Trigeminal-based Temperature Illusions
Jas Brooks, Steven Nagels and Pedro Lopes
University of Chicago
Chicago, United States{jasbrooks,stevennagels,pedrolopes}@uchicago.edu

Figure 1: We engineered a thermal display that does not utilize heat or cold but instead creates an illusion of temperature bystimulating the trigeminal nerve in the user’s nose with different scents. This nerve is responsible for detecting temperature shifts
but also reacts to chemicals such as menthol or capsaicin, which is why breath mints feel cold and chilis feel hot. Here, how our device
renders (a) the warmth of a desert by emitting a capsaicin-based scent and (b) the coolness of an icy mountain by emitting eucalyptol.
ABSTRACT

We explore a temperature illusion that uses low-powered
electronics and enables the miniaturization of simple warmand cool sensations. Our illusion relies on the properties ofcertain scents, such as the coolness of mint or hotness of
peppers. These odors trigger not only the olfactory bulb, butalso the nose’s trigeminal nerve, which has receptors that
respond to both temperature and chemicals. To exploit this,we engineered a wearable device based on micropumps andan atomizer that emits up to three custom-made “thermal”
scents directly to the user’s nose. Breathing in these scents
causes the user to feel warmer or cooler. We demonstrate
how our device renders warmth and cooling sensations in
virtual experiences. Participants rated VR experiences with
our trigeminal stimulants as significantly warmer or cooler
than the baseline conditions. Lastly, we believe this offers analternative to thermal feedback devices, which unfortunately
rely on power-hungry heat-lamps or Peltier-elements.
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DOI: https://doi.org/10.1145/3313831.3376806

Author Keywords

VR; trigeminal; smell; thermal; illusion; haptics.
CSS Concepts• Human-centered computing~Human computerinteraction (HCI); Haptic devices; Virtual Reality.
INTRODUCTION

Haptic feedback allows virtual reality experiences to match
our sensorial expectations beyond just visual realism [4, 64].
Today, researchers focus on engineering wearable devices
that deliver realistic haptic sensations, such as touch [4, 7],
force feedback [23, 29]. More recently, researchers started
also focusing on stimulating new senses, such as olfaction[25, 28, 79] and thermoception—the sense of changes intemperature [52]. The latter has proven extremely hard tominiaturize into wearable devices because, in order to create
the sensation of temperature, researchers rely on heatlamps [20] or thermoelectric materials [55] (e.g. Peltiers),
which consume substantial power and require large batteries.
In this paper, we explore a temperature illusion that can beachieved with low powered electronics and enables theminiaturization of simple ambient warm or cool sensations.
This illusion, depicted in Figure 1, is based on the propertiesof certain scents, such as mint, which not only stimulate ourolfactory bulb (which feels the “smell” of mint) but also
stimulates our trigeminal nerve-endings in the nasal cavity,
which feel the “freshness” of mint. Our approach allows

users to feel ambient temperature changes, much like a
Peltier element would achieve. However, unlike thetraditional thermoelectric materials that consume a lot of
power (e.g., 4-13W per Peltier [54, 58]), our approach uses
low-power electronics (only uses 0.25W), and, thus, enables
the miniaturization of simple warm or cool sensations.
WALKTHROUGH: A HOT/COLD VR EXPERIENCE

To help readers understand the applicability of our device,
we demonstrate its usage in a virtual reality (VR) experiencefeaturing several temperature changes. To explore thistemperature illusion, we engineered a stand-alone device,
shown in Figure 1, that attaches to the front of the VR
headset. Our device delivers the trigeminal stimuli bypushing the scents using micropumps onto a vibrating meshatomizer, which in turn emits the scents as a fine spray. The
scents used in this experience were engineered to minimize
their odor recognizability while maximizing their trigeminal“thermal” effect (refer to Implementation for details).
In our VR experience, the user finds themselves in a cabin in
the woods, during the winter. Figure 2(a) depicts what theuser experiences through the headset (1st person). However,for the sake of visual clarity we will depict a 3rd personcomposited image, shown in Figure 2(b), which allows us tohighlight the different temperature effects that our system
adds to this VR experience. Furthermore, on the right side of
our illustrations we depict an indicator of whether our device
renders a scent that induces a warming or cooling sensation.

Figure 3: As the user stands at the doorstep of the cabin without
heat and electricity, they feel the temperature drop as themountain cold creeps in; here, our system atomizes eucalyptol,
which stimulates the cold receptors of the trigeminal nerve.

As the user walks outside the temperature drops, which is
depicted in Figure 4. Our device atomizes eucalyptol atshorter intervals of 6s, increasing the trigeminal stimulationof the cold receptors, which increases the cooling sensation.

Figure 4: As the user walks outside into the cold of themountain, our device atomizes eucalyptol faster to induce acolder ambient temperature than at the doorstep.

Figure 2: (a) User’s first person view of our “cabin in thewoods” scene. (b) Composited mixed reality view, showing theuser warmed by a furnace, our device atomizes a cayenne
pepper tincture (capsaicin) to create a warming sensation.

As the cabin is ambiently heated by a furnace, our wearable
device induces a warm sensation by stimulating the user’s
trigeminal nerve via intermittent atomization of a cayenne
pepper tincture solution (mostly based on capsaicin), every
6s. As the user is breathing in this trigeminally-rich scent,they feel warm. Suddenly, the winds outside in the mountain
range blow the cabin door wide open, as depicted in Figure
3. The strong winds break the power lines outside, cutting off
the cabin’s lights, and the wind puts out the furnace’s fire.
This brings in an influx of cold from outside. Immediately,
our device stops atomizing capsaicin and switches toatomizing eucalyptol intermittently, every 12s. This creates
emulates the drop in the cabin’s temperature. To survive thiscold, the user must restore electricity and heat to the cabin.

Outside, the user finds out that the powerlines were cut by
the storm but, luckily, they find a backup generator, depicted
in Figure 5. The user turns the backup generator on bypulling a lever, as depicted in Figure 5(a). When thegenerator starts, it exudes a little heat and some smoke,
depicted in Figure 5(b). As a response, our device atomizes
two new scents, in addition to the 6s-interval eucalyptol usedthe cold ambient temperature: (1) it atomizes short bursts of
CPT every 12s, which creates a slight warm sensation that
renders heat from the backup generator; and, (2) it also
atomizes 8-Mercapto-p-menthan-3-one (CAS1 33281-91-3)
for a few seconds as the generator starts up; this gives off arubbery fuel-like odor. The latter demonstrates that ourdevice can increase the realism of traditional smell displaysby adding temperature sensations (e.g., not only it does
smells like fuel, but it feels like the fuel is burning).

1 CAS is a unique identifier of chemical substances; allowing researchers toquickly find these ingredients while replicating our work.

Trigeminal system & nasal pungency

The trigeminal nerve (cranial nerve V) is a somatosensory
nerve primarily known for sensing mechanical and thermal
stimulation, i.e., it senses changes in pressure, vibration ortemperature [44]. Of the receptors of the trigeminal nerve,
we focus on the family of transient receptor potential (TRP)
ion channels that are activated by specific temperatures(thermoTRP channels). These channels comprise themolecular mechanisms of thermosensitivity [81].

Figure 5: (a) Users turns on the backup generator; (b) The
generator starts warming up and burning fuel; our device
renders this by simultaenously mixing a fuel odor (olfactorystimulant) with the capcaisin (trigeminal stimulant).

Now the power is restored and the user walks away from thegenerator, therefore our device ceases to emit capsaicin. As
the user re-enters the cabin and finds the furnace back on, ourdevice slowly fades out the interval of eucalyptol anddecreases the interval for capsaicin. Finally, creating thesensation of the room heating back up.
BENEFITS, CONTRIBUTION AND LIMITATIONS

Our key contribution is that we propose, explore, andengineer a device to create an illusion of temperature without
actually requiring to cool/heat the room or the user.
Our approach has the following four benefits: (1) it is the firstinteractive device dedicated to the underutilized trigeminal
nerve; (2) it provides an alternative hardware means fortemperature sensations, which might result in new types ofdevices in the future (similar to how several technologies
emulate touch sensations: vibrotactile, electrotactile, etc.);(3) it requires substantially less power than traditional
approaches (such as Peltier modules, air conditioning, or heatlamps), in fact, it requires 20x less power than a TEFC103112 Peltier module; lastly, (4) it enables existing olfactoryinterfaces, such as [33], to easily upgrade to olfactory andtemperature by simply incorporating the trigeminal
stimulants from our paper.
Our approach is limited in that: (1) currently, the range of ourillusion are limited, i.e., it can simulate a “warm firepit” butnot a “hot coal ember”; (2) like any wearable chemicalolfactory display, our device relies on reservoirs that must be
refilled, though one reservoir lasts for approximately 2hours; (3) our trigeminal scent changes act primarily on thetrigeminal nerve at the nasal pathway, and therefore are notdistributed over the user’s entire skin; and, lastly (4) thetemperature sensation is tied to the user’s breathing.
Furthermore, as a first work venturing in stimulating thetrigeminal nerve, there are still many unexplored avenues.
However, we do believe this offers a solid first step inalternative modalities for temperature.
RELATED WORKThe work presented in this paper builds primarily on the fieldof haptics, in particular on thermal and olfactory displays.
Also, to familiarize the reader with the underutilized
trigeminal nerve, we present an overview of its mechanisms.

What is striking about these receptors is that they also
respond to airborne particles [44]. In fact, it is their ability tosense both airborne particles and temperature shifts that
explains why scents such as peppermint feel cold [43]. It is
not the olfactory quality of peppermint that feels cold, butinstead a trigeminal-effect of menthol, the principal active
ingredient stimulating the TRPM8 receptors—thesereceptors respond to both temperatures under 25°C andchemical compounds such as menthol [71]. Similarly, chili
peppers feel hot when we eat them because their active
ingredient, capsaicin, notably activates our TRPV1
receptor [81], which conversely responds to temperatures
above 42°C. Our approach hinges precisely on thisperceptual duality to simulate temperatures.
While no interactive device has been designed to interface
with the trigeminal nerve, its effects are widely studied in
psychophysics [8], neuroscience [32] and medicine [46].
Also the effects of the trigeminal nerve is readily
recognizable in everyday life, such as in situations involving
spicy foods, minty flavors, breathing cold air, and so forth;these effects are all attributed to the trigeminal nerve [1].
Thermal feedback

To maximize VR realism, researchers have been adding
more haptic sensations. Recently, attention has been given to
rendering a realistic depiction of a virtual environment’s
temperature [14, 24, 34, 48, 55, 58]. The earlier forms ofthermal feedback in VR were achieved via air conditioning
units [78] or heat lamps [14, 30]. Unfortunately, due to theirsize and power requirements, these cannot be mounted to amoving user, which limits their use in VR/AR.
As a response to this disadvantage, researchers turned tothermoelectric elements, also known as Peltier elements.
These devices create a temperature difference across twojoined conductors in the presence of an electric current.
Peltier elements quickly became the most adopted thermal
actuator in VR due to their compact size (e.g., ThermoVR
uses modules of 4 Peltier elements, each 8mm x 8mm [55]).
There are several examples of thermal stimulation using
Peltiers. Vito et al. provide thermal navigation cues on asteering wheel [13]. ThermalBracelet [54] attached Peltiers
to the user’s wrist to explore eyes-free feedback.
ThermoVR [55] adds Peltiers to the VR headset, thus
providing temperature sensations to the user’s face. Season
Traveler [59] uses heating elements on the back of the user’sneck, as well as an olfactory display. Affecting tumbler [68]is an augmented beverage lid with Peltier elements that affectthe perceived flavor. Lastly, Thermotaxis [50] uses Peltierelements on the user’s ears to render temperature changes.
Alternative methods to generate temperature changes include
hydraulics (i.e., moving cold or hot water by means of tubes
that touch the user’s skin) [24], gel packs [34], or resistive
heating [37, 74]. Much like all the remainder, these cannot
be easily miniaturized and/or require a lot of power.
Olfactory displays

Since our device utilizes the same hardware backbone as
typical olfactory displays, we also review the technologies
used to project scents. Typically, olfactory displays are used
for the sake of immersion [14, 38, 56, 79] as well as
notifications and communication [2, 41, 75, 77]. Olfactory
displays deliver odors by: projecting vapors [45, 51, 59, 80];
heating fragrant solids [9, 15, 36]; evaporating a fragrantliquid [12, 39]; or, aerosolizing a fragrant liquid [3, 6, 27, 42,
47, 67]. The resulting gas is most commonly delivered nearor in the nostrils, but can also be guided using fans, air vortex[80], bubbles [63], or ultrasound arrays [26].
Unlike previous research focused on scents that stimulate theolfactory bulb (smell), our wearable projects custom scents
specifically targeting the trigeminal nerve (temperature).
IMPLEMENTATION

To help readers replicate our design, we now provide thenecessary technical details. Furthermore, to accelerate
replication, we provide all the source code, firmware, andschematics of our implementation2.
Hardware Design

Figure 6 shows our device. It measures 24mm x 85mm x
88mm and weights 119g. At the core of our device is acombination of three piezoelectric diaphragm micropumps
and a vibrating mesh atomizer.

Figure 6: Front and back view of our device and its components.
Controlling atomization

One of the key technical challenges in engineering olfactory
displays is achieving a fine control over the atomization.
2 https://lab.plopes.org/projects/trigeminal

We incorporate a vibrating mesh atomizer, which uses anannular piezo element to vibrate a perforated membrane inresonant bending mode [53, 70]. The mesh itself has conicalperforations at the center (aperture), which generates a
micro-pumping action pushing the liquid in contact with theunderside of the aperture and creating an aerosol.
We designed our device to deliver small drops (of 1µL) tothe back of the atomizer’s aperture, which in turn nebulizes
the liquid. To ensure these small drops, we limit the outputof the micropump using a capillary tube with an innerdiameter of 0.127mm. To push the drops to the atomizer, we
activate our micro-pump using small bursts of 70ms.
Reducing cross-contamination, while enabling mixing

To keep device size to a minimum, we designed it around
one single transducer because these are large components at
16mm of diameter. Other devices utilize one entire atomizer
per channel [60], which results in larger hardware footprints.
When using a single atomizer for multiple channels of liquid,
it becomes paramount to reduce cross-contamination, i.e.,ensure that only the selected channels are atomized. Most
devices using a vibrating mesh use a cotton wick to draw theliquid up to the transducer via capillary forces [2, 3].
However, this results in cross-contamination as the wickmixes all liquids. Our tubes extend independently from eachpump to the atomizer, and each has its own capillary tube.
As such, in our design, mixing of the liquids only occurs at
the atomizer level. Additionally, by pumping out very small
droplets, we can effectively avoid cross-contamination
because these small droplets are quickly atomized and do notlinger in the transducer. Furthermore, out capillary tubes are
so narrow that the surface tension of the inner liquid will notallow any of the other samples to flow backwards.
Battery life & sample life

Our complete prototype uses only 0.25W of power, which is
20 times less than generating actual temperature changes
with typical Peltier elements, as these use at least 5W (e.g.,
the TEFC1-03112). For direct comparison with prior work,
Ambiotherm uses two Peltier elements requiring up to 13W,
Hutchison et al. use four of the aforementioned TEFC103112 [62], and Shoichi et al. use four Peltier elements(KSMH029) each requiring up to 6.8W [49]. Thus, generally
speaking, our approach is around 20-52 times more efficient.
However, unlike Peltiers, heat lamps, or air conditioning
units, our device has limited operation due to the need forrefilling reservoirs. A test revealed that three 1mL vials can
induce a sustained temperature change for 5.84 hours. The
device itself can run for nearly 18 hours, using a 3.7V,
1200mAh Li-Po battery. To achieve sustained temperature
our prototype does not require continuous delivery, as once
exposed to a trigeminal scent (e.g., warming), users feelwarm for a few seconds (this time depends on scent used, see
Discussion). We optimized sample life at an inter-stimulusinterval of 6 seconds followed by 1 second of atomization.
Operating latency

The total latency of our device is around 200ms, i.e., from akeyboard trigger in the Unity3D to an actual mist starting tobe atomized. This overall latency includes: Unity latency, theBluetooth channel latency, microcontroller latency, physicalaction of micropump, liquid flowing to the atomizer.
ENGINEERING OUR TRIGEMINAL STIMULANTS

As this is a first work venturing in stimulating the trigeminal
nerve, we detail three methods that we created to prepare
trigeminal stimulants. Each of these methods equips future
researchers using our device with different approaches
depending on their desired goals. In this section, we take as
examples the six candidate trigeminal scents (all obtained
from safe and off-the-shelf ingredients) that we evaluated ina later study: capsaicin, methyl salicylate, thymol, linalool,
eucalyptol and menthol.
1. Odorless trigeminal stimulants

The ideal trigeminal stimulants should be odorless, i.e., theycannot be identified by smell but only by their temperature.
While odorless trigeminal stimulants are not abundant in offthe-shelf and safe ingredients, we were able to find one:
capsaicin—the active component of hot peppers. Capsaicin
is an odorless warming trigeminal stimulant that requires nomodifications when used in small doses [65]. Note that hot
peppers do have an associated faint smell which is notcapsaicin itself but the other spices. Capsaicin can be diluted
in odorless vehicles, for example ethanol, dimethyl
sulfoxide, or hypotonic saline, or even added to existingfragrances dispensed by olfactory displays.
2. Diluting the stimulus

Most common trigeminal stimulants also stimulate theolfactory bulb when in high concentrations. Though not a
perfect solution, one can iteratively adjust the concentrationof a trigeminal stimulant to reduce its olfactory stimulation.
However, dilution will not preserve trigeminal stimulation
while only lowering olfactory stimulation. In fact, olfactory
thresholds are much lower than trigeminal thresholds [11].
In all of our trigeminal scents, except capsaicin, we usedserial dilution to minimize their odor. This involves a
stepwise dilution of a substance in another solution to reduce
its concentration in a logarithmic fashion. The main
disadvantage of dilutions is that it will soften the trigeminal
impact of the scent.
3. Obfuscation via olfactory pink noise

Obfuscation is a last-resort method we offer for the extreme
case in which designers might want to maintain maximum
trigeminal effect of a scent yet render its odor unidentifiable.
To achieve this, designers command our device to emit an“olfactory pink noise” anytime that the trigeminal scent isemitted. The effect is that, much the same way that auditory
pink noise prevents hearing other frequencies, olfactory pinknoise will obfuscate the smell of the trigeminal scent but will
not decrease its trigeminal effect. This olfactory pink isuseful, for instance, in obfuscating the strong smell of mint,
which we also found to be the most powerful coolingtrigeminal scent in our later study. The main disadvantage ofobfuscation is also its advantage: it creates an olfactory
environment where nothing is identifiable. This is limiting inthat it prevents designers from using it in scenarios where
olfactory realism is important, such as in some VR scenes.

Engineering an olfactory pink without trigeminal stimulation

When creating our olfactory pink noise, we ensured two key
aspects: (1) maximum olfactory intensities in a wide range,
with (2) minimum trigeminal effects. The latter was criticalbecause otherwise, the olfactory pink noise would induce
trigeminal effects that could confound its purpose. Thus, weselected seven compounds selected from Weiss et al.’solfactory whites [73] that had shown no trigeminal effect instudies with anosmics (individuals with no sense of smell,but intact chemesthesis) [16]. Our olfactory pink iscomposed of seven ingredients diluted in Everclear: 100%w/v3 vanillin (CAS 121-33-5); 0.1% v/v amyl acetate (CAS628-63-7); 50% v/v geraniol (CAS 106-24-1); 50% v/v
phenyl ethyl alcohol (CAS 60-12-8); 10% w/v coumarin(CAS 91-64-5); 0.5% w/v indole (CAS 120-72-9); and, 5%v/v limonene (CAS 5989-27-5). Lastly, while we empiricallyverified with olfactory experts that this obfuscation is very
powerful (e.g., it renders menthol unidentifiable), it is a lastresort solution we provide for extreme cases and we do notuse it in our VR applications for realism’s sake.
USER STUDY 1: THE BEST TRIGEMINAL SCENTS

In our first study, we aimed at identifying the most effective
trigeminal stimulants to achieve simple hot and cold
sensations. We focused on six off-the-shelf stimulants thatcan be easily obtained by other researchers to increase thereplicability and applicability of our findings.
Our hypothesis was that different trigeminal stimulants
would exhibit different levels of cooling and warming
sensations, which can be perceived by comparison against
each other. Our study was approved by our Institutional
Review Board (IRB19- 0907).
Stimulant Preparation

First, we determined concentrations of the stimuli with equal
intensity, also known as isointensity. We started by utilizing
the concentrations for eucalyptol and menthol suggested by[17], and then calibrated the other stimulants to be isointenseusing the methods detailed in [17, 19].
Stimulants

We used the following stimulants (all solutions used
Everclear as a solvent as it is available off-the-shelf andmostly odorless): cayenne pepper tincture (a capsaicin-based
solution), methyl salicylate, thymol, eucalyptol, linalool, andpeppermint essential oil (a menthol-based solution). Figure 7
depicts each stimulants’ composition, preparation anddenotes which trigeminal (TRP) receptors it targets.
For each chosen stimulant, we checked its safety-dosage bycomparing to the concentrations used in over-the-counterproducts on DrugBank [76] as well as reviewing relevant
toxicology literature [10, 21, 35]. Our stimulants
composition and preparation are depicted in Figure 7.
3 Units % v/v and % w/v are concentration ratios. % v/v is a ratio of twovolumes when combining liquids (e.g., 50% v/v eucalyptol in Everclear is
50mL of eucalyptol in every 100mL of solution). % w/v is a weight (g) tovolume ratio (ml).

Warming scents

1. Capsaicin-based (cayenne pepper tincture).
Triggers the TRPV1 receptor.

Ultimately, participants completed 48 comparison trials
each, for a total of 480 trials across all participants.

Composition: 10% v/v cayenne pepper tincture.

Apparatus

Preparation of tincture: We let 2g of cayenne
pepper soak in 30mL of Everclear for 20 min,while on the magnetic stirrer. We let the mixture
sediment for 30 mins. Then, we collected theresulting tincture by filtering out any sedimentwith a fine filter (10 to 15μm).
2. Methyl salicylate. Triggers the TRPV1 andTRPA1 receptors.
Composition: 100% v/v methyl salicylate solution(CAS 119-36-8) in Everclear.
3. Thymol. Triggers the TRPV3 and TRPA1.
Composition: 0.05% w/v thymol solution (CAS89-83-8) in Everclear.
1. Eucalyptol. Triggers the TRPM8 receptor.
Composition: 60% v/v eucalyptol solution (CAS470-82-6) in Everclear.

A simplified version of our wearable device was used. This
device utilized allowed participants to press a button tocontrol when to start the atomization. The device was placedon a stand, under participant’s nose. Lastly, to ensure that alltrials had precisely the same volume, all stimuli were
pipetted at a volume of 1µL onto the surface of the atomizer.
Our experiment was conducted in a ventilated laboratory,
with one perforated duct ventilation (500CFM) and two fumehoods. The ambient temperature was kept at 24°C.
Participants

10 participants (3 self-identifying as female, 7 as male) wererecruited from our local institution and received a 10 USD
compensation. No participant reported any allergies.
Results

Our main finding was the perceived temperature of each of
the simulants, depicted in Figure 8. We conducted a one-way
within-subjects ANOVA and then used Bonferroni-correctedt-tests.
110

Composition: 80% v/v linalool solution (CAS 7870-6) in Everclear.
3. Menthol-based (peppermint essential oil).
Triggers the TRPM8 receptor.
Composition: 20.05% v/v peppermint essential oil
solution in Everclear. The concentration of thepeppermint essential oil was determined to reflect
an approximate concentration of 17.85% w/v Lmenthol (as in [19]), as L-menthol accounts forapproximately 36.0-46.0% of peppermint essential
oil’s composition according to [69].

Figure 7: Composition and preparation of our stimulants.

Lastly, to understand whether Everclear itself has a strong
trigeminal effect that could bias the perception of our six
scents, we added three solvents to the study’s stimulants:
Everclear (75.5% ethanol); dimethyl sulfoxide (DMSO,CAS 67-68-5); and, saline (0.65% v/v sodium chloride).
Task and Procedure

For each trial, participants were presented a random pair ofstimuli (warm, cool, or baseline) with a brief pause inbetween (10s). Stimulants were presented in a randomized inorder, but counter balanced for within-trial order acrosstrials (42 trials). After each trial, participants were asked torate both stimuli on a visual-analog scale with only
annotations on the extremes, denoting cool and warm.
Upon concluding all the trials, participants were thenpresented two consecutive solvent stimuli (Everclear,
DMSO, or saline) in random order, counter-balanced for thewithin-trial order (six trials). After each trial, participants
were again asked to rate both stimuli on the same scale.

visual-analog distance(mm)normalized to everclear

Cooling scents

2. Linalool. Triggers the TRPM8 and TRPA1.

n.s.
n.s.
n.s.

55

0

-55

-110

n.s.
n.s.

cooling stimulants

warming stimulants

Figure 8: Perceived temperature of our six trigeminal scents(peo stands for peppermint essential oil; euca for eucalyptol;
lina for linalool; evcl for Everclear; mesa for methyl salicylate;
cpt for cayenne pepper tincture; and thy for thymol). All
pairwise comparisons were significant unless indicated.

We found a significant effect of trigeminal stimulants on thereported temperature [F(2,826)=37.01, p<0.001]. Out of six
candidate scents, we found that two stimulants wereperceived as warm (thymol and capsaicin), while theremainder four were perceived as cooling; all of thesecomparisons, except five, were statistically significant(p<0.05) using an ANOVA for multiple comparisons.
Firstly, we found that peppermint (M=−53.01, SD =43.86)
and thymol (M=29.11, SD=54.48) were rated as the mosttemperature inducing scents. These were followed byeucalyptol (M=−34.77, SD=48.01) and the capsaicin(M=13.99, SD =46.30). Lastly, participants rated methyl
salicylate (M=−17.17, SD=61.95) and linalool (M=−6.49,

SD =50.61) as weaker cooling stimulants and we did not see
a statistical difference between both.

that these hot and cold sensations would increase one’s senseof immersion.

Despite participants rating all solvents around neutral, we
found a significant effect of solvents on reported temperature[F(2,105)=5.65, p=0.0046]. DMSO was perceived towards
the warming side (M=9.75; SD=46.77), hypotonic saline
towards the cooling side (M=−21.16; SD=30.56), andEverclear around neutral (M=2.00; SD=40.55). No statisticalsignificance was found between DMSO and EVCL. On theother hand, hypotonic saline was statistically significant in
its difference with DMSO (p=0.000) and Everclear(p=0.007).

Interface conditions

Discussion

The study confirmed our hypothesis that certain scents
induce simple hot/cold sensations; this forms the basis of ourtemperature illusion. Of all our initial expectations, depicted
in Figure 8 via its color coding, only methyl salicylate fell
short, as it instead induced cooling sensations for mostparticipants. In fact, linalool and methyl salicylate activate
both a warm TRPV receptor and the cold TRPA1 receptors,
it is difficult to predict what perceptual properties either
have. We would advise against using these for temperatureillusions. The unpredictability of this combination wasemphasized in comments from participants (P4, P5, P8) who
experienced “simultaneously cold and burning” sensations.
When compared to cooling scents, warm scents appeared
weaker in intensity. This warrants further investigation inengineering stronger warming scents, though it will likely
require ingredients that are not store bought.
From participants’ comments, both thymol and peppermint
were easily identifiable. In contrast, participants could not
identify eucalyptol or capsaicin. When describing qualitiesof the trigeminal experience, P3 remarked that “cold ones
seemed almost to clog [my] nose like cold air; hot ones open[my] nose like hot air”. Similarly, P7 and P3 described the
eucalyptol as breathing in “fresh air” and “cold air”.
Regarding our solvent of choice, the perceived temperatureof Everclear was around neutral but distinct from hypotonic
saline. We still deemed Everclear as our best solvent because
most ingredients are not soluble in water or hypotonic saline.
USER STUDY 2: RENDERING VIRTUAL TEMPERATURE

We conducted a second user study in to verify our key idea:
our stimulants can induce simple hot/cold sensations in VR.
During this study, participants explored two distinct VR
scenes while breathing in our trigeminal hot scent, cold
scent, or a no-stimulant (baseline). Our study was approvedby our Institutional Review Board (IRB19-0281).
Hypotheses

Our main hypothesis was that our trigeminal cold and hot
stimulants would be perceived respectively as cooler andwarmer than the no-trigeminal condition, in which we
atomized Everclear. Because our trigeminal stimulants are
diluted in Everclear, any perceived temperature difference
between the trigeminal and baseline would be a result of theeffect the trigeminal ingredients. Secondly, we hypothesized

We presented three scents. As the goal of this study is toevaluate immersion, we opted for stimulants that are harder
to identify by means of their odor but were still effective in
our previous study: (1) cooling: our eucalyptol scent, (2)warming: our capsaicin scent; and (3) baseline: Everclear.
The order of conditions was randomized across all subjects.
Apparatus

Participants explored the VR scenes using a Wireless HTC
Vive and headphones. Our prototype was attached to theheadset. The tracking area was 5 x 5 meters. This study was
conducted in the same location as our previous experiment.
Task

For each task, participants were asked to explore a VR scene.
Our scenes were either a patch of grass in a desert oasis or aflower patch on an icy mountain, as depicted in Figure 9.
The instructions were to find a flower of a unique color.
a icy mountainb desert oasis

Figure 9: Study VR scenes: (a) desert and (b) icy mountain.

Our VR scenes were designed to ensure that the participants
experienced them for a minimum of two minutes, to have acomparable experience across participants. Thus, the flower
only appeared after two minutes. To ensure that the flower
did not appear unrealistically in the user’s view, we analyzed
the user’s viewport in real-time and displayed the flower ina random location behind the user. No participant reported
that the flowers appeared in an unrealistic manner.
After each trial, we asked participants to judge the perceived
temperature using the ASHRAE scale [31], which is abipolar 7-point Likert scale, specifically designed to measuretemperature; it ranges from -3 (cool) to 3 (warm).
Participants

We recruited 12 participants (5 self-identifying as female, 7
as male, 25 ±5 years old). 10 participants had tried VR, but
only two tried it in conjunction with smell at art exhibitions.
No participant reported any allergies or sensitivity to theingredients in our trigeminal stimulants. Two trials wererepeated due to depleted battery in the VIVE headset.
Participants were given an intake form at the start of thestudy. Using the 5-point Likert questions from the NOSE
questionnaire [66], we confirmed that no participant reported
nasal congestions (M=0.55, SD=0.52), nasal obstructions(M=0.18, SD=0.40), or difficulty breathing (M=0, SD=0).

Results

We conducted a one-way within-subjects ANOVA and used
Bonferroni-corrected t-tests.
Figure 10 depicts our main findings. We found a statisticallysignificant effect of trigeminal stimulants on perceived
temperature [F(3,44)=46.57, p=0.0046]. Using a paired
t-test, we found a statistically significant difference between
our cooling scent and the baseline (p<0.001). Similarly, we
found a statistical difference between our warming scent andthe baseline (p<0.002). In the mountain scene, participants
perceived a cooling temperature with the eucalyptol scent(M=−1.92, SD=0.67) when compared to the baseline(M=−0.67, SD= 0.98). In contrast, participants perceived a
warming temperature in desert with the capsaicin scent(M=0.60, SD=1.32), while the baseline was perceived ascold (M=−1.08, SD=1.084). This validates our first
hypothesis.

temperature rating(compared to room)

warm 31* p = 0.0019459* p = 0.0005489-1

cool -3

Mountain

Desertbaseline (everclear)with stimulants

Figure 10: Rating of perceived thermal experiences across all
participants. We found temperature sensations to bestatistically more intense in with our trigeminal scents.

Figure 11 depicts our findings regarding induced immersion.
The effect of trigeminal stimulants on immersion was
significant [F(3,44)=17.41, p<0.001]. We found out astatistically significant difference between the immersion
induced by our cooling scent and the baseline (p<0.022);
and, a statistically significant difference between theimmersion induced by our warming scent (p<0.008).
Participants rated the mountain environment with the
eucalyptol scent (M=5.33, SD=0.98) as more immersive thanthe baseline condition (M=4.67, SD=1.15). Similarly,
participants felt the desert environment with the capsaicin
scent (M=5.0, SD=1.13) as more immersive than the baseline
condition (M=4.25, SD=1.14).

7level of immersion

Participants reported low spice sensitivity (M=1.45,
SD=0.82), regular consumption of spicy foods (M=2.54,
SD=0.82), and rare use of products with substances likementhol (M=0.64, SD=1.03). With consent of theparticipants, we videotaped their trials. Participants werecompensated for their time with 20 USD.

* p = 0.0222518* p = 0.0082869

5

3

1

Mountain

Desertbaseline (everclear)with stimulants

Figure 11: Rating of immersion across all participants. We
found statistically stronger immersion with trigeminal scents.
Qualitative feedback

When asked whether they preferred such thermal
experiences with or without the trigeminal stimulants, 8 of
the 12 participants preferred with stimulants. P6 preferredwithout stimulants but clarified that it was because theexperience was “too intense”.
Several participants remarked that our cooling trigeminal
scent increased their sense of immersion in the environment
and felt as though the “air breathed in was cold”. P3
additionally remarked that the cooling sensation “spread toher chest (…) as if there was air conditioning” and that they
“paid more attention to environment details such as snow
patches and the desert sun”. Outside of P3’s remark,trigeminal stimulation was perceived as mostly localized to
the nose or face.
As seen in the previous study, the warming sensations wereless pronounced. This manifested also in less comments
regarding the desert experience. However, P3 mentioned thatwith the warming scent the desert made them “feel thirsty”.
Lastly, no participant in this study identified the capsaicin
scent by its odor. Only one participant (out of 12) identified
eucalyptol by its odor.
DISCUSSION

We now condense our guidelines, limitations andsuggestions for engineering new trigeminal scents.
Summary of guidelines for designers

We suggest that designers use thymol or capsaicin for warm
sensations and peppermint essential oil or eucalyptol forcooling. That said, the stronger warm and cooling scents that
we engineered here have a caveat: they are easily identified
by their odor traces. For this reason, we advise they are usedin congruency with the virtual environment they are
depicting, e.g., a VR scene in hot day at the park might fit
well with the smell and heat sensation of thymol (as it willremind users of thyme plants); conversely, a hot desert
without any vegetation would not be fitting to thymol. As asolution to this problem, we encourage designers to use
capsaicin for warming (has no smell) or diluted eucalyptol
for cooling (less recognizable than peppermint).

Limitations of Everclear as solvent

The utilization of ethanol (the main ingredient in Everclear)
as a solvent poses minor limitations because it lightly
triggers the TRPV1 receptor; also shown in our study. Future
researchers may consider substituting ethanol for DMSO or
propylene glycol for ingredients that are solvable in it.
Potential desensitization effect

Repeated stimulation of the trigeminal nerve may have timedependent effects. Prior work on habituation focused mostly
on oral desensitization from repeated exposure [61]. The key
factor here is the inter-stimulus interval, which determines
whether capsaicin sensitizes or desensitizes [22, 57].
Intervals under approximately 3 minutes cause sensitization
while intervals above that can cause desensitization.
Future trigeminal scents

We believe our research is only opening the door to theunderutilized trigeminal nerve. Many more ingredientsmight be worth investigating; for instance, the WilkinsonSword cooling ingredients [5, 40]. These novel ingredients,
such as FEMA-4684, are expected to induce longer-lasting
sensations. Similarly, icilin is odorless [72] and has beenreported to be twice as cooling as menthol [18]. In contrast,
warming scents have not been as abundantly researched.
FURTHER APPLICATIONS

While we explored using hot/cold trigeminal stimulants at
the example of inducing temperature sensations in virtual
reality, we believe this is just the tip of the iceberg to a range
of applications. Furthermore, our hardware is simple in thatthe atomizer can be mounted to a variety of situations bysimply extending its tubing.
Untethered thermal experiences

Thermal research has been mostly conducted in lab
environments with the devices tethered to power outlets.
While our device is limited in many ways, it can still enable
a new range of experiments that take thermal sensations tomobile contexts, such as Augmented Reality (AR/MR).
Accessibility: experiences for anosmics

Our trigeminal-based thermal illusions, unlike mostolfactory experiences, can be experienced by anosmic (smell
impaired) individuals. While this requires further validation,
there are many studies in psychophysics illustrating that
anosmics can perceive many trigeminally-rich odors [16].
CONCLUSIONSIn this paper, we proposed, explored, and engineered adevice that creates an illusion of temperature without
actually requiring to cool/heat the room or the user. Our
illusion is based on the properties of certain scents, suchmint, which not only stimulate our olfactory bulb (whichfeels the “smell” of mint) but also our trigeminal nerveendings in the nasal cavity, which feel the “freshness” ofmint. Furthermore, we contributed with the design andimplementation of a self-contained wearable device thatemits scents using an ultrasonic transducer and micropumps.
In our first user study, we found out that our trigeminal scents
induce simple warming/cooling sensations. We extracted
guidelines for their application and condensed insights on

preparation methods for creating new scents. Then, in our
second study, we demonstrated how our device enables
participants to feel temperature changes in virtualenvironments, such as experiencing a desert or an icy
mountain.
Lastly, our device achieves this temperature illusion withlow powered electronics, enabling the miniaturization of
simple temperature sensations. We see this as the firstalternative to the traditional thermoelectric materials (e.g.,
Peltier elements) that consume a lot of power in order togenerate temperature changes.
FUTURE WORK

We believe this paper represents a first step towards
empowering a broader set of HCI researchers to explore therole of the trigeminal nerve. Now that we know that thetrigeminal nerve can be interfaced to create temperaturesensations, future research might want to investigate finer
details of these illusions: the speed of change from one
temperature to another, the number of different temperaturelevels, and compare these with other existing approaches.
ACKNOWLEDGMENTS

This material is based upon work supported by the National
Science Foundation Graduate Research Fellowship Program
under Grant No. (DGE-1746045). Any opinions, findings,
and conclusions or recommendations expressed in thismaterial are those of the author(s) and do not necessarilyreflect the views of the National Science Foundation.
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