Stereo-Smell via Electrical Trigeminal Stimulation
Jas Brooks

Shan-Yuan Teng

Jingxuan Wen

University of Chicago, USAjasbrooks@uchicago.edu

University of Chicago, USA
tengshanyuan@uchicago.edu

University of Chicago, USA
jingxuanw@uchicago.edu

Romain Nith

Jun Nishida

Pedro Lopes

University of Chicago, USA
rnith@uchicago.edu

University of Chicago, USA
junnishida@uchicago.edu

University of Chicago, USA
pedrolopes@cs.uchicago.edu

Figure 1: We propose a novel type of olfactory device that renders readings from external odor/gas sensors into trigeminal
sensations by means of electrical stimulation. By stimulating the trigeminal nerve, it allows for smell augmentations or substitutions without the need for implanting electrodes in the olfactory bulb. To realize this, we engineered a self-contained
device that users wear across the nasal septum, it communicates with external gas sensors using Bluetooth. In this example, it
enables a user to perceive the gas’s direction (i.e., to their left or right) by varying the pulse-width and current polarity of theelectrical impulses. The result is that this user can quickly locate their gas leak using our device as a stereo-smell augmentation.

ABSTRACT
We propose a novel type of olfactory device that creates a stereosmell experience, i.e., directional information about the locationof an odor, by rendering the readings of external odor sensors
as trigeminal sensations using electrical stimulation of the user’s
nasal septum. The key is that the sensations from the trigeminal
nerve, which arise from nerve-endings in the nose, are perceptually fused with those of the olfactory bulb (the brain region thatsenses smells). As such, we propose that electrically stimulating
the trigeminal nerve is an ideal candidate for stereo-smell augmentation/substitution that, unlike other approaches, does not require
implanted electrodes in the olfactory bulb. To realize this, we engineered a self-contained device that users wear across their nasal
septum. Our device outputs by stimulating the user’s trigeminal
nerve using electrical impulses with variable pulse-widths; and it
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https://doi.org/10.1145/3411764.3445300

inputs by sensing the user’s inhalations using a photoreflector. It
measures 10x23 mm and communicates with external gas sensors
using Bluetooth. In our user study, we found the key electrical
waveform parameters that enable users to feel an odor’s intensity(absolute electric charge) and direction (phase order and net charge).
In our second study, we demonstrated that participants were able tolocalize a virtual smell source in the room by using our prototype
without any previous training. Using these insights, our device
enables expressive trigeminal sensations and could function as an
assistive device for people with anosmia, who are unable to smell.

CCS CONCEPTS• CSS CONCEPTS; • Hardware → Emerging technologies;

KEYWORDS
Electrical stimulation, trigeminal, olfaction, intranasal
ACM Reference Format:
Jas Brooks, Shan-Yuan Teng, Jingxuan Wen, Romain Nith, Jun Nishida,
and Pedro Lopes. 2021. Stereo-Smell via Electrical Trigeminal Stimulation.
In CHI Conference on Human Factors in Computing Systems (CHI ’21), May
08–13, 2021, Yokohama, Japan. ACM, New York, NY, USA, 13 pages. https://doi.org/10.1145/3411764.3445300

CHI ’21, May 08–13, 2021, Yokohama, Japan

1

INTRODUCTION

The sense of smell is what allows us to understand a wide rangeof everyday experiences, from pleasurable ones (such as enjoying
good food) to detecting potential hazards (e.g., smell of rotten food,microbial threats, and non-microbial threats such as from hazardous
gases) [53].
Traditionally, olfactory interfaces physically push odor
molecules into the user’s nose. This methodology is limiting: it
requires olfactory interfaces to be built from bulky actuators (tubes,
pumps, and transducers) that redirect smell from containers to theuser’s nose. While this works in scenarios where one can hide alarge device (e.g., smells as notifications while driving [11]) theseactuators are impractical for ubiquitous everyday use because
their form-factor is dictated by the size of reservoirs and actuators.
One of the most promising ways to break from size-limitations
is to shift from analog (i.e., pushing odor molecules to the nose)
to digital (i.e., stimulating the olfactory bulb to generate the odor
perception) actuation. This approach has gained some attention in
the HCI community [52]. While this approach digital actuation iscertainly promising, it involves risky insertion procedures withlittle success at reproducing smells.
Instead, we approach digital actuation from a new angle bysearching for a less-invasive location to apply electrical nose stimulation, one that still works without the need for invasive and risky
electrodes attached to the olfactory bulb.
The principle behind our approach is the fact that while, previously, the sense of smell was attributed to the function of theolfactory bulb alone, this notion has been revised once the contributions of the trigeminal nerve to smelling were understood. In
fact, the trigeminal nerve, which has nerve-endings lodged in thenasal cavity, works in tandem with the olfactory bulb to detect
the warmth, freshness, astringency, etc. of incoming odors (e.g.,
the freshness of mint) [5]. In other words, we do not perceive the“freshness of mint” in isolation from the “odor of mint” because
the experiences of the trigeminal nerve and the olfactory bulb areoften fused together into what we experience as a “smell.” In fact,some of our examples for the role of smell included situations inwhich the trigeminal sensations are key, for instance, in detecting
potential hazards via the smell of noxious gases [53].
Building on this perceptual fusion between the sensations of thetrigeminal nerve and olfactory bulb, we propose a novel type ofolfactory substitution device that renders readings from odor or gas
sensors as trigeminal sensations by means of electrically stimulating
their trigeminal nerve. Our device is a new alternative to olfactory
devices that stimulate the olfactory bulb via electrodes deep inthe nose [59], which unfortunately require trained personnel. Our
device sits across the user’s nasal septum, where it can alreadyaccess the trigeminal nerve, unlike other devices that require theinsertion of electrodes up the nasal cavity. While our device does
not create an entire “digital smell,” because it does not extend to theolfactory bulb, it does create sensations typically associated withsmelling: trigeminal sensations.
Our device measures 10x23 mm and communicates with external sensors via Bluetooth, allowing it to fit entirely inside theuser’s nose (including battery, sensors, wireless, stimulator, electrodes) like a nose-ring across the septum. While our device is

Jas Brooks et al.

self-contained, it is only capable of electrical trigeminal stimulation
and requires a connection to external odor sensors (via Bluetooth)to obtain odor or gas readings. It stimulates the trigeminal nerve
using an electric, biphasic waveform (using a positive and negative
current) with computer-controlled pulse-width, depicted in Figure
1. This electrical waveform design is key to how our device allows
users to feel via trigeminal cues: not only the intensity of the odor
but also its direction, i.e., stereo-smell.

2

WALKTHROUGH: A USER SMELLING THE
DIRECTION OF A GAS LEAK
(STEREO-SMELL)

Figure 2 shows an example of a user, who while making toast for
breakfast uses our device to find a gas leak. In this case, our device
augments their perception, making a gas that humans cannot detect,methane, interpretable via the trigeminal sensation. To sense thisgas, our device is connected (via Bluetooth) to a combustible gas
sensor (see Implementation for details). In everyday life, gas leaks
are recognizable because gas providers inject an additional odor
molecule called mercaptan, which smells of “rotten eggs”, to alert
consumers to the presence of a leak. Unfortunately, mercaptan often
fails to alert users due to: temporary loss of smell (e.g., as a causeof other illnesses, such as COVID-19); smell impairments (suchas anosmia, loss of sense of smell, or hyposmia, reduced sense of
smell); or simply due to the unfortunately common odor masking(i.e., other smells that are stronger and mask the mercaptan) [46].
To this user these limitations are not a problem, since our device
allows this user to perceive the gas leak (the methane sensed by
the external combustible gas sensor) by feeling a tingling sensation
inside their nose which is synchronized with their breathing-in,
depicted in Figure 2 (b). This sensation is a trigeminal cue, like thesensation when smelling white vinegar.
To render this trigeminal sensation: (1) our device detects inhalation by measuring the flare of the user’s nostril with a photoreflector(i.e., as the user breathes in, their nostrils enlarge); (2) then, as theuser inhales, our device stimulates their trigeminal nerve with anelectrical impulse which renders both the intensity of the gas, andits direction (i.e., to the left or right of the external odor sensor). The
latter is particularly useful to locate odors in space and allows thisuser to experience stereo-smell for methane. As such, as depicted
in Figure 2 (c), they find that it is their stove, which is to their left,that is leaking the gas and it is not the burnt toast smell to theirright. As we will detail in our implementation and study, we induce
this stereo-smell sensation by varying the electrical characteristicsof the stimulation waveform (pulse-width and polarity).

3

RELATED WORKThe work presented in this paper builds primarily on the fields ofelectrophysiology, olfactory interfaces, and sensory substitution.
Also, to familiarize the reader with the underutilized trigeminal
nerve, we present an overview of its functions.

3.1

Sensory Substitution

Sensory substitution involves a translation of information from one
sensory modality to another. Sensory substitutions for vision are
most common and translate images into other modalities, usually

Stereo-Smell via Electrical Trigeminal Stimulation

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Figure 2: (a) Our user burnt their toast while making breakfast, which unfortunately emits a strong odor that masks the actual
danger, a gas leak in their stove. (b) As the user inhales, our device renders the smell of methane by stimulating the user’s
trigeminal nerve electrically and to the left—indicating the location of the methane gas leak, which is tracked via an external
odor sensor that relays these readings to our sensor via Bluetooth. (c) This allowed the user to find the leak.
as assistive devices for visually impaired individuals. For instance:
SmartTouch is a system that uses electrotactile stimulation on theskin to render an image [25]; and, the tongue display unit is atactile-vision sensory substitution system which translates an image
from a camera into electrotactile stimulation on the tongue [2].
Other popular sensory substitutions involve image-to-sound, suchas Meijer et al.’s device [38].

3.2

Olfactory Sensory Substitution

While sensory substitution for vision has been extensively researched, little research has investigated the translation of olfactory
cues into other sensory modalities. Two key contributions in olfactory substitution have investigated the translation of non-olfactible
gaseous compounds (e.g., combustible gases, such as methane) into
detectable stimuli for humans. The Olfactory Assist Mask is a wearable that converts metal-oxide gas sensor responses to combustible
gases that cannot be smelled into actual smells, such as the fragranceof apple [36]. Choi et al. engineered a wearable augmentation thatconverts detected combustible gases into vibrotactile stimulationon the forearm [10]. Unfortunately, as far as we know, no otherexisting works explore the substitution of olfactory cues to othersensory modalities.

3.3

Trigeminal System

The trigeminal nerve (cranial nerve V) is a somatosensory nerve
primarily known for sensing mechanical and thermal stimulation,
sensing changes in pressure, vibration, or temperature [39]. A lesser
known property of the trigeminal nerve is that its receptors (thetransient receptor potential ion channels) also respond to airborne
particles [39]. It is their ability to detect both airborne particles andtemperature shifts that explains why scents such as peppermint
feel cool [37]. The effects of the trigeminal experiences are readily
recognizable in everyday life, e.g., eating or smelling spicy food,pungent flavors, and breathing cold air [1]. What is remarkable
is that humans rarely experience pure olfactory or pure trigeminal
sensations in isolation—in other words, these senses are uniquelyintertwined.

3.4

Trigeminal-Olfactory Fusion

Though the trigeminal and olfactory bulb have different structuresand functions, most smells simultaneously activate both systems,
each contributing to the overall perception of smell (e.g., freshnessof menthol being inseparable from its minty odor). There are few
chemicals that selectively stimulate only one of the two: carbon
dioxide only stimulates the trigeminal nerve and vanillin only stimulates the olfactory bulb. Several electrophysiological and imaging
studies have demonstrated that the perception of trigeminal andolfactory systems is intertwined and that these systems influence
one another [5, 20, 32, 33]. For further detailed information, Brand
et al. present a thorough review of literature on the interactions
between the olfactory bulb and the trigeminal nerve [5].

3.5Trigeminal Interfaces

Our device is inspired by four recent interactive systems that stimulate the trigeminal nerve. The first two interfaces leverage theinteractions between the thermal-side of the trigeminal nerve andolfactory bulb to modulate the perception of an odor: Fujino et al.
change the temperature of the air to modulate the perceived odor[17]; and, similarly, affecting tumbler is a cup lid that thermally stimulates the skin around the nose during drinking to modulate theperceived flavor [56]. The other two interfaces leverage the chemical side of the trigeminal nerve, creating a temperature sensation.
In Season Traveller, Ranasinghe et al. utilize peppermint essential
oil (which contains menthol) to stimulate a cooling sensation for
their VR winter environments alongside thermoelectric materials[44]. Similarly, Brooks et al.’s wearable device induces temperatureillusions, both hot and cold, by delivering aerosolized trigeminal
stimulants to the user’s nose [6].
Unlike these devices, our approach is the first wearable device toelectrically stimulate the trigeminal nerve endings in the nose. Thishas several advantages when it comes to sensory substitution: (1)it is a much smaller device (no need for liquids, reservoirs, pumps,
large batteries, etc.); and (2) stimulation patterns can be controlled
at the microsecond level, which is difficult to achieve using liquids
or gases.

CHI ’21, May 08–13, 2021, Yokohama, Japan

3.6

Intranasal Electrical Stimulation of
Olfactory Bulb and Trigeminal Nerve

Previous neuroscience and physiology research have investigatedthe perceived sensations of electrical stimulation of the nasal mucosa and olfactory epithelium.
Researchers have been exploring reproducing smell via electrical stimulation of the olfactory bulb. Such stimulation requires a
trained experimenter to insert a long prong up the user’s nostril
to touch the olfactory bulb (brain region at eye-level) with an electrode, holding the electrode-prong in place to maintain constantcontact with the olfactory bulb. Unfortunately, most researcherseither fail to produce such smell percepts via electrical stimulationof the olfactory bulb [19, 55, 59] or fail to produce consistent sensations among participants [9, 19]. Nonetheless, they all succeeded
to produce several unintended trigeminal percepts, such as warm,
tingly, painful, etc.
In contrast, less research has been devoted to electrical stimulation of intranasal trigeminal nerve-endings. Most notable is Iannili
et al.’s studies that confirmed that electrical stimulation consistently
elicited trigeminal sensations, rather than olfactory ones [21]. The
sensitivity threshold for the trigeminal nerve was determined to be
at 0.6 mA and upper-threshold stimuli to be 1.8 mA for electricalstimulation [21].

3.7

Stereo Olfaction and Odor Lateralization

Humans can track an odor and follow its route by searching andcomparing different locations in an odor field, i.e., by sampling different locations one builds a mental map of odor intensities. However, this process is not what we typically would call “stereo-smell”
as the user needs to move to sample the space. Surprisingly, there
is recent evidence that humans do exhibit stereo-smell even when
standing still, however these sensations are pre- or non-conscious
and cannot be verbalized [60]. When non-trigeminal smells arriveat the olfactory bulb via different nostrils, they moderately bias
humans’ perceived direction of self-motion towards the nostril
with a higher concentration of the smell, but when asked users
cannot verbalize which nostril smells stronger [60]—this is called
stereo-olfaction (as in olfactory bulb).
On the flipside, the trigeminal nerve is capable not only of recognizing the concentration of trigeminal-odors in the different nostrils,
but its user is capable of verbalizing it, i.e., we can consciously tellin which nostril we smell more of a particular trigeminal odor
[12, 27, 51]—this is called trigeminal lateralization.
In contrast, pure odorants such as vanillin (the core ingredientin the smell of vanilla) cannot be consciously used for spatial navigation, but trigeminally irritants such as menthol (minty freshcompound) yield lateralization rates of more than 96% [27].
As discussed, the trigeminal system responds to mechanical,
chemical, and electrical stimulation [21]. When electrically stimulating the olfactory epithelium (which includes trigeminal enervation[50]) in one nostril, participants reported localizing the stimulation
to the stimulated side [13, 18, 19, 21, 22], suggesting one may also
accurately lateralize electrical stimulations detected by the trigeminal system. We take advantage of these neuroscientific observations
to propose using the electrical-sensitivity of the trigeminal nerve
for stereo-smell.

Jas Brooks et al.

3.8

Framing Our Approach: Lateralization Via
Electrical Stimulation Of The Intranasal
Trigeminal Nerve

This lateralization, which is a unique feature of the trigeminal
nerve, is what our device leverages to allow users to experience akind of “stereo-smell”; one that is not driven by the olfactory bulb
but instead by the electrical stimulation of the trigeminal nerve.
To leverage this lateralization, we designed our novel device to beinside the nose and stimulate the nasal septum, where the trigeminal
nerve can be easily found [16]. While theoretically one might be able
to access the trigeminal nerve by electrically stimulating different
areas of the nose, we found that not only the septum was moreeasily accessible, but other areas of the nose did not work in our
initial experimental pilots.
Furthermore, it is worth emphasizing that we found that while
our approach shares some similarities with electrotactile skin stimulation (e.g., [23, 24, 58]), our approach is in fact specific to thetrigeminal nerve inside the nose. While the area where we attach
the electrodes is also skin (hence the apparent similarity to electrotactile), yet, as we will demonstrate in our studies, electricalstimulation of the septum creates lateralizable sensations. Feelingthe direction of the current is something that we found to be uniqueto the nose’s trigeminal nerve; in fact, to support this assertion, weplaced our device across the finger pad, but we failed to elicit any
lateralization in the finger pad’s skin.

4

BENEFITS, CONTRIBUTIONS, AND
LIMITATIONS

Our key contribution is the first wearable device that fits in theuser’s nose to electrically stimulate their trigeminal sense. Our device enables users to feel, via trigeminal cues, not only the intensityof an odor but also its direction (i.e., to their left or right).
Our approach has four key benefits: (1) as we observed in our
second study, users are able to make use of our device to locate
smells without any training; (2) this new modality (electricaltrigeminal stimulation) can also be employed as an assistive device
for smell-impaired individuals, because it electrical stimulating thetrigeminal nerve, which remains functional in most anosmics, instead of the olfactory bulb; (3) it is an safe alternative to electricallystimulating the olfactory bulb with electrodes inside the user’s nose,
which require surgical and invasive procedures; and, (4) it is thefirst wireless and self-contained interactive device that can be worn
nasally, providing an alternative means for trigeminal stimulation
without the use of chemical reservoirs, utilized in prior research[6, 44].
However, our approach is not without its limitations. (1) We
purposely explored electrical stimulation of the trigeminal nerve
rather than the olfactory bulb for accessibility reasons. This meansthat our device is limited to the creation of trigeminal sensations—as such our stereo-smell is trigeminally induced: in other words,
this is not a traditional olfactory device (one with chemical-basedodors). (2) While our device is entirely self-contained (including
battery, sensors, wireless, stimulator, electrodes), it is only capableof trigeminal stimulation and depends upon a connection to external
odor sensors (via Bluetooth) to obtain odor readings.

Stereo-Smell via Electrical Trigeminal Stimulation

CHI ’21, May 08–13, 2021, Yokohama, Japan

Figure 3: (a, b) Our device is comprised of two printed circuit boards (PCB), one in each nostril. (c) Small magnets on each side
allow the device to hold securely to the user’s nose by means of magnetism.

Figure 4: (a) High-level schematic for our device’s hardware and (b) a stimulation that creates trigeminal sensation to the left
nostril (i.e., a lateralized sensation to the left; more details of how we found these waveforms in our first user study).

5

IMPLEMENTATION

To help readers replicate our design, we now provide the necessary
technical details. Furthermore, to accelerate replication we provide
all the source code, firmware, and schematics of our implementation.1
Figure 3 shows our complete and self-contained prototype, including its battery. It measures 10x23x5 mm in one nostril, 10x23x7
mm in the other, and weighs 3.4 g.
To implement our device, we utilized an nRF52811 (Nordic Semiconductors) as the system’s microcontroller, because of its small size
as well as integrated ARM Cortex M4 and Bluetooth Low Energy(BLE) radio. We leverage the BLE to communicate with external
devices, such as gas sensors or even mobile phones.

5.1

Design of Our Intranasal Stimulator(Output)

To create an electrical stimulation of the user’s trigeminal nerve, we
designed our own stimulator based on the results from our Studies 1
and 2; its circuit design is depicted in Figure 4 (a). We did so because
no existing stimulator would fit in the user’s nose. Additionally, theseptum is an ideal stimulation location as trigeminal stimulation isperceived as stronger in the front of the nasal cavity [16]. To create
the stimulation waveform depicted in Figure 4 (b), we utilize a
DRV8847 (Texas Instrument), a dual h-bridge capable of outputting
up to 18 Vpp in a 3x3 mm footprint. The voltage supply for the
stimulation is generated from a MT3608 (Aerosemi Technology)
boost-converter with adjustable voltage output, which we set at
18V. The parameters of the generated waveform (such as pulsewidth or current polarity) are generated by the microcontroller bycontrolling the h-bridge’s enable and direction.
Lastly, to deliver the electrical impulses from our stimulator to
the user’s trigeminal nerve, we utilize two silver/silver chloride(Ag-AgCl) disc electrodes, which protrude on the back side of each
PCB. Thus, these act as points of contact with the nasal septum forstimulation. Ag-AgCl electrodes were chosen because they do notcorrode over time, as it is common with copper or other types ofelectrodes.

1 https://lab.plopes.org/#stereo-trigeminal

CHI ’21, May 08–13, 2021, Yokohama, JapanJas Brooks et al.

Figure 5: (a) To stimulate the during inhalation, we use a photo-interrupter. (b) The sensor measures how nostril flares during
inhalation. (c) In turn, this flare creates an increase in the distance between the photo-interrupter and the nostril.

5.2

Design of Our Intranasal Nostril Movement
Sensor (Input)

To allow users to feel the trigeminal sensations in a realistic manner,
we coupled the stimulation to the user’s breath-in phase. To track
the breathing patterns, we added a SG-105F reflective-type infrared
photo interrupter (Kodenshi Corp) on the PCB, angled directly
outwards to one of the user’s nostrils, as depicted in Figure 5. We
sample this sensor at over 100 Hz (at a 12-bit resolution) to measure
the distance between nostril and PCB as the user’s nose flares during
the breathing cycle; it is known that nostrils enlarge as we breath in[28]. Furthermore, users can turn on/off the stimulation by quickly
inhaling three times; moreover, we acknowledge there are othermethods to handle nose gestures as input [31].

5.3

Securing Our Device Inside The User’s NoseTo ensure the device makes good contact with the user’s septum
without the need for implantation (e.g., such as an actual nose ring)
we use two magnets, one on each side of the PCB. The magnets
hold the device in place by attracting each other, even across thenasal septum. The use of magnets also allows a user to snap thedevice on or off their nasal septum, without any overly complicatedinsertion procedure.

5.4

Battery Life

We power our device using a 10 mAh lithium polymer battery(PGEB201212, General Electronics Battery Co.). It draws an average
0.2 mAh while connected wirelessly to an external odor sensor
using BLE. To this, it adds an additional current draw of 0.05-1
mAh during electrical stimulation. Thus, at an average of 12-20
stimulations per minute (human’s average respiratory rate [47]),
our device runs for around one hour using a single battery. Notethat a second one can be added to the other PCB to double thebattery life.

5.5

Pairing Our Device With An Existing Stereo
Gas Sensing Device

While we chose to focus our contribution on the stimulation sideof our device, which is entirely novel, the sensing side is important
to close the loop between odor-sensing and stimulation. There is awide range of approaches that previous engineers and researchers
have devised to sense the intensity of a gas, such as methane, in

the air. The most popular approach is to use metal oxide sensors(often also referred to as combustible gas detector) to determine
the location of a gas source [8, 48, 49, 57]. Additionally, researchers
have employed gas sensors in wearables, such as wristbands, forpersonal air quality monitoring [34, 35, 43].
As such, to complement our device and to demonstrate it in aclosed-loop fashion, we replicated the sensor design of Kohnotoh
and Ishida’s ASNose [29], using two 3.0 V pumps (SC3101PW) andtwo metal-oxide sensors (MiCS-5524); this configuration is referred
to as an “active stereo nose” as it detects the gas’s direction (i.e., tothe left or to the right) by comparing intensity of the readings on
both sensors. We sample the metal-oxide sensors systems using a
RedBear DUO BLE microprocessor. The gas’ direction and intensity
are sent directly from the sensor-device to our intra-nasal device
with Bluetooth.

6

USER STUDY 1: INTENSITY & DIRECTIONUSING ELECTRICAL TRIGEMINALSTIMULATION

Our first study aims to understand the relationship between thestimulation parameters and the perceived direction and intensity.
We hypothesize that different stimulation patterns result in different
trigeminal sensations. Our study was approved by our Institutional
Review Board (IRB20-0308). User Study 1 and 2 were completed
before our device implementation, as we used these studies toinform the stimulator’s design.

6.1

Apparatus

A simplified version of our wearable device was used. Participants
wore a 3D-printed nose clip with Ag-AgCl electrodes (same asused in our final device), but this time connected to a Hasomed RehaMove 3, a medically compliant functional electric stimulator. We
used this stimulator as it allowed us to vary all possible waveform
parameters prior to designing our own device based on the study
findings. Once we found the relevant waveform parameters fromthis study, we started the design of our own stimulator to support
those waveforms.
Prior to the start of the study, we asked participants to insert thenose clip into their nose such that the bridge was flush against thebottom of the nose and the pressure felt even across the septum.

Stereo-Smell via Electrical Trigeminal Stimulation

CHI ’21, May 08–13, 2021, Yokohama, Japan

Figure 6: Two example pulses, of which (a) is monophasic and (b) is biphasic. (c) The Rehamove signal generator, which weused in our studies.

6.2

Stimulus Design

Since our study was the first to explore the sensations that arise
from electrical stimulation across the septum, we aimed to vary as
many waveform parameters as possible to maximize our insights
into this novel modality. Following, we varied the three parametersof a typical squared stimulation waveform: (1) pulse width, (2)polarity, and (3) phase order. We fixed the frequency of stimulation
at 62 Hz and the intensity of each pulse was at 1 mA, within therange of noticeable yet pain-free electrical stimulation as shown in[21].
Pulse width: We utilized pulse widths of 100 µs, 250 µs, and 500µs for each pulse phase. All 0mA phases were reduced to a pulse
width of 0µs to avoid redundancy.
Waveform polarity: We designed our study around the two
traditional types of electrical stimulation waveforms, which are
depicted in Figure 6 (a) monophasic (only one polarity, as in DC
currents); or, (b) biphasic (i.e., current switches direction, as in AC
currents).
Phase order: Additionally, the phase order was variable, e.g.
positive first or negative first, which is depicted in Figure 6 (a) and
Figure 6 (b) respectively.
Thus, all combinations of the parameters above result in a totalof 25 different waveforms (the total was in fact 75, but to avoid
redundant stimulations, we removed all waveforms that starts witha pulse of 0 mA intensity).

6.3

Trial design and Procedure

Each participant performed a total of 75 trials: 25 possible waveforms x 3 repetitions. Waveforms were presented in a randomizedorder across participants.
Per each trial, participants felt one brief electrical trigeminal
stimulation (out of the possible 25), delivered to across their nasal
septum. Each stimulation contained a sequence of 20 impulses,
lasting a total of 322 ms. After each trial, participants rated thestimulation intensity (1=“did not feel”; 7=“most intense”) and direction (-3=left; +3=right) using 7-point Likert scales.

6.4

Participants

We recruited 10 participants from our local institution (4 women,
5 men, and 1 non-binary person) aged 20-25 years old (M=22.7,
SD=1.9). Participants received 25 USD compensation for their time.

Participants were given the Nasal Obstruction Symptom Evaluation (NOSE) 5-point Likert Scale [54] at the start. With it weconfirmed that no participant had nasal congestions or stuffiness(M=0.5, SD=0.53), blockages or obstruction (M= 0.2, SD=0.42), trouble breathing through nose (M=0.5, SD=0.53), trouble sleeping(M=0.5, SD=0.53), or difficulty breathing through their nose during
exercise or exertion (M=0.1, SD=0.32).

6.5

Metrics for Analyzing Electrical
Waveforms

Towards our results, we describe the aggregated electrical properties of our waveforms using two existing metrics, which are
exemplified in Figure 7 the total electrical charge, which represents
the sum of all charges over time regardless of sign; and the net
electrical charge, which represents the charge imbalance.

6.6

Results

6.6.1 We conducted a one-way MANOVA and used Bonferronicorrected t-tests. Figure 8 depicts ratings for (a) lateralization and (b)intensity for all 25 pulse candidates. First, we examined the perception of the intensity of the stimulation for the different waveforms.
We found a statistically significant effect of total electrical charge
on reported intensity [F(7,719)=10.5, p=1.2e-12]. Of the eight total
electrical charges, all were statistically significant, except for 250
mAµs and 500 mAµs. This suggests that, as total charge increases,
we tend to observe an increase in perceived intensity, as depicted
in Figure 9
Secondly, we examine the perception of direction (also known
as trigeminal lateralization). We found a statistically significant effect of net electric charge on reported lateralization [F(10,719)=16.8,
p<2.2e-16], as shown in Figure 10 (a). Of the 11 possible net charge
values, all were statistically significant, except for -100 mAµs, 0mAµs, and 100 mAµs; suggesting that net charge influences perceived lateralization.
Additionally, we found that the phase order (whether the pulse
began with a negative or positive stimulation) also had a significant
effect on perceived lateralization [F(1,719)=41.3, p=2.3e-10]. As
depicted in Figure 10 (b), pulses beginning with a positive phasewere typically perceived as right-sided stimulus.

CHI ’21, May 08–13, 2021, Yokohama, JapanJas Brooks et al.

Figure 7: Metrics used to analyze the electrical waveforms are total charge (yellow) and net charge (red).

Figure 8: Participants reported the (a) intensity and (b) lateralization of all 25 pulse candidates. The left-most column andtop-most row represent the pulse’s first and second phases, respectively.

6.7

Figure 9: Perceived intensity ratings for the different waveforms according to their total charge.

Resulting Waveforms That Achieve
Multiple Intensities And Directions

To make use of our findings we chose nine pulses that allow us torender three intensity levels (low, middle, high) and three directions(left, center, right), which are depicted in Figure 11
The final set of waveforms is: at a low intensity (-1, 250), (1,250) for left; (-1, 100), (0, 0) for center; and (-1, 100), (1, 100) for
right; at a medium intensity (-1, 250), (0, 0) for left; (1, 100), (-1,250) for center; and (-1, 100), (1, 250) for right; lastly, at a highintensity (-1, 500), (0, 0) for left; (-1, 500), (1, 500) for center; and(1, 500), (-1, 100) for right. Next, we will put these waveforms toan actual test in our User Study 2.

6.8

Qualitative Feedback

All participants commented being interested in and entertained by
the sensation; no pain was reported by any participant. P6 reported
a watery nasal drip and had to blow their nose. This was consistentStereo-Smell via Electrical Trigeminal Stimulation

CHI ’21, May 08–13, 2021, Yokohama, Japan

Figure 10: (a) Perceived lateralization (direction) of the different waveforms based on their net electric charge. (b) Pulse candidates were categorized as either starting with a negative or positive current; (0,0)-pulses are excluded. Participants ultimately
rated pulses starting with a positive phase as more to the right.

Figure 11: Our suggested pulses are divided into three directions and three intensities to render both changes in source locationand proximity. Each pulse shows its reported intensity (vertical plot) and reported lateralization (horizontal plot).
with the type of dripping that occurs when eating trigeminal stimulants in food, such as in spicy food [45]. The drip ended as soon
as the study was completed, and no other participants reported
similar experiences.

7

USER STUDY 2: FINDING AN ODOR USINGOUR (ELECTRICAL) STEREO-SMELL

In our first study, we found the waveform parameters that allow
us to render the intensity and direction of an odor via electricaltrigeminal stimulation. As such, in our second study, we aimed todetermine whether participants new to this modality, could makeuse of these trigeminal cues to locate an odor source. As such, ourstudy’s objective was explorative: we were not aiming to show
which condition improved gas source localization, but instead wereinterested in seeing whether localization using electrical trigeminal
stimulation had similarities to how we ordinarily navigate gas
sources.

7.1

Apparatus

Participants explored a simplified, virtual reproduction of the testing space using a Wireless HTC Vive, headphones, and the same
stimulator and nose clip from our previous study. The tracking area
was approximately 5 x 5 meters, and this study was conducted in
the same location as our previous study. The virtual space and VR
headset served as a “blindfold” so that the participant was unable
to see where the odor source was placed but would be alerted by
the headset to walls as they approached them. Our experiment was
conducted in a ventilated laboratory, with one perforated duct ventilation (500 CFM) and two fume hoods. The ambient temperature
was kept at 24{°}C.
A fan was placed at a 45-degree angle, approximately 2.5 meters
away from the participant at one of 3 pre-selected locations. Usingan anemometer, the fan’s wind speed was 2.7 m/s at 5 cm awayfrom the fan and 0 m/s after 50 cm. From pilot studies, participants
were not able to feel the tactile stimulation of the fan, and as thefan produced no measurable wind space after 50 cm, unintentional

CHI ’21, May 08–13, 2021, Yokohama, Japan

tactile stimulation from the fan was ruled out as a confounding
factor in navigating the odor field.

7.2

Conditions

Participants were presented with two possible conditions: navigating a real or virtual odor field.
7.2.1 Real Odor Field. In the real odor field condition, a small cup
filled with a 50% v/v peppermint essential oil in ethanol solution
is placed behind the fan to inject a steady stream of mint/menthol
throughout the task. The participant was asked to localize thesource with their sense of smell, while wearing the nose clip but
not receiving any stimulation. Note that adding a real odor field
condition setup was not unique to our study design, as this setup
is often used in gas source localization studies, which typicallyinvolve gas searching robots [7].
7.2.2 Virtual Odor Field. The participant was asked to localize thevirtual odor source using the trigeminal stimulation. In this virtual
odor field condition, as there was no odor. Instead, we simulated aturbulent virtual odor field in a simulated laboratory space matching ours as closely as possible. We did this to exclude any potential
confounds from tracking odor sources using external sensors, allof which have tracking errors that are not what we are interested
in measuring. On the contrary, simply guiding participants via
perfect left/right cues to the source is unrealistic as odors move
fluidly and turbulently through air, and as we smell around in thereal world we also deal with these turbulences. As such, a simulated odor field with turbulences proved the best setting to gathera realistic understanding of our device. Adding these turbulences
allowed us to make the conditions more comparable. The inclusionof a simulated odor field condition is also not unique to our study
design, previous olfactory work has leveraged CFD for modelingvirtual conditions: "[CFD] enables us to have the dynamic odor
concentration distribution even if we have complicated obstaclesin the virtual environment" [41].
To simulate the virtual odor field, we used ANSYS, a computational fluid dynamics (CFD) software, to mimic as many parameters
as possible from a real odor field: (1) we added turbulent air sourcesfrom the fan, HVAC, and participant movements in the space; (2) we
added the odor’s dynamic viscosity, which was estimated by that ofliquid ethanol at 24{°}C and 1 bar, approximately 1.1 cP. The simulation’s timestep results (around 41M air flow intensity samples fromANSYS) were then exported, segmented using a simple Bounding
Volumes Hierarchy algorithm, and imported into Unity3D. From
Unity3D, a script simulates the detection of the odor, comparing
the odors concentration captured in two Unity3D colliders, one
for each nostril. To emulate the tidal breathing, each collider isapproximately 0.25 L in volume. We then separately sum the concentrations of any odor samples within the colliders for each nostril
to get estimated inhaled odor concentrations. The estimated intensity of the odors uses the following equation based on Murphy’s
results for varying menthol concentrations [40], c  0.617
i = 2.5 ×
1.7
where i is the estimated intensity and c is the total inhaled menthol
concentration. Additionally, we cut off any concentrations below

Jas Brooks et al.

1.4e-3 ppm to reflect human odor detection threshold of menthol[11].
Finally, we map the estimated intensity to three intensity levels for stimulation obtained in our User Study 1 (see Figure 11).
Estimated intensities under 3 were mapped to the low intensity
stimulations. Estimated intensities between 3 and 6 were mapped
to medium intensity. Finally, estimated intensities above 6 were
mapped to high intensity. The Unity3D script then stimulates theparticipant’s nose 24 times per minute with a short trigeminal cue.
Our CFD simulation and study scripts are provided to assist withreplication of our study1

7.3

Task and Procedure

Participants performed a total of four trials: 2 conditions x 2 repetitions; the order of trials was randomized.
Per trial, participants were asked to navigate the room and identify where the odor source (either real or virtual) was by declaring
when they finished and standing at the estimated location. The
examiner then measured the distance between the user and the target. Participants were given 5 minutes to complete the task, and–at
the end of the allocated time–were given a few seconds to make aforced choice.
Most importantly, we designed this procedure to have no training. We never shown the stimulation to the participants beforehand
nor there were any “habituation trials” or explanations. We just letthem locate the odor.
7.3.1 Deodorization. After each trial, the participant was asked
to take a 12-minute break outside of the testing room. During thistime, the examiner deodorized the space. For odor control withozone oxidation, we used a portable ozone generator that outputs
50 mg/h. To comply with OSHA exposure limits in a workspace(0.2 ppm for no more than 2 hours exposure [30]), we only emit 10
mg of ozone for the entirety of a study with one participant (notcounting the ventilation). Following, the ozone generator is placedat the gas source location and turned on for only 6 minutes (5 mgof ozone) after real odor field condition trials.

7.4

Participants

We recruited four participants from our institution (two selfidentified as women and two as men) aged 20-40 years old (M=30.3,
SD=6.55). Participants were given an intake form at the start ofthe study. Of the four participants, three participants reported
using VR before and one reported experiencing VR with smell
before. Participants were given the Nasal Obstruction Symptom
Evaluation (NOSE) 5-point Likert Scale [54] at the start. With it
we confirmed that no participant had nasal congestions or stuffiness (M=0.5, SD=0.58), blockages or obstruction (M= 0.25, SD=0.5),
trouble breathing through nose (M=0.5, SD=0.58), trouble sleeping(M=0.75, SD=0.96), or difficulty breathing through their nose during exercise or exertion (M=0, SD=0). Additionally, we asked thatparticipants estimate how well they detect odors in their daily lives:of the 4 participants, 2 reported normal smell function, 1 reported
higher sensitivity, and 1 reported not detecting odors frequently.
Finally, we asked participants to rate how often they experienced arunny nose after eating spicy food on a 5-point Likert Scale, which
they reported experiencing every so often (M=2.25, SD=1.26).

Stereo-Smell via Electrical Trigeminal Stimulation

CHI ’21, May 08–13, 2021, Yokohama, Japan

Figure 12: On average, participants were closer to the odor source in the virtual condition than in the real. Note that there wasno training phase whatsoever in our study and these participants had no experience with our electrical trigeminal stimulation.

7.5

Results

Considering the small sample size (N=4), we opted to discuss themeasured data without invoking statistical tests. Our results are
depicted in in Figure 1. We observed that even without any trainingor calibration phase participants in the virtual condition halved theiraverage distance (m) to the target (M=0.631, SD=0.337) comparedto the real condition (M=1.20, SD=0.585).

7.6

Qualitative Feedback

Of the four participants, only one reported experiencing some nasal
drip after one virtual odor trial. Participants reported that the electrical stimulation was clear and easy to use. P1 and P2 shared that
their confidence locating the virtual odor field was much higher
than when locating real odors. Similarly, P3 describes the electricalstimulation as “strong, loud and clear,” and P1 refers to the experience as “clear zaps”. Most participants reported that the electricalstimulation felt like a new sensation (P1, P3). Participants were able
to distinguish levels of intensity and direction at different rates: P3
specifically mentioned the directional sensation and P2 reported
distinguishing two intensities as well as two “nostril type stimuli”;
P2 shared that they wished the stimulation were more constant,which is why our final device is tied with the user’s breathing cycle.
Finally, most participants reported interest or excitement about thesensation (P1, P2, P4), though P2 shared that the stimulation in fact
made them want to avoid the smell instead of pursuing it but did
find the experience fun.

7.7

Discussion

As we observed in our second study, users are able to make use ofour device to locate smells without any training, we expect this tobe the case as these sensations are similar to those elicited by thetrigeminal nerve upon chemical or thermal stimulation (e.g., eating
spicy food or inhaling cold air), which previous research has shown
to be easily lateralizable. Additionally, we believe this contributes
to the potential success of the system’s olfactory substitution, as
users would not need a long learning phase to use the directional
sensation’s information. This is in stark contrast to other types of
sensory substitutions – such as that of the tongue display unit [2]–which require careful training to make sense of the new mapping.

As P1 reported, the sensation is unique and does not feel likesmell (echoing results from prior papers such as [59]). This may
be due to the waveforms not being designed to elicit more subtle
stimulation (as 1 mA is already perceived quite intensely [21]), no
variation during a stimulation itself, and because the RehaMove 3
does not provide stimulation in phase with inhalation nor varies
stimulation based on the participant’s sniff strength (e.g., P2 desiringa “more constant” stimulation).
P3’s comment regarding “two nostril type stimuli” may stemfrom the limitation in our first user study. Although we characterize
relationships for intensity and lateralization with total and net
charge, stimulations have perceivably different trigeminal qualities(e.g., prickling, warm, tickling, pressure, etc.). Following, despite thevirtual odor field getting rendered for intensity and lateralization,
the virtual odor’s quality may be non-uniform as the participantnavigates the plume.
Most participant’s confidence in the stimulation may come fromthe fact that they are experiencing verbalizable stereo-smell via
electrical stimulation or may be due to clear differences between
intensity levels and lateralization directions (as compared to smaller
variations with the real odor).

8

POTENTIAL BENEFITS AS AN ASSISTIVE
DEVICE

Smell is often leveraged in the detection of hazardous situations,such as expired food or chemical risks. Unfortunately, an individualwith anosmia (loss of olfactory function) cannot detect gas, smoke,or fire through their sense of smell [3, 42]. In Keller and Malaspina’sreports, 72% of their subjects reported fearing exposure to dangers
due to their olfactory dysfunction [26]. The main concern was theinability to detect a fire, volatile compounds, or a gas leak. (Thelater stems from people with anosmia not being able to smell thecompound–methyl mercaptan–added to odorize natural gas, which
is normally odorless.) Bonfils et al. reported similar results fromincidents with their survey population [4]. 63% regularly burned
their food, 47% had incidents where they did not detect a gas leak,and 26% had not detected active fires. These consequences ripple
across many aspects of a person’s lives such as active avoidance of
housing with natural gas, food poisoning, prolonged exposure tohazardous volatile chemicals, reduced quality of life from lack offlavor, and more.

CHI ’21, May 08–13, 2021, Yokohama, Japan

In contrast to their olfactory dysfunction, people with anosmia
often retain trigeminal function, though it may be dampened insensitivity [14, 15]. Our device may serve as an assistive technology
for people with anosmia, as it can enable them to sense gases in the
air. The specificity of the gaseous response is directly tied to thesensor, and the commercially cheap option (metal oxide sensors)often detect smoke and natural gas at incredibly low concentrations.
If using a stereo sensing system, people with anosmia may use ourdevice to also lateralize the direction of the gas and navigate towards
the odor source to hopefully prevent potential harm from exposure.
Though our device may serve as a technical foundation to achieve
trigeminal-based stereo-smell, additional research on the design
space of trigeminal stimulation and co-design with people withanosmia is necessary to further validate our system as an assistive
tool.

9

CONCLUSIONS & FUTURE WORKIn this paper, we proposed, explored, and engineered a novel typeof olfactory device that creates a stereo-smell experience, i.e., directional information about the location of an odor, by rendering
the readings of external odor sensors as trigeminal sensations using electrical stimulation of the user’s nasal septum. Our device
outputs by stimulating the user’s trigeminal nerve using electricalimpulses with variable pulse-widths; and it inputs by sensing theuser’s inhalations using a photoreflector.
In our first user study, we found the key electrical waveform
parameters that enable users to feel an odor’s intensity (absoluteelectric charge) and direction (phase order and net charge). In our
second study, we demonstrated that participants could immediately
make use of these stimulations to find a target (a virtual odor)
without any previous training with it.
We believe this opens exciting new interactive uses. Most importantly, we believe this device is useful for users with loss of smell(anosmia) because our technique relies on electrical stimulation of
the trigeminal nerve, and not on stimulation of the olfactory bulb;
as such it is expected to work for most people with anosmia.
Moreover, while we think of our device as a sensory substitution
device to create a proxy of smell sensations, other researchers might
be inspired to use it for more general interactive uses, for example tobuild new types of haptic interfaces. For instance, feeling navigation
cues, such as walking directions as though one were “smelling their
way home.” Finally, we expect researchers to explore combinationsof our device with chemical stimulation of the olfactory bulb.

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