in the simulation of actions (mirror systems), including
the premotor cortex (BA6) and adjacent BA44 (34, 35). The
primary somatosensory cortex (BA2) is also commonly activated
during action observation (34), but also plausibly could code the
sensory aspects of itch. In all three of these regions, activity was
greatest in the earlier half of the stimulus presentation. This is
more consistent with involvement of these regions in the per-
ception of itch than in, say, the generation of scratching urges.
The latter would be expected to build up over the duration of the
stimulus (although we did not explicitly measure how the sub-
jective experience unfolds over time). However, each of these
regions likely has a relatively different functional contribution
that remains to be fully elucidated. The area of activity in pri-
mary somatosensory cortex lies in the left hemisphere hand area
(36), suggesting that it may code the sensory effects of scratching
(rather than the location being scratched). Along with a role in
the simulation of actions, the premotor cortex also responds to
somatosensory stimuli (37–39) and the sight of touch (40–42);
thus, in principle, its role also may be sensory-based rather than
action-based. However, premotor and somatosensory cortices
differ in the degree to which they are also activated by the
control condition of tapping. The premotor region does not re-
spond to this control action relative to fixation, whereas so-
matosensory cortex does respond (Fig. S3). This could reflect
different motoric demands that affect primarily the premotor
cortex; for example, scratching requires complex manipulation of
fingers, but tapping is a far simpler wrist-based action.
Itch-related activity in the left BA44 is correlated with neu-
roticism, and neuroticism itself has been identified as the sole
reliable trait predictor of individual differences in subjective
feelings of itch contagion. This trait is known to exacerbate
certain clinical symptoms, such as chronic pain (43), and is
a predisposing influence in various psychopathologies (44). The
importance of neuroticism as opposed to empathy might reflect
a key difference between the social processing of itch versus pain
that may originate in distinct motivational biases toward social
proximity (pain) or distance (itch). The prefrontal cortex is
generally implicated in the control of cognition and behavior,
and in the present context it may serve a gating (attention-re-
lated) function that modulates the degree of contagion.
Finally, some patients report persistent itch sensations (often
accompanied by a belief of infestation) but appear dermatolog-
ically normal (45). It is likely that the same central mechanisms
responsible for itch sensations induced by observing itch in
others (an essentially normative response) is responsible for itch
induced by self-generated thoughts of itching or infestation
(which may become established as dominant overvalued repre-
sentations in a minority of persons). Individual differences within
this network, also related to personality traits, may modulate the
extent to which this contagion is triggered by environmental cues
versus occurring spontaneously and habitually (46, 47). Further
research is warranted to explore the link between contagious
itching and compulsive itching.
Participants. The participants included 51 healthy volunteers. Eighteen par-
ticipants took part in the fMRI procedure (9 males, 9 females; mean age,
20.9 y; range, 18–29 y), and the remainder completed the behavioral ratings
and questionnaires outside of the scanner (8 males, 25 females; mean age,
21.2 y; range, 18–35 y). All participants provided written informed consent.
The study was approved by the Research Governance and Ethics Committee
of the Brighton and Sussex Medical School. Participants received financial
compensation at a rate of £5/h.
Stimulus Materials. Short (20-s) video clips were created in advance for this
experiment, showing either body scratching or a control movement.
Scratching consisted of continuous scraping of the target site using four
curled fingers of one hand. Five different target sites were used: left forearm,
left upper arm, chest, right forearm, and right upper arm. The control videos
showed continuous tapping of a target site. Two different models were
filmed (one male, one female) from the waist up to the neck, ensuring that
the head was never visible. The total stimulus set comprised 20 videos [2
conditions (scratch vs. control) × 5 target sites × 2 models (male, female)].
Procedure. The same basic procedure was used for all participants. However,
participants completing the task in the scanner underwent four experimental
runs (instead of two) to maximize the number of brain volumes acquired.
Those tested outside the scanner completed the study seated at a computer
screen in a testing room. They were also filmed during the experiment.
Participants tested in the scanner were placed in a supine position, and
visual stimuli were projected on a screen behind the scanner, which the
participant could view via a mirror mounted in the head coil. The experiment
had a blocked fMRI design. At the beginning of each block, one video (lasting
20 s) was shown, followed by the acquisition of one brain volume [repetition
time (TR) of 3.3 s] during which a fixation cross was displayed. Next, the
participant was asked to rate the intensity of itchiness (if any) induced by the
Table 1.Regions showing significant activation in the contrast of scratch vs. control
Left inferior parietal cortex (40% BA2*, 40% hIP3)
Left superior parietal lobule (80% 7A*)
Left inferior parietal cortex (60% PFt*, 50% BA2)
Left inferior frontal gyrus (BA44, extending into BA6)
Left superior frontal gyrus (20% BA6)
Left precentral gyrus (50% BA6)
Left superior frontal gyrus
Right insula (anterior)
Ventral aspect of thalamus
Left middle occipital gyrus (40% hOC5 V5/MT*)
Right middle occipital gyrus
The most probable anatomic region in the Anatomy Toolbox 1.8 (28) is in parentheses. k, cluster size in voxels;
x, y, and z, MNI coordinates.
*Indicates assigned regions.
Holle et al.PNAS
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| vol. 109
| no. 48
preceding video. The participant recorded his or her rating via button press
on a scale of 0 (not at all) to 7 (extremely), with 4 indicating moderate
itchiness. The display prompting a response remained on the screen for one
TR (3.3 s), followed by another 3 TRs during which a fixation cross was shown.
One experimental run consisted of 20 blocks and lasted approximately 12
min. It was essential that the participant remain still during the scanning
session and refrain from scratching during experimental runs. (No such
instructions were given in the behavioral part of the study.) Each participant
was observed by the experimenter to ensure compliance. The participants
also completed several questionnaires, including the Big Five Inventory (BFI)
(25), the Empathy Quotient (EQ) (26), and the Interpersonal Reactivity Index
(IRI) (24). One participant failed to complete the IRI, and the EQ was in-
troduced only after the first 23 participants had been tested.
Imaging Data Acquisition. To minimize signal artifacts originating from the
sinuses, axial slices were tilted 30° from the intercommissural plane. Thirty-six
slices (3 mm thick, 0.75 mm interslice gap) were acquired on a 1.5-T Siemens
Avanto magnetic resonance scanner with an in-plane resolution of 3 × 3 mm
(repetition time = 3.3 s per volume, echo time = 50 ms).
Imaging Data Analysis. FMRI data were analyzed using SPM8 (www.fil.ion.ucl.
ac.uk/spm) and Matlab R2007b (MathWorks). Standard spatial preprocessing
[realignment, coregistration, segmentation, normalization to Montreal
Neurological Institute (MNI) space, and smoothing with an 8-mm FWHM
Gaussian kernel] was performed. Voxel size was interpolated during pre-
processing to isotropic 3 × 3 × 3 mm.
Two statistical models, a categorical model and a parametric model, were
calculated for each participant. For the categorical analysis, the two exper-
imental conditions (scratch and control) were modeled separately as stimu-
lation blocks time-locked to the entire duration of each video (20 s each). Six
movement regressors were also included to regress out any residual variance
from head movement.
The parametric model included three regressors: a boxcar regressor cov-
ering the duration of each video presentation (scratch and control), a re-
gressor modeling the parametric modulation of these periods by the linear
effect of itchiness (as indicated by the rating obtained after each video), and
a regressor modeling the quadratic effect of itchiness (to allow for curvilinear
relationships). Three participants had a least one run in which no variation in
rating response occurred (all zero ratings, meaning that no itch was induced
by any of the visual stimuli). These three participants were excluded from the
parametric analysis, because itwas not possible to estimate astatisticalmodel
in these cases.
Statistical parametric maps of contrast estimates of experimental effects
from individual participant analyses were entered into second-level group
analyses performed using SPM8. To protect against false-positive results,
a double threshold was applied in which only regions with a z-score ex-
ceeding 3.09 (P < 0.001, uncorrected) and a volume exceeding 378 mm3were
considered. Thresholds were determined in a Monte Carlo simulation using
a Matlab script provided by Scott Slotnick (https://www2.bc.edu/sd-slotnick/
scripts.htm). This approach provided a statistical correction for multiple
comparisons corresponding to P < 0.05, corrected.
To ensure that the parametric analysis and all reported correlations were
unbiased (48, 49), different data were used for selecting the regions of in-
terest (run 4) and computing the correlations (runs 1–3). Regions of interest
were created using the MarsBaR toolbox.
ACKNOWLEDGMENTS. This study was supported in part by a donation from
the Dr. Mortimer and Theresa Sackler Foundation through the Sackler
Centre for Consciousness Science and by a research grant from the Economic
and Social Research Council (to J.W.).
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