Electrical Head Actuation: Enabling Interactive Systems to
Directly Manipulate Head Orientation
Yudai Tanaka

Jun Nishida

Pedro Lopes

University of Chicago
yudaitanaka@uchicago.edu

University of Chicago
junnishida@uchicago.edu

University of Chicago
pedrolopes@uchicago.edu

Figure 1: (a) We propose a novel interface concept that enables interactive systems to directly actuate the user’s head orientation around its yaw (i.e., left/right) and pitch (i.e., up/down) axis by applying electrical muscle stimulation (EMS) to the neckmuscles. Actuating the neck muscles with EMS is uncharted territory for interactive systems, yet, enabling it opens up a range
of applications not possible before: (b) directly changing the user’s point-of-view to locate target objects in a fre safety training; (c) a sound controller that uses neck movements as both input and output; (d) transmitting one users’ neck movement
to another; and, (e) rendering force feedback from VR punches on the head. (f) Finally, we explored users’ experience on ourelectrical head actuation.

ABSTRACTCCS CONCEPTS

We propose a novel interface concept in which interactive systems directly manipulate the user’s head orientation. We implement
this using electrical-muscle-stimulation (EMS) of the neck muscles, which turns the head around its yaw (left/right) and pitch(up/down) axis. As the frst exploration of EMS for head actuation,
we characterized which muscles can be robustly actuated. Second,
we evaluated the accuracy of our system for actuating participants’head orientation towards static targets and trajectories. Third, we
demonstrated how it enables interactions not possible before bybuilding a range of applications, such as (1) synchronizing headorientations of two users, which enables a user to communicate
head nods to another user while listening to music, and (2) directly
changing the user’s head orientation to locate objects in AR. Finally,in our second study, participants felt that our head actuation contributed positively to their experience in four distinct applications.

• Hardware → Emerging technologies.

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CHI ’22, April 29–May 05, 2022, New Orleans, LA, USA© 2022 Copyright held by the owner/author(s). Publication rights licensed to ACM.
ACM ISBN 978-1-4503-9157-3/22/04. . . $15.00
https://doi.org/10.1145/3491102.3501910

KEYWORDS
Electrical Muscle Stimulation, Haptics, Augmented Reality, Virtual
Reality
ACM Reference Format:
Yudai Tanaka, Jun Nishida, and Pedro Lopes. 2022. Electrical Head Actuation:
Enabling Interactive Systems to Directly Manipulate Head Orientation. In
CHI Conference on Human Factors in Computing Systems (CHI ’22), April
29–May 05, 2022, New Orleans, LA, USA. ACM, New York, NY, USA, 15 pages.
https://doi.org/10.1145/3491102.3501910

1

INTRODUCTION

While head movements are integral in life and, in many interactive
systems (e.g., scanning one’s surroundings in VR/AR or as nonverbal cues in telepresence), researchers have rarely explored ways
to directly utilize head movements as output. When it comes tohaptic actuation, researchers predominately focus on actuating theuser’s hands [6, 59] arms [36, 45, 60], or feet [17, 48]. Even though
actuating the limb’s extremities has proven promising for guiding
user’s touch [6, 66], movements [35, 41] and even walking direction[48], very few interactive systems explored the potential of directly
actuating head movement.
There have been four types of explorations in actuating thehead: (1) emergent exoskeletons used only in medical rehabilitation[69]; (2) mounting a fywheel to the VR headset to render inertial

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Yudai Tanaka et al.

Figure 2: Main muscle groups that control the human neck, which we explored to understand how to robustly actuate the headwith EMS (muscle names in gray denote deep muscles that are hard-to-reach with non-implanted EMS).
forces, which uses the gyroscopic efect as the user’s head is actively
moving [14]; (3) mechanically applying pressure to the head causing
it to jerk in one direction, denoted as hanger refex [27]; and (4)using air jets mounted to the VR headset to render accelerations of
the head [32].
Unfortunately, while all the above haptic interfaces are promising, they are built around the assumption that one can just attach
actuators to a headset or, even more dramatically, that all users will
be excited to attach actuators on their face, forehead, etc. Thus, theseare mostly limited to VR, where a headset is already present.
Instead, in this paper, we explore a novel approach that empowers interactive systems with direct control of the user’s headorientation by actuating the user’s neck muscles with electricalmuscle stimulation (EMS). Our system is depicted in Figure 1 (a).
The fact that it builds on EMS results in a haptic device capableof actuating the user’s head without requiring any attachmentson the user’s face/head or above the neck muscles—e.g., a simple
turtleneck would cover our system’s EMS electrodes entirely.
As the frst investigation of interactive head actuation, we frst
explored which muscles and electrode placements robustly actuate
the user’s head orientation. Then, in our frst study, we characterized our system’s accuracy while actuating participants’ head orientations to static targets and trajectories. Then, we demonstrated
how our electrical head actuation enables interactions not possible
before by building a range of applications, such as directly changing the user’s point-of-view to locate targets in a mixed-reality
fre safety training and transmitting head nods from one user toanother. Further, to gather more qualitative feedback about participants’ experience with this novel type of haptics, we conducted asecond study, in which participants explored our applications. We
found that our head actuation positively shaped the participants’experience.

2

OUR APPROACH: ACTUATING HEAD VIAEMS TO NECK
2.1 Neck = uncharted territory for interactive
EMS
While EMS to the neck muscles has been practiced in the medical
feld (mostly for rehabilitating swallowing disorders [4, 10, 29, 51]
or neurological conditions such as hemispatial neglect [23, 49, 61];
pain therapy [39]; and even as an epilepsy treatment [9]), there are
no prior works on interactive head actuation with EMS, hence

there is little information about which neck muscles and electrode placement should a realtime, interactive system use to robustly actuate the head. In most interactive systems based on EMS[26, 34, 46, 48, 59], the approach to fnding how to actuate a particular limb with EMS is: (1) consult the anatomical chart; (2) fnd at
least two large muscles that pull and push the limb (i.e., a fexor andan extensor) (3) repeat this for each degree of freedom of this limb’s
joint; and, (4) iterate over each of these electrode placements to fnd
suitable locations that actuate the limb while minimizing cross-talkbetween the diferent degrees of freedom and minimizing unwanted
movements in adjacent muscles [47, 57, 59]. These two well-known
limitations of EMS pose stark consequences for unexplored EMS
regions. Often, the frst researchers to interactively actuate an unexplored body part with EMS map the territory, i.e., explore whichmuscles and electrode placement can actuate robustly and ensure
pain-free operation. As an example, the PossessedHand [59] wasa pioneer interactive system in using EMS for individual fnger
movements; as such, much of their emphasis, even across multiple papers [58, 59], was characterizing electrode placement andits resulting accuracy. This is precisely the work that we present
for the case of the neck muscles, which we hope will accelerate
other researchers in exploring this new terrain for interactively
controlling head movements.

2.2

Building the frst map of neck muscles forinteractive head actuation

The neck is a complex target for non-implanted EMS due to its
12 main muscles (and 6 additional minor muscles) [53], multiple
attachment points (skull, hyoid bone, clavicles, and the sternum),and movement in three axis: cervical rotation (turning the headto the left/right), cervical fexion (nodding the head down to thechest) & cervical extension (nodding the head up to the back) andcervical lateral fexion (tilting the head to the shoulder) [22]. Figure
2 summarizes the 12 main muscles that are involved in each ofthese movements [53].
To gather intuition on which muscles are useful for neck EMS,
we placed electrodes at all skin locations atop each of the ninesuperfcial muscles (three muscles were found to be too deep for
non-implanted EMS actuation) and observed the resulting neckmovement during EMS actuation. These preliminary pilot tests
allowed us to characterize which placements can robustly actuate
the user’s head without unwanted movements and enabled us to,later, build a neck-PID controller for each axis, which we describe
in Implementation.

Electrical Head Actuation: Enabling Interactive Systems to Directly Manipulate Head Orientation

2.3

Tilting

As depicted in Figure 3, we found that tilting left/right (cervicallateral fexions) induced unwanted movements in the shoulders, i.e.,
involuntary shoulder shrug. To perform this movement with EMS,
we explored all available muscles: scalenes and levator scapulae.
Unfortunately, we concluded that their proximity to the shoulder
muscles allows EMS currents to run through the shoulder andcause unwanted movements. Thus, we decided against using thisdirection of movement as it did not provide robust results.

Figure 3: Electrode placement for head tilting induces large
unwanted movements in the shoulders.

2.4

CHI ’22, April 29–May 05, 2022, New Orleans, LA, USAfrom both sides must be precisely equalized; otherwise, the neckwill move down but also tilt to the stronger side (Figure 5c). We
found this efect to be so strong that even a slight unbalance inelectrode placement will cause unwanted movements. To mitigate
this, as depicted in Figure 5 (b), we actuate the two symmetricsternocleidomastoid muscles via one EMS channel across both.
We observed that this causes the EMS current to hit both sides ofthe muscles, in a more balanced fashion, resulting in a uniform
downwards head nod; to the best of our knowledge, this electrodeplacement has never been explored before.

Figure 5: We found an electrode placement that robustly
nods the head down with EMS.

Turning left/right
2.6

Nodding up

As depicted in Figure 4, we found that turning left/right (cervical rotations) can be robustly achieved with EMS. We explored allavailable muscles: splenius capitis, sternocleidomastoid, and splenius
cervicis. These muscles are bilateral, i.e., they run symmetrically inboth sides of the neck, thus turning the head to one direction withany of these muscles is achieved by actuating only the corresponding side. More importantly, we observed that the splenius capitis
was the only muscle to achieve this movement robustly (Figure4b). The remainder muscles were found to be unsuitable: actuating
sternocleidomastoid turns the head but induces unwanted tilting(Figure 4c); actuating splenius cervicis resulted in head nodding up(Figure 4d).

As depicted in Figure 6, we found that nodding up (cervical extension) can be robustly achieved with EMS. We explored all available
muscles: splenius cervicis, splenius capitis, and upper trapezius. Moreover, since all these muscles run symmetrically on both sides of
the neck, we followed the same strategy used for nodding down:using one EMS channel for both sides. We found that both splenius
cervicis (Figure 6b) and splenius capitis (Figure 6c) achieved upwards
nodding robustly. In addition, we found that splenius cervicis outperformed splenius capitis with more range of upwards movement
even at a lower EMS intensity. Conversely, we found that the upper
trapezius did not result in upwards motions (Figure 6d).

Figure 4: We found a placement that robustly turns the headleft using EMS.

Figure 6: We found an electrode placement that robustly
nods the head up with EMS.

2.5

Nodding down

As depicted in Figure 5, we found that nodding down (cervical
fexion) can be robustly achieved with EMS to sternocleidomastoid.
As depicted in Figure 2, sternocleidomastoid was the only available
muscle for this movement because the remainder muscles wereunable to be stimulated via non-implanted EMS due to their depth(i.e., they are behind the larynx). More importantly, we also foundthat attaching one pair of electrodes (i.e., one EMS channel) per
muscle, which is typical of interactive EMS systems [34, 38, 48] is
not ideal for the sternocleidomastoid muscles. This is because twosternocleidomastoid muscles run symmetrically on both sides of
the neck, thus, with one channel on each side, the resulting forces

2.7

Safety during EMS to the neckTo ensure operational safety during our head actuation via EMS
to the neck, we follow the established safety guideline around
EMS [28] by using a medically-compliant stimulator throughout
the study [50]. The device has the following safety features: (1)electrode error detection (i.e., if the electrodes fall from a user,
the device immediately halts all impulses); (2) current control (i.e.,the output current amount is independent of skin resistance); (3)galvanic isolation (i.e., no physical connection between the user
and other electrical sources). Moreover, we make sure the properelectrode placement before the stimulation so that electrodes are of
from carotid sinus, which is, to our knowledge, the only nerve under

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Yudai Tanaka et al.

the neck that can be stimulated by an EMS device and has a risk of
causing side efects (prior research implied electrical stimulation to
the nerve could decrease blood pressure [21]). Note that none of
our electrode placements are atop the nerve in principle.

2.8

Summary

Our exploration provided the frst map of which muscles can robustly actuate the neck in four directions (nodding up/down &turning left/right), which we use to demonstrate interactive uses ofhead actuation.

3

WALKTHROUGH: USING ELECTRICAL
HEAD ACTUATION TO GUIDE ONE’S
POINT-OF-VIEW

To illustrate the new possibilities enabled by our electrical headactuation, we demonstrate it in the example of a mixed reality(MR) application for fre safety training. Here, our user wears ourelectrical stimulation system on their neck as well as a HoloLens
2 MR headset, which displays the virtual content and tracks their
head.

Figure 7: (a) The user sees a fre in the solder station; but, (b)cannot locate the fre extinguisher. (c) Our device changes
the user’s point of view by actuating their head orientation
by means of EMS. Allowing the user to spot the extinguisher.
The user is at a laboratory, learning about fre safety from a frstperson perspective, instead of with training videos. As depicted
in Figure 7 (a), they see a fre breaking out at the solder station
through the MR headset. Their goal is to put out the fre. The
MR interface visually depicts the next instruction to the user, “Putout the fre”. However, the user is having difculties locating thefre extinguisher, which we depict in Figure 7 (b). Instead of the
GUI elements that MR typically make use of to indicate of-screen

Figure 8: (a) While the user is putting out this fre, they forget to scan the surrounding for runaway small fres. (b) Our
device uses EMS to change the user’s point of view so thatthey see a fre on the upper shelf, which (c) the user tries toextinguish.

Figure 9: Our system actuates the user’s head, in a trajectory,
that follows the evacuation route. The exit symbol is also
seen in MR by the user; here, for the sake of visual clarity,
we added it in post-production.

targets, our system relies on a novel approach: it actuates the user’s
head to the lower-left, which is depicted in Figure 7 (c) so that thefre extinguisher comes into their view—in other words, our system
actuates the head, which in turn guides the user’s point-of-view(POV).
Now that the user has located the fre extinguisher with thehelp of electrical head actuation, they put out the fre. Our training

Electrical Head Actuation: Enabling Interactive Systems to Directly Manipulate Head Orientation

CHI ’22, April 29–May 05, 2022, New Orleans, LA, USA

Figure 10: Our music volume controller depicts how a UI can use neck movements for both input and output.
simulator renders a virtual extinguishing smoke coming out of
the fre extinguisher prop. While putting out a fre, safety guides
indicate one must scan the surroundings for other runaway fresthat might have spawned from the original fre. Unfortunately, theuser does not see that the shelf above the soldering station is on
fre. Figure 8 (b) depicts how our system assists in the training by
actuating their head upwards, to the new fre.
Figure 8 (c) depicts the user trying to extinguish the fre. Unfortunately, the fre is breaking out in the ceiling, emitting toxic
smoke. The rule in such a situation is: “once a fre starts to spread,your best option is to evacuate the building” [8]. As such, visual
instruction urges the user to get out of the laboratory by meansof the evacuation route. Amidst the smoke (virtually projected inMR), it is harder to fnd the route. Thus, as shown in Figure 9, oursystem assists the user by actuating their head in a trajectory tofollow the exit symbol tracing out the evacuation route. This also
demonstrates how our head actuation can be combined and complement visual cues by assisting the user to follow the moving visualtarget (i.e., exit symbol), that otherwise could be missed.

4

BENEFITS AND CONTRIBUTION

Our key contribution is that we propose a novel interface concept
where interactive systems directly manipulate the user’s head orientation and realize it by actuating the user’s neck via electricalmuscle stimulation. Our approach has the following three benefts.
(1) A new channel for interactive systems to interface with theuser’s head, providing it diferent types of haptic feedback, suchas guidance or force-feedback—this channel does not rely on any
visual, auditory, or vibrotactile cues and can be used either as analternative modality or in conjunction with existing approaches.
(2) Our device does not require any facial attachments or bulky
components, and it does not add any weight above the user’s head.
This enables applications outside of VR, in which users might not
want to wear actuators on their faces. (3) Moreover, because ourapproach controls the user’s head orientations in more directions(nod up/down, turn left/right, and all its diagonals) than prior work
using head-mounted devices [14, 27, 32], we can build applications
that were previously not possible.

5

DEMONSTRATING HOW DIRECT HEADACTUATION ENABLES NEWAPPLICATIONSOur electrical head actuation enables many types of applications,
including some that were not previously possible. To demonstrate

this, we implemented four classes of applications: (1) snapping tospatial information by guiding POV; (2) neck-based I/O interface; (3)transmitting neck movement to another user; and (4) force feedback
around the head for immersive experiences. Although we exhibit
each application in either mixed reality, virtual reality, or the realworld, these applications can be implemented across all of them.
Since our walkthrough covered (1) already, we describe the otherthree below.

5.1

Neck-based I/O Interface for ControllingSound Volume

In this application, we depict how we built a type of application, in
which a UI uses head movements not just as input but also as output,
which demonstrates how our technique can be employed to close
an I/O loop in the neck. Here, we demonstrate this with the example
of a simple music volume controller that the user can adjust using
head movements. As depicted in Figure 10 (a), the user activatesthe system by using a trigger gesture (a triangular head motion
detected by the user’s AirPods Pro earbuds with IMU tracking),
while listening to music; (b) our system immediately takes over andrenders the current value of the music volume by moving the user’s
head up with EMS; (c,d) now, by moving their head voluntarily
up/down, the user adjusts the volume, and as they hit the ends ofFigure 11: Our device can be used to transmit the head orientation between two users. (a) Two users nod to a beat in sync.
(b) A drone’s rotation dictates the user’s neck movement.

CHI ’22, April 29–May 05, 2022, New Orleans, LA, USA

the slider scale (i.e., maximum/minimum volume), the EMS pulls
their head back in the corresponding direction to notify this; and, (e)when fnished adjusting it, the user turns their head right to indicate
the interaction is fnished. Finally, while this example is a type ofsymmetric proprioceptive interaction (as in [34]), it is the frst to
extend this to the neck, which enables hands-free interaction.

5.2

Transmitting Neck Movement to AnotherUser

We leverage our head actuation to transmit the movement of one
user to another’s neck. In Figure 11, we depict a simple applicationthat allows a user to nod their head to a beat and transmit thatrhythmic neck movement to a remote user wearing our system(Figure 11a). This is inspired by bioSync [46] but allows sharing
neck movement rather than that of the arms. By attaching a tracker
to a remote-controlled drone, it is even possible to transmit thedrone’s rotational movement to the user’s neck (Figure 11b); thisallows a new type of human-drone interaction by inverting theconcept shown in Flying Head [18].

Figure 12: (a) The user is experiencing VR boxing. Our system renders force feedback of the (b, c) left/right hooks and(d) uppercut.

5.3

Rendering the Force Feedback of Punches
in VR Boxing

While EMS has been explored in VR for creating force feedback,all existing systems mostly only provide forces to the user’s arm,
forearm, wrist, etc. Instead, in Figure 12, we demonstrate a boxing
simulator in which the user feels the impact of left/right punches
and uppercuts. This is inspired by Impacto [33] but depicts a novel
use of EMS in that it allows the user to feel directional force feedbackon their head.

6

RELATED WORK

Our work is built on interactive systems that physically actuate thehuman body, particularly those for the head and neck. Our work isalso inspired by the more traditionally used modalities (e.g., visual,auditory, or tactile cues) to inform a user to voluntarily change
their head orientation.

Yudai Tanaka et al.

6.1

Traditional Modalities to Inform the User
to Voluntarily Change Their Head
Orientation

First, we review the more traditionally employed approaches that,instead of directly actuating the user’s head orientation involuntarily, inform the user via visual/auditory/tactile cues that they should
voluntarily change their head orientation. In HCI, these approaches
are typical in interactive systems for navigation or view-fnding.
Displaying visual icons (e.g., arrows, circles) that indicate spatial
targets is a powerful method to guide a user’s head orientation.
For instance, JackIn utilizes these visual signs to allow a remotecollaborator to communicate spatial targets from the local user’s
point of view [25]. Visual Guidance also utilizes these visual signs toguide a user’s point of view in 360° videos [30]. Outside-In guides auser’s POV in 360° videos by visualizing the regions of interest thatare outside of their feld of view with picture-in-picture previews[31]. Peripheral LED displays attached to head-mounted-displays(HMDs) can also be utilized for guiding the user’s POV [12, 13, 63].
Similarly, Spider Vision blends the views from two cameras attachedto the front and the back of the user’s head on the user’s HMD sothat they can see the rear view without looking back [7].
Auditory stimuli have also been leveraged for guiding head orientation. For instance, Yang et al. demonstrated that spatialized
auditory beacons are useful to fnd target objects in MR remote collaboration [65]. Taking this further, HindSight automatically detects
both static and moving target objects in the user’s 360° surroundings and notifes the user of their locations by assigning spatializedsound cues to the target objects [54].
Furthermore, vibrations can also be used to guide head orientation; typically, researchers engineer neck-worn or head-worn
vibrotactile devices. For instance, HapticPointer uses 16 neck-worn
vibrotactile actuators to notify the user of 3D targets’ directions[43]. Haptic Collar uses eight neck-worn actuators to assist users
in navigating to a target destination [52]. Neck Strap renders twodimensional spatial directions by modulating waves of vibrations
from one neck-worn actuator [64]. Moreover, larger haptic devices
capable of strong forces can even rotate the user’s whole bodyto face a new direction. For instance, SwiVRchair directly rotates
the user’s body with a motorized swivel chair [15]. These motion
platforms provide new automatic ways to guide head orientation,
but they, unfortunately, are bulky and only suited for experienceswhere the user is sitting or standing inside the grounded device;moreover, these devices actuate the user’s full body and not thejust their head; Thus, as we will see next, researchers have been
exploring ways to mechanically and automatically move the user’s
head orientation.

6.2

Mechanical Devices that Actuate the User’s
HeadAlbeit robotic exoskeletons are the mainstream way to directly
actuate one’s body, robotic head actuation is still an emerging
topic; so far, only used in medical rehabilitation. Only recently,
a few robotic exoskeletons have been engineered for actuating apatients’ neck [62, 67, 68]. In particular, Zhang et al. proposed the
Robotic Neck Brace, the frst and only wearable exoskeleton thatmanipulates the user’s head around three rotational axes (i.e., roll,Electrical Head Actuation: Enabling Interactive Systems to Directly Manipulate Head Orientation

CHI ’22, April 29–May 05, 2022, New Orleans, LA, USA

pitch, yaw) using large mechanical linkages attached to the user’s
forehead and shoulders that are controlled by servo motors [67].
These researchers have characterized the neck motion of patientswith amyotrophic lateral sclerosis (also known as ASL) using thisneck-worn exoskeleton [69]. So far, no robotic neck actuators havebeen utilized for building interactive systems.
In the feld of virtual reality haptics, researchers have developed devices that provide force feedback around the user’s head.
For instance, GyroVR is a fywheel device attached to the user’s
VR headset that can render inertia around the head during headrotations, leveraging the gyroscopic efect [14]. HangerOVER is aforce illusion that generates a yaw-axis pseudo-force by applying
multiple points of normal force around the head, which causes theuser to rotate their head involuntarily; this efect is known as thehanger refex [27]. Finally, HeadBlaster utilizes air propulsion forces
around the head to create left/right and front/back accelerations
by emitting compressed air from nozzles attached to the HMD [32].
While these devices have their unique afordances, they also comewith severe limitations: (1) the gyroscopic efect cannot render force
when the user’s head is static [14]; (2) as with most illusions, thehanger refex is notoriously difcult to control for precise rotation
angles [27]; and (3) devices based on air propulsion force requires
large air compressors (200L), which limits the user’s mobility [32].
We take inspiration from all these works but, instead, are thefrst to explore how electrical muscle stimulation (EMS) allows
interactive systems to directly actuate the user’s head.

of balance using galvanic vestibular stimulation (known as GVS)
[40, 56]. This type of electrical stimulation induces a sense of imbalance, which in turn, causes a refex where the user shifts theircenter of balance with their whole body, subsequently, this also
causes movement of the user’s head.
The critical diference between our work and these prior works
is that our system directly actuates the user’s head by actuating theneck muscles, which—to the best of our knowledge—have neverbeen explored for EMS-based interactive systems. Moreover, it is byactuating the neck that our system also gains access to manipulating
the user’s point-of-view (POV), enabling expressive applications
other than just force feedback.

6.3

To help readers replicate our design, we now provide the necessary technical details. Additionally, all source code and materials
are made publicly available1 .

Interactive Systems that Actuate the User’s
Body via Electrical Stimulation

In contrast to mechanical-based haptic devices [2, 6, 11, 45, 70]
that, inevitably, are constructed from heavy and bulky actuators,
researchers started engineering interactive systems based on electrical muscle stimulation (EMS). Unlike exoskeletons, EMS allows
for unencumbered actuation, which is increasingly popular formobile haptics systems, in which the user roams freely in theirenvironment (e.g., as in untethered VR or AR experiences).
Over the last decade in HCI, researchers built a plethora of theseinteractive systems based on EMS, with the largest emphasis beingon actuating the arms, wrists, and fngers with EMS. For instance,
PossessedHand is an EMS-based system that assists users in learninga new musical instrument by actuating their fngers [59]. Pose-IO
is an EMS-based eyes-free I/O interface that reads and writes the
position of the wrist [34]. Afordance++ is an EMS-based systemthat assists users in interacting with objects they have never seen
before [35]. Researchers have even actuated the user’s hand or index
fnger to follow 3D targets [26, 44], which we take inspiration frombut expand to the neck. Finally, a number of EMS-based systems
have been engineered as a means to provide force feedback in VR[33, 36] or AR [37].
While most of the aforementioned EMS systems focused on actuating the user’s upper limbs, EMS has also been used to interactively
manipulate lower limbs. For instance, Footstriker assists the user
to keep a proper running form by correcting their mid-air leg posture while running [17]. Moreover, CruiseControl is an EMS-based
interactive system that actuates the user’s leg rotation to achieve
redirected walking [48]. Finally, prior work also actuated the sense

7

IMPLEMENTATION

Figure 13: Our wearable system setup.

7.1

HardwareFigure 13 depicts our wearable hardware setup that ensures a user’s
mobility. The system stimulates the user’s neck muscles at 30 Hz
using a medical-grade electrical muscle stimulator (HASOMED
RehaMove3 [50]) which the user carries in a slim backpack. Our
system utilizes four channels of the EMS stimulator to actuate thehead accordingly to the electrode placement found in our preliminary exploration: (1) turn left by stimulating the left side of thesplenius capitis; (2) turn right by stimulating the right side of thesplenius capitis; (3) nod up by stimulating the splenius cervicis; and(4) nod down by stimulating the sternocleidomastoid.
For tracking head orientation, we made our system compatible
with HoloLens 2 [20], HTC VIVE, and AirPods Pro earbuds, suchthat the user equips one of them based on the application type (i.e.,
MR, VR, and real environment). The interface between the EMS
stimulator and the tracking device(s) is a laptop (MSI GL65 Leopard)
running our Unity3D applications; the laptop is also stored in theuser’s backpack.

7.2

EMS Control Loop

PID-based control for static targets. To actuate the user’s headto a target orientation, we implemented a PID controller, designed
specifcally for neck EMS. The PID controller regulates the pulsewidth of the EMS impulses between the range of 0 µs and 300 µs.
1 https://lab.plopes.org/#electrical-head-actuation

CHI ’22, April 29–May 05, 2022, New Orleans, LA, USA

In each cycle, the system calculates the error (e x , ey ) between theuser’s head orientation, given by the current orientation (obtained
from tracking), and the target orientation. Note that the x axis here
represents head rotations around the yaw axis (i.e., turn left/right)
and the y axis represents head rotations around the pitch axis (i.e.,
nod up/down). With the input of e x and ey , our PID controller
individually computes PWx and PWy , which are fnal EMS pulsewidths for actuating the user’s neck to turn horizontally and/or
nod vertically towards the target respectively. Our controller isdual in that, if PWx is positive, it will be set as the pulse-width forthe right-channel; if PWx is negative, it will be negated and set as
the pulse-width for the left-channel; similarly, if PWy is positive,
it will be set as the pulse-width for the up-channel, and vice-versa
for the down-channel. For instance, in one cycle, (PWx , PWy ) =(−100, 200) dictates that left-channel outputs a 100 µs pulse and upchannel outputs a 200 µs pulse (i.e., no pulses are output from rightchannel and down-channel). Via initial pilots, we settled on thefollowing PID coefcients (Kp, Ki, Kd): (16, 4, 2) for static target
acquisition. Finally, note that our PID controller works without any
voluntary movement by users; if the neck’s own springiness or theuser moves away from the target, the PID will push it back without
user intervention.
PID tuning for pursuing trajectories. To actuate the user’s
head to pursue a trajectory, our system moves the target orientation
along the trajectory on a real-time basis. Unlike the static targets,
we adopted the PID coefcients of (Kp, Ki, Kd): (64, 4, 2), since
we found that they provide faster acquisition, which is important
for trajectories, without much noticeable error increase. We also
found that, even with the increased Kp, the head actuation often
falls behind the target’s movement and the errors accumulate overthe pursuit. To address this issue, the software-side of our system
control the target’s speed in real-time so that the angular distance
between the user’s head orientation and the target orientationis always smaller than a certain threshold, i.e., when the error
surpasses the threshold, the target stops and waits for the user’s
head orientation to catch up, and once the error becomes smaller
than the threshold, it starts moving again.
PID discussion. From these empirically determined coefcients,
we can intuit that the main factor driving the pulse-width is the Kp
coefcient (the linear adjustment based on an error to target) withminimal contributions from the Ki and Kd coefcients. However,
we also noted in our pilots that the neck muscles tend to dampen
movement more than the extremities (e.g., arms), which might
explain the relatively low values of I and D coefcients while preventing the resulting actuation from overshooting or oscillations.

7.3

Extending Our System in Unity3D for
Developers

We provide two types of Unity3D prefabs that encapsulate thefunctionality of our system, making it easy to add to existing applications developed in Unity 3D (not limited to VR). Figure 14 showsa screenshot of a user using our prefabs to add electrical head actuation to their VR safari. UI developers drag and drop the prefabof their choice to a 3D object that they wish the user to acquire
using EMS-based head actuation; our system will automaticallyhandle the target orientation calculation, initiate communication

Yudai Tanaka et al.

Figure 14: Our Unity3D prefabs in action while developing
a VR safari.
with the EMS device, and start the PID controllers. As depicted in
Figure 14, these simple Unity3D prefabs allow developers to assign
both static targets (e.g., the elephant) as well as trajectories (e.g., thefying bird). After assigning the latter, the prefab allows a developer
to defne the shape of the trajectory by directly drawing it using
Bézier Path Creator [3]. Then, they can fne-tune the speed of themoving target.

7.4

Tracking, Display, Communication and
Latency

Our head actuation system was implemented to be agnostic of
the tracking system as long as it provides rotational tracking of
the user’s head. We implemented bridges between our system andthe three external tracking systems, which aford diferent use
cases. Note that in all our applications, users tend to wear thecomplete system on their body, typically in a backpack. Naturally,
the choice of a tracking system impacts the quality of the resulting
applications, i.e., using room-level positional tracking (e.g., HTC
VIVE or Hololens 2, which we use for our VR and MR applications)allows applications to actuate the user’s head to point to targets
in physical space; conversely, using only inertial-based tracking
systems (not positional) allows applications that actuate the user’s
head with respect to the inertial frame of reference (e.g., IMUs from
AirPods Pro, which we use for our neck-based I/O or transmittingneck movement applications).
MR tracking. We leverage the tracking of the Hololens 2 to track
both the user’s head position in space and its rotation. This datais streamed in real-time from the headset to the laptop running theUnity application via Wi-Fi, built on top of the Holographic Remoting
Player API [19]. Using the same communication channel, the laptop
streams and displays visual MR scenes to the user’s headset.
VR tracking. We leverage the tracking from the HTC Vive system, using its base stations and the headset.
Mobile tracking. In our most mobile and minimal tracking
implementation, we leverage AirPods Pro, which tracks only theuser’s rotation with its inertial measurement unit. The sensor data
is retrieved via Apple Core Motion API [5]. Since the API can only
be accessed from iOS, the sensor data is frst streamed from an
iOS device (iPhone 11) and relayed to the laptop via Open Sound
Control over Wi-Fi.
End-to-end latency. In our most extreme applications that run
over the Wi-Fi (e.g., transmitting head nods to a remote user) our
system has an end-to-end latency of around 246 ms (calculated
from high-speed video).

Electrical Head Actuation: Enabling Interactive Systems to Directly Manipulate Head Orientation

CHI ’22, April 29–May 05, 2022, New Orleans, LA, USA

ensured that the EMS was able to robustly actuate all the participant’s heads by this quantifable threshold. We repeated the above
calibration procedure for all four EMS channels (i.e., left, right, up,
and down).

8.1

Figure 15: The results for acquiring static targets in eight directions with two distances. The dashed lines represent the
distance between the targets (black points) and the overallmean output head orientations across all participants (bluepoints). The blue ellipses represent the standard deviationsof the mean outputs.

8

USER STUDY #1: EVALUATING THE
ACCURACY OF OUR HEAD ACTUATION

To understand the quality of our system, we conducted a user study
comprised of two tasks, each focused on evaluating a diferent
function of our technique: (1) acquire static targets and (2) follow
trajectories.
Participants. We recruited seven participants (fve identifed as
male, two as female, average age = 25.6 years, SD=1.4) from ourlocal institution. Six participants had previously experienced EMS
on their arms, but none had experienced it in the neck. We excluded
one participant from the analysis who, unfortunately, did not haveenough time for the completion of all trials as each study session
took about 1.5 hours to complete.
Apparatus. Participants wore our EMS electrodes, stimulator,
and a Hololens 2. We used the headset only for tracking and nothing
was displayed during the study. According to a prior work that
evaluated the accuracy of this tracking device on participants’ heads[16], we established that its accuracy was sufcient for our study.
Calibration procedure. Prior to the actual trials, we frst calibrated the amplitude of EMS currents for each participant. For calibration, we applied a constant EMS pulse-width of 300 µs. Startingfrom 0 mA, we increased the current by 1 mA steps, while ensuringpain-free operation prior to the next increase. We repeated these
1mA increase steps until the participant’s head rotates at least up to
30° for left/right/up movements and 15° for down movement. This

Task #1: Acquiring Static Targets in Eight
Directions

In this frst technical evaluation, we measured how our system actuates and holds the participants’ head orientation at 16 static target
positions: eight “cardinal” directions (N, NW, W, SW, S, SE, E, NE)× two distances (the darker points in Figure 15). We denote targets
by their cardinal direction and the numeral 1 or 2 for their distance(i.e., NE2 stands for the furthest target in the Northeast direction).
Target placement was inspired by the anatomical range of the neckand the actuation range observed in our pilots. Importantly, to evaluate the system’s actuation ability, we asked participants not tovoluntarily move their neck during the trials.
Procedure. We asked participants to relax their neck in its neutral position, which we defne as the origin for a trial (the center
black dot in Figure 15). Then, after a randomized waiting period, the
EMS was applied for six seconds, regulated by our PID controller, sothat the head reaches and stays on the target. We compute the fnalhead orientation by averaging the head angles in the fnal secondof the trial and calculated its error (i.e., distance) to the target. Eachparticipant performed 32 trials in a randomized order (16 targets ×
2 repetitions).
Results. Figure 15 depicts the overall mean outputs (blue points)
and their standard deviations (blue ellipses) for acquiring all 16
targets across all trials (6 participants × 2 repetitions).
We observed a mean error of 7.55° (SD=5.89°) across all targets
and trials. Secondly, we observed a smaller error for the closer
targets than for the further away distances. Moreover, we observed acompounding efect of PID errors when actuating in two directions
simultaneously; in other words, the diagonal directions tend to
exhibit more error to target than N, S, W, E. To provide readers witha detailed understanding of the error for each target, we also report
the mean error and its standard deviation (SD) for each target in
Figure 16. Finally, we found that on average, for all 16 targets, it
took 2.60 s (SD=0.79) for our system to acquire a static target. To
calculate this, per trial, we measured the elapsed time from thestart of a trial until the participant’s head reached the fnal headorientation (i.e., the average of the participant’s head rotation in thelast second of a trial; as such, this measure discounts stabilization
or oscillation time).

8.2

Task #2: Pursuing Trajectories

In the second task of our technical evaluation, we measured how oursystem actuates the participants’ head orientation to follow three
simple types of trajectories: a horizontal back-and-forth trajectory,

Figure 16: Mean errors and their standard deviations (SD) for all 16 targets.

CHI ’22, April 29–May 05, 2022, New Orleans, LA, USA

a vertical back-and-forth trajectory, and a sinusoidal trajectory,
which are shown in Figure 17. Again, to focus on evaluating thesystem’s actuation ability, we asked participants not to voluntarily
move their neck during the trials.
Procedure. We follow the same procedure as the previous task.
When we start EMS stimulation, our PID controller actuates theparticipants’ head orientation to follow the moving target along a
trajectory. We stop the stimulation at 0.2 seconds after the target
reaches the end of the trajectory to account for the delay between
the head’s movement and the target. Note that stimulation periods may vary across trials since the speed of the target changed
according to the error. We implemented these moving targets asfollows: the target moved at 60 degree/sec, and when the error
surpassed 15°, the target stopped until the error was below 15°. We
track the participants’ head orientation and calculate the error to
the target point on the trajectory at every time step during thestimulation. Participants performed six trials in a randomized order(3 trajectories × 2 repetitions).
Results. Figure 17 depicts average trajectories (thick blue lines)
and raw trajectories (light blue lines) derived from all participants’
data for the three target trajectories. To obtain the average trajectories, we equalized the data length of each trial by means of timenormalization.
For the horizontal back-and-forth trajectory, depicted in Figure
17 (a), we observed a mean error of 8.92° (SD=4.56°) across all time
steps and all trials. As shown in the box plots in Figure 17 (a), we
also quantifed horizontal under/overshoot of the PID around the
trajectory’s leftmost and rightmost points based on the leftmost
and rightmost points that the participants reached. We found thata mean output horizontal angle for the leftmost point was -25.21°(SD=4.98°) and that for the rightmost point was 25.3° (SD=6.87°)
across all trials.
For the vertical back-and-forth trajectory, depicted in Figure 17(b), we observed a mean error of 8.0° (SD=3.32°) across all time steps
and trials. Again, we also quantifed vertical under/overshoot of
the PID around the trajectory’s top-most and bottom-most points
depicted in the box plots in Figure 17 (b) based on the top-most andbottom-most points that the participants reached by calculating
the standard deviation of the trajectory’s top-most and bottommost points, which depict sharp (180°) turns in motion. We foundthat a mean output vertical angle for the left-most point was 25.8°(SD=2.79°) and that for the right-most point was -16.1° (SD=4.57°)
across all trials. In line with our previous study, this suggests thatthe vertical axis (up/down) has the best precision, compared to thehorizontal axis (left/right).
For the sinusoidal trajectory, depicted in Figure 17 (c), we observed a mean error of 11.41° (SD=3.76°).
Finally, the technical evaluation also demonstrated that our system is relatively fast for EMS-based dynamic target acquisition. Our
system actuated participants’ heads to acquire these trajectories
in < 10s (for comparison, Muscle-plotter takes 16.2 s (SD=4.9 s) toplot trajectories by actuating the wrist [38]), specifcally taking an
average of: 7.83 s (SD=3.5 s) for the horizontal trajectory; 5.58 s(SD=4.69 s) for the vertical trajectory; and, 6.03 s (SD=5.33 s) for
the sinusoidal trajectory.

Yudai Tanaka et al.

Figure 17: The results of the trajectory acquisition (a) horizontal back-and-forth; (b) vertical back-and-forth; and, (c)sinusoidal. (black lines depict target trajectories, blue curves
depict average trajectories and light-blue curves are raw trajectories).

Electrical Head Actuation: Enabling Interactive Systems to Directly Manipulate Head Orientation

8.3

Framing Our Results in the Context of Our
Proposed Applications

We now discuss the resulting accuracy found in our study (ranging
from an average of 7.55° for static target acquisition up to an average of 11.41° for acquiring targets on a sinusoidal trajectory) with
our four applications presented in Section 5. First, for applications
in which the neck-based interactions are the only available modality, such as our Neck-based I/O or our Transmitting Neck Movement
applications, the resulting accuracy directly limits the applications’
resolution. In this case, we recommend that these applications aredesigned with the accuracy limitation in mind. For instance, a designer can space static haptic events within the user’s range of headorientation at least 7.55° apart. On the other hand, for applications
in which the head actuation provides an additional modality, theresulting accuracy might not dramatically impact the applications’
resolution. For instance, the resulting accuracy is well suited forforce-feedback-based applications, such as our VR Boxing application, since these require a coarser type of head actuation. Moreover,for POV guidance applications, such as our AR Fire Safety application, one can frame the above results while taking the size of auser’s feld-of-view (FOV) into consideration. Specifcally, one canconsider the focused area of the FOV, which roughly comprises
a square of ±15° (horizontal angle) × ±15° (vertical angle), whereusers reliably recognize symbols [42]. While the overall mean error
such as 7.55° (for static targets) and 11.41° (for the sinusoidal trajectory) might appear as a large error when thinking of gaze (eyemovements), it is relatively small for one’s point-of-view since it
is still within the focused area of FOV. In other words, missing atarget by 7.55° implies this target can be still seen as a symbol in
one’s POV [42]. However, those of NW2 (Mean: 18.07°, SD: 6.14°)
and NE2 (Mean: 16.27°, SD: 6.58°) are proximal to this boundary,
implying that a user might miss those targets.

9

USER STUDY #2: EXPLORING THE
EXPERIENTIAL SIDE OF ELECTRICAL
HEAD ACTUATION

While in our frst study, we validated the accuracy of actuating
participants’ heads to diferent angles, we now turn into understanding users’ experiences while wearing our system. This study
was not designed with the intention of evaluating performancespecifc metrics, or comparison to other modalities, but instead, of
extracting insights from participants’ comments or behaviors while
using our system. This study was approved by our Institutional
Review Board (IRB21-1158).

9.1

Study Design

Participants. We recruited eight participants (fve identifed as
male, three as female, mean age = 22.1 years old; SD=2.1) fromour local institution; none had partaken in our frst user study andnever experienced EMS on their neck. Each study took about 1.5hours to complete. Participants received 50 USD as compensation.
Tasks. Participants explored our four applications adapted toft the study format: (1) AR fre safety, we asked participants to
fnd a real hidden fre extinguisher and put out four virtual freshidden around the room. Our system guided their head to the next

CHI ’22, April 29–May 05, 2022, New Orleans, LA, USAtarget when they were less than 2.5 meters away from it. Also, tofacilitate participants’ input while holding a heavy fre extinguisher
as a prop, we asked participants to verbally say “press” and theexperimenter sent a message to the HoloLens application via Wi-Fi
that triggered a virtual extinguishing smoke and put out the fre.
(2) Sound control, we asked seated participants to use our system toadjust the sound volume of a background music track to specifc
volume settings, while also watching YouTube on a tablet device;
for simplicity, we omitted the activation gesture and instead thesystem activated itself at four pre-determined moments. (3) Nod tothe tempo, this task was a variation of our previous transmittingneck movement application, but we asked participants to drum
along to the tempo that they felt via the EMS-induced head nods.
To make this experience comparable across participants, instead oftransferring the head nods from the experimenter to the participant,
we utilized a pre-scripted tempo, which participants had no priorknowledge of. This tempo started at 45 bpm and shifted every 30
seconds: frst to 55 bpm, then down 30 bpm, and fnally, up to 45
bpm. (4) VR boxing, we asked participants to play a boxing matchagainst a virtual avatar controlled by the experimenter. To makethe experience’s duration consistent across participants, the avatar
fell on the mat once participants punched it after two minutes haveelapsed.
Procedure. We calibrated our EMS device to robustly actuate
the participant’s head in all directions, while remaining in painfree operation, following the same procedure as in our frst study.
Then, they experienced all four applications in a counterbalanced
order. We made sure that they interacted with each application for
at least two minutes. Moreover, since the objective of this study
was to extract insights from the participants’ experience, we asked
participants to "think out loud" and voice any comments also during
the trials. With their consent, we videotaped and transcribed thestudy.
Interviews. We followed each trial with a semi-structured interview, which started with two general experience-related questions
using a 7-point Likert scale: (1) "How much did the head actuation
contribute to your experience?” and "How much did you enjoy
the experience?”; for each question, we followed by asking “why?”.
Then, we followed with a set of questions that allowed us to elicit
comments about the participants’ direct experience: (3) "In yourown words, can you tell us what you thought the head actuation
was trying to depict in this application?"; (4) "How it felt when
the EMS frst moved your head in this application"; (5) "How did
you feel about the remainder head actuations in this application".
After participants fnished all four trials, we invited them for a
fnal interview, which was guided by the following questions: (6)"What else could you imagine yourself using this technology beingfor?"; (7) "What aspects of this technology do you think need to
be improved?"; (8) "Any other aspect of your experience that you
would like to share with us". Finally, we conducted a post-study
survey to collect feedback (in the form of open-ended questions) regarding the participants’ comfort and perceived safety while using
our system.

CHI ’22, April 29–May 05, 2022, New Orleans, LA, USA

Figure 18: Participants raring to the contribution of head actuation to their experience and enjoyment (black line: median).

9.2

Results

Figure 18 depicts participants’ overall ratings aggregated across all
applications regarding the contribution of head actuation to theirexperience (M=5.4; SD=1.2) and their enjoyment (M=5.6; SD=1.1),
suggesting that participants overall felt that the head actuation contributed positively to their experience and resulted in an enjoyable
experience. As for the contribution of head actuation to their experience, per application, we observed: AR fre safety (M=5.6; SD=1.3),
sound control (M=5.8; SD=1.3), Nod to the tempo (M=5.0; SD=1.2),
and VR boxing (M=5.3; SD=1.3). As for participant’s enjoyment, perapplication: AR fre safety (M=5.8; SD=1.3), sound control (M=5.4;
SD=1.2), Nod to the tempo (M=5.4; SD=1.1), and VR boxing (M=5.8;
SD=1.2). Now, we turn our attention to the participants’ comments
and observed behavior.
AR fre safety (Figure 19 a,b). All participants described how
the head actuation helped them by guiding their point of view;the comments included “the [actuation] helped me pinpoint where

Yudai Tanaka et al.

exactly I need to look at” (P2) or “it defnitely helped by directingme (...) so if I didn’t have that I would never be able to fnd the fre”
(P3). P6 described their impression on the head actuation: “initially,
I was expecting [the actuation] to feel unnatural, (...) but when I
saw the extinguisher, it felt natural”. P7 reasoned that the system’s
helpfulness was akin to a remote collaborator: “felt like somebody
in a command center who has a better view of the environment (...)
was guiding me”.
Sound control (Figure 19 c,d). Six participants described thehead actuation as useful in adjusting the volume; the comments
included “in terms of multitasking (...) it was very efective” (P1).
Three participants mentioned the usefulness of the system depicting
the endpoints of the volume scale, such as P6: comments included“it’s very useful because it tells me when the volume goes overthe maximum or below the minimum by pulling my head back”.
By utilizing this haptic information, P7 playfully maximized andmuted the sound according to the beat of one of the songs (seeour video fgure). P5 and P7 justifed their enjoyment from theirbidirectional relationship with the system; for instance, P5 stated“I really enjoyed me being in control with the system (...) but also
me controlling the system with my head”.
Nod to the tempo (Figure 19 e,f). Five participants described
how the head actuation helped them feel the tempo. Their comments included “it was pretty efective (...) I was able to clearlyfeel which bpm [beats per minute] I was supposed to play” (P4),or “after a couple [of actuations], I felt I was able to anticipate thetempo” (P2). P2 also stated how they felt during the head actuation:“the head bobbing made me feel more physically in sync with therhythm”. Surprisingly, P8 regarded the head actuation as not only
depicting the beat but also another person’s musical intentions:“[the actuation] let me know what the other individual is thinking(...) and the music rhythm”. We observed that, after a while, someparticipants enjoyed amplifying the EMS-induced head nods byvoluntarily nodding stronger at the same time, while others let the
EMS do all the nodding.
VR boxing (Figure 19 g,h). Six participants described how thehead actuation added realism; their comments included “without

Figure 19: Photos from participants using our system (reproduced with participants’ consent). The system actuates the participants’ heads to look at the fre extinguisher, or the fre above (a,b). The system moves the head upward and downward tocommunicate the sound volume of background music (c,d). The participants enjoy playing the drum while feeling the tempo
via up-down head actuation (e,f). The participants feel the haptics of left and right punches in VR (g,h).

Electrical Head Actuation: Enabling Interactive Systems to Directly Manipulate Head Orientationit, (...) it would be so much more boring [because of] no genuine
physicality to the person” (P1), or “the punches feel material and [it]defnitely feels more like boxing environment” (P8). We observedthat P2 was so immersed that they stepped back and kept a distancefrom the boxer in VR as if it was a real fght (see our video fgure),
which they also alluded to during their interview: “I did feel really
immersed at one point when there was a kinda standof where
I wasn’t punching and he wasn’t (...) it felt like the stakes werehigher”.
How did you feel regarding the actuation? First, regarding
the sensations evoked by EMS actuation, all participants mentionedthat they became increasingly “used to” the head actuation as theexperiences progressed. We saw this at the single application level,as highlighted by comments such as “after a few times, it became a
lot more fuid” (P1, during AR fre safety), and “it was surprising at
frst and I felt a little bit rigid but, after a while, I got more used toit” (P2, during nod to the beat). Moreover, we also saw this efectacross the duration of the study, as highlighted by comments suchas “at this point, [since this was their fourth trial] (...) I’m pretty
used to it” (P4, during AR fre safety) and “it was quite expected, (...)
I already knew how the system works, so I didn’t get surprised” (P5,during sound control). With regards to perceived safety and comfort, which we probed via our post-study survey, all participants
described they “felt safe” during the study; the comments included“due to its adjustability it was a safe and easy process” (P3), or “I
thought the incremental increase of the charge level [during thecalibration] (...) gave me a sense of safety” (P7). As for their comfort,
four participants mentioned directly that “it was comfortable”; conversely, the remaining participants had some mention of discomfort
mostly associated with the tingling sensation caused by EMS; thecomments included “the electric pulse produces discomfort during
usage [while it] doesn’t cause pain” (P8). Moreover, P8 also addedthat “the involuntary muscle contractions feel quite disturbing”, atype of reaction that is not uncommon to EMS, likely attributed tolosing the sense of agency [24].
Application ideas. Five participants described applications related to guiding one’s point of view, such as “narrating VR theatercontents by directing user’s attention” (P2) and “street navigation
using [head actuation]” (P4). P6 and P7 proposed the use of oursystem for potentially improving the user’s incorrect neck posture.
Improvement suggestions. Six participants felt the EMS calibration needs improvement. Five suggested to make it faster, suchas P4: “would need to be quicker (. . .) it would deter people from casually using it” or P7 who suggested eliminating manual calibration
for an automatic calibration: "you can use sensors to auto-calibratethese things [EMS]". Moreover, P1 and P7 suggested that the device
could be further miniaturized to "something like a necklace or aneck brace" (P1) and " make these things (. . .) invisible" (P7).

10

LIMITATIONSFirst, like any other interactive systems based on EMS, our system requires calibration and has limited accuracy/range, which
we found in our study to be overall 7.6° error for pointing static
targets and 9.4° error for following trajectories within the following range: 30° in the left/right directions; 30° in the up direction;and 15° in the down direction. Second, our system can only move

CHI ’22, April 29–May 05, 2022, New Orleans, LA, USA

the user’s head orientation to anatomically reachable targets (e.g.,
it cannot rotate the user’s head by extreme angles, such as 180°),
instead camera+HMD combinations can show a view of what lies
behind the user [1, 7, 31]. Third, while actuating the user’s headorientation might prove useful for interactive systems to guide theuser’s point of view, we acknowledge this is not the only factor
that determines where a user is looking since the user’s gaze area key factor to navigate inside their feld of view determined bytheir head orientation. Lastly, as a consequence of our explorationof which muscle sites are robust for actuation of the head around
independent degrees of freedom, we found that tilting the head tothe left/right shoulders was not independently actuated without
inducing parasitical motion on other directions; thus, unlocking
these might require future research.
Finally, it is important to note that we do not think of our novelneck-EMS technique as a means to replace existing (visual, auditory,tactile) cues to guide head movements but rather as a new andcomplementary modality that will also enable novel applications
and can be combined with prior work.

11

CONCLUSIONS AND FUTURE WORK

We proposed, engineered, and explored a novel interface concept inwhich the interactive system actuates the user’s head orientation.
We implemented this concept by applying electrical muscle stimulation (EMS) to the user’s neck muscles, enabling our system toactuate the user’s head orientation around its yaw and pitch axis.
As the frst exploration of EMS for head actuation, we characterized which muscles can be robustly actuated. Second, we evaluatedthe accuracy of our system for actuating participants’ head orientation. Third, we demonstrated how it enables new interactions
by building a range of applications. Finally, in our second study,
participants felt that our head actuation contributed positively totheir experience in four distinct applications.
We believe our work will inspire researchers to further delve
into this avenue of directly controlling the user’s head orientation.
We believe there are several ways that researchers can further
expand on our work, and, to accelerate this, we have provided all
the source code1 . Possible future investigations include exploring
how electrical head actuation (1) interacts with eye-tracking andvisual gaze redirection, (2) impacts the sense of agency (i.e., “whois looking at the target” [24]), (3) might enable to control the user’s
neck posture over time (e.g., which has been explored also via arobotically actuated computer monitor [55]). Our work provides afoundation for researchers to explore these challenges and expand
on our fndings or system in novel directions.

ACKNOWLEDGMENTS
This work was supported by NSF grants 2047189. Any opinions,fndings, and conclusions, or recommendations expressed in thismaterial are those of the authors and do not necessarily refect theviews of any funding agencies. We are grateful for the feedback
from our reviewers.

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