Haptic Source-effector: Full-body Haptics via Non-invasive Brain
Stimulation
Yudai Tanaka

Jacob Serfaty

Pedro Lopes

University of Chicago
yudaitanaka@uchicago.edu

University of Chicago
jserfaty@uchicago.edu

University of Chicago
pedrolopes@uchicago.edu

Figure 1: We propose & explore a novel concept in which a single on-body actuator renders haptics to multiple body parts—even
as distant as one’s foot or one’s hand—by stimulating the user’s brain (a). We implemented this by mechanically moving a coilacross the user’s scalp. As the coil sits on specific regions of the user’s sensorimotor cortex it uses electromagnetic pulses to
non-invasively & safely create haptic sensations, e.g., touch and/or forces. For instance, (b) recoil of throwing a projectile, (c)impact on the leg, (d) force of stomping on a box, (e), impact of a projectile on one’s hand, or (f) an explosion close to the jaw.

ABSTRACT
We propose a novel concept for haptics in which one centralized
on-body actuator renders haptic effects on multiple body parts bystimulating the brain, i.e., the source of the nervous system—we callthis a haptic source-effector, as opposed to the traditional wearables’
approach of attaching one actuator per body part (end-effectors).
We implement our concept via transcranial-magnetic-stimulation(TMS)—a non-invasive technique from neuroscience/medicine inwhich electromagnetic pulses safely stimulate brain areas. Our
approach renders ∼15 touch/force-feedback sensations throughout
the body (e.g., hands, arms, legs, feet, and jaw—which we foundin our first user study), all by stimulating the user’s sensorimotor cortex with a single magnetic coil moved mechanically acrossthe scalp. In our second user study, we probed into participants’experiences while using our haptic display in VR. Finally, as thefirst implementation of full-body haptics based on non-invasive
brain stimulation, we discuss the roadmap to extend its interactive
opportunities.

CCS CONCEPTS• Human-centered computing → Haptic devices; • Hardware→ Emerging interfaces.

KEYWORDS
Transcranial Magnetic Stimulation, Electrical Muscle Stimulation,
Haptics
ACM Reference Format:
Yudai Tanaka, Jacob Serfaty, and Pedro Lopes. 2024. Haptic Source-effector:
Full-body Haptics via Non-invasive Brain Stimulation. In Proceedings of
the CHI Conference on Human Factors in Computing Systems (CHI ’24), May
11–16, 2024, Honolulu, HI, USA. ACM, New York, NY, USA, 15 pages. https://doi.org/10.1145/3613904.3642483

This work is licensed under a Creative Commons Attribution International 4.0 License.

1

CHI ’24, May 11–16, 2024, Honolulu, HI, USA© 2024 Copyright held by the owner/author(s).
ACM ISBN 979-8-4007-0330-0/24/05
https://doi.org/10.1145/3613904.3642483

Enabling users to leverage all their senses while immersed in interactive experiences has been one of the driving visions in HumanComputer Interaction—as Sutherland put it “[the interface] should

INTRODUCTION

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

serve as many senses as possible” [80]. To this date, an immensebody of work in haptics has engineered specialized actuators todeliver skin sensations to the user’s body (e.g., contact [75], texture[10, 96], pressure [37]).
The overwhelming majority of haptic devices are engineeredusing the same principle, i.e., an actuator is mounted atop thepart of the skin that is meant to be stimulated, e.g., finger-worn
haptic devices deliver tactile cues to the fingerpad [37, 55, 75],
foot-worn devices deliver tactile cues to the soles [79, 93], headworn devices deliver tactile cues to the face [15, 65], and so forth—this approach can also be referred to as haptic end-effector [42],a term borrowed from robotics literature, where it denotes how
the actuator is attached at the very endpoint of a robotic extremity.
Similarly, in this context, a haptic end-effector denotes how theactuator is attached at the very endpoint of the user’s nervous
system, e.g., a vibration motor attached atop the tactile receptors
at one’s fingertips [31]. Users feel haptic sensations delivered by
end-effectors via the following process: (1) mechanical stimuli
are translated to electrical impulses (by mechanoreceptors at theendpoint), (2) electrical signals propagate via the peripheral nervous
system (via neurons from limbs to the spinal cord), and, finally, (3)signals are processed in the source location where all the neurons
flow (i.e., the brain, in particular, the somatosensory cortex).
This end-effector approach has been successful in that devices
can deliver precise temporal and spatial cues. However, few examples of these devices lend themselves well to full-body haptic
experiences, which reveals a research question to which our field
has not found new answers: how might we scale up haptic devices
given that the current approach requires many devices to rendersensations on many locations of the user’s body?
Thus, to propose a new approach, we take the inverse conceptual
turn. Instead of placing haptic end-effectors on the endpoints of theuser’s nervous system, we propose delivering full-body haptics via
a single haptic device that directly stimulates the user’s brain areas
responsible for creating haptic sensations—we call this a sourceeffector.
To create the first instantiation of this concept, we turned to transcranial magnetic stimulation (TMS)—a technique from medicine
that allows for safe & non-invasive brain stimulation. To translate the TMS technique into readily usable knowledge for humancomputer interaction, we (1) characterized, by means of a user study,
which areas of the body TMS can elicit haptic sensations; then, (2)engineered a haptic device that sits atop the user’s head and is able
to position the TMS coil at strategic locations to generate haptic
sensations (i.e., either touch or touch & force) in nine locations of
the users’ body: hands, arms, legs, feet, and jaw, as shown in Figure
1—all by stimulating the user’s sensorimotor cortex with a single
magnetic-coil moved mechanically across the scalp; furthermore,(3) we characterized, by means of a second user study, the user
experience created by our device in a virtual reality application;
finally, (4) we mapped out the possible evolution of this concept,
including its hardware challenges.
Finally, we see this approach as the first step towards exploring new methods to scale haptic feedback to multiple body areas
without encumbering users’ bodies. Naturally, as a first step in thisdirection, our current implementation using transcranial magnetic
stimulation is not without shortcomings. We will discuss these in

Yudai Tanaka et al.

detail later as we believe they are crucial for identifying the next
research challenges to be tackled. However, we believe that ourwork provides technical insights that will inspire researchers to
explore new approaches for haptics.

2

RELATED WORK

Our work is built primarily on approaches towards full-body haptic
feedback, which we organize by their conceptual takes on delivering
haptic sensations: to the endpoint (e.g., attaching a haptic endeffector to the palm, or even sending an air) or to a midpoint (e.g.,
electrically stimulating nerves in the arm to create touch on thepalm). We discuss the differences between these approaches andour source-effector. Finally, since our approach is one of the very
few that makes use of transcranial magnetic stimulation (TMS)
in an interactive context, we briefly review the working principle
behind this non-invasive brain stimulation technique.

2.1

Full-body Haptic Feedback by Stimulating
the Endpoint

N:N haptics (N actuators, N locations): The most popular way
to realize full-body haptics has been to attach an actuator per location where a sensation should be felt—hence N:N haptics. Someof the first explorations of this approach date back to the 1990s,
when researchers used 10 vibrotactile actuators placed at the hands,
feet, legs, and head [97]. Later, Lindeman et al. took this further
with 16 vibrators, even including solenoids and fans [47]. Because
attaching/removing individual actuators on many parts of one’s
body is extremely time-consuming for the user and requires many
fixation points that encumber the user (e.g., straps), researchers
opt to integrate these vibrotactile actuators in haptic suits—both[97] and [47] are canonical examples of haptic suits, while many
more exist in research [19, 40] and even in industry (e.g., bHaptics[60]). This haptic-suit approach improves wearability & comfort
by trading it off with design flexibility (e.g., it is harder to reconfigure the location of the devices depending on body size, individual
preferences, goals of the application, and so forth).
1:N haptics (1 actuator, N locations): Counter to the previous approach, an emergent and more experimental way to deliver
haptics to the endpoint, is to use a single actuator. In these systems, there is only one actuator but the user feels sensations in
multiple locations—hence 1:N haptics. One way to enable this is
to utilize the air around the user: as demonstrated by AIREAL, an
air vortex that stimulates the skin up to 1 m away [77]. This approach has been then extended to present tactile sensations at thehead, shoulders, and legs [29, 81]. Another approach is to attacha single haptic actuator to an endpoint but then move it mechanically to reach different limbs. For instance, Calico is an on-cloth
robotic platform that moves a small end-effector across the user’s
clothes, providing vibrations from arms to legs [73]. As emergent
techniques, these are not without limitations. In the case of air
vortexes, these require line of sight, intensity decays over distance,and have a limited interactive volume, etc. Conversely, moving
wearables on the body is relatively slow and can require rails added
to one’s clothing, etc. Most importantly, all of these techniques
cannot deliver force feedback—these provide only tactile cues (e.g.,
soft puffs of air or vibrations). That said, these devices conceptuallyHaptic Source-effector: Full-body Haptics via Non-invasive Brain Stimulationalign with our goal, i.e., building a centralized haptic device capableof delivering haptics to multiple locations. In addition, these are
flexible in adding new points of sensation easily (i.e., pointing the
air cannon or moving the on-body robot). Thus, we are inspired bythese centralized devices, yet seek to explore a novel approach thatbreaks away from having to stimulate the endpoint.

2.2

Haptic Feedback by Stimulating the
Midpoint (Peripheral Nerves)

An approach, which we draw inspiration from, is to create sensations in a target area, but using actuators attached to another patchof skin that communicate via peripheral nerves. These techniques
typically work by electrically stimulating nerves to induce sensations ”remotely” at the endpoint of this nerve. This can take theform of touch-feedback (electro-tactile) or force-feedback stimulation (electrical muscle stimulation).
Tactile sensations from the midpoint. One way to create
tactile sensations without attaching a haptic end-effector to theendpoint is to stimulate the nerve that leads to it—this is known
as inducing a referred sensation, i.e., “somatosensory feelings thatare perceived to emanate from a body part other than, but in association with, the body part being stimulated” [53]. While thiswas initially only explored in prosthetics to provide tactile feedback to the prosthetic hands or feet of amputees [14, 48, 64], it
has also been adapted to enable users to feel tactile sensations intheir hands or fingerpads without attaching electrodes directly tothese areas; instead, electrodes are placed on the wrist to create
sensations in the hand [63, 66], or other strategic locations more
proximal to the peripheral nerves such as the back-side of the hand[86], palm, [1] or base of the fingers [99, 100] to induce sensationsin the fingerpads.
Force feedback from the midpoint. Similar to this, withelectrical muscle stimulation (EMS), it is possible to create force
feedback without requiring mechanical actuators (e.g., motors or
exoskeletons). Electrical impulses are sent to electrodes placed atop
a muscle (midpoint), which in turn cause a movement on the limb
(endpoint). For example, stimulating the forearm can cause thefingers [83, 84] or the wrist to move [36, 49]. Similarly, stimulating
the neck can move the head [85], or even stimulating the calf [32]or thigh [68] can move the foot.
Pushing past the midpoint. While we take inspiration fromthese techniques, we ask the natural question: how would an interactive system look like if we moved past the midpoint? The logicalconsequence of this is to actuate the source of the nervous system,
i.e., the user’s brain, to generate sensations even at larger distancesand from a centralized haptic device. To find a suitable method tostimulate the brain, we turn to the field of neuroscience where anumber of brain stimulation techniques are used in research & therapeutics. There are several established techniques to achieve this,however, they range in their applicability outside of the medical
domain. First, invasive techniques involve implanting electrodes inthe brain, via a surgical procedure. Given this stringent requirementof implantation, this technique (known as intracortical microstimulation [11]) is typically used for extreme medical cases, such as
researching prosthetics with tactile feedback for amputees [24].
On the other side of the spectrum stand non-invasive techniques,

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

which trade off accuracy for safety and wide applicability. One
such non-invasive technique that has been extensively explored forthe last decades is transcranial magnetic stimulation (TMS) [30].
Before we turn into the details of this technique, we note that TMS
is not the only non-invasive brain stimulation approach; others
include transcranial direct-current stimulation [62] or transcranial
focused ultrasound stimulation (tFUS) [44]. With regards to tDCS,
it is a neuromodulation technique that uses small electrical currents
delivered via electrodes on the scalp; as a modulation technique, it
typically does not have the accuracy nor amplitude to induce haptic
sensations and is used for neuropsychiatric conditions [61]. On
the other hand, tFUS is a nascent stimulation technique that uses
focused ultrasound, delivered by a transducer placed atop the scalp[12]. While tFUS might eventually resolve some limitations of TMS(which we will discuss later in detail, e.g., device weight), its safety
protocols and guidelines are still under development, unlike thoseof TMS, which have been established for decades [71]. Therefore,
we decided to implement our concept using TMS.

2.3

Transcranial Magnetic Stimulation

Transcranial Magnetic Stimulation (TMS). TMS is a techniquethat originated in medicine/neuroscience [27], in which a magnetic
coil placed atop the user’s head produces a magnetic field that oscillates rapidly. This, in turn, creates electrical currents, known
as eddy currents, inside the brain [30], without the need for implanted cables or electrodes [24] (as with other brain stimulation
techniques)—it is thus, non-invasive.
TMS in neuroscience as a tool to understand brain function.
TMS has been popularized as a tool for neuroscience since first
demonstrated by Barker et al. in 1985 [5]. It is frequently used
to map brain functions to cortical areas, e.g., motor control [95],vision [9], language processing [23], working memory [57], andmore.
TMS as a medical intervention/treatment. TMS has also
been extensively used as therapy for improving motor function in
Parkinson’s patients [43], reducing epileptic seizures [26], and as adepression treatment [28].
Peripheral magnetic stimulation (e.g., muscles or touch).
While magnetic stimulation finds its primary use in stimulating thebrain, it can also be used for peripheral nerves, placing its coil atopmuscles (similar to EMS) or skin (similar to electro-tactile). Whilethe latter (tactile) is rather rare, it has been used for creating tactile
sensations in mid-air without the need for direct contact [38, 39].
The former (magnetic muscle stimulation), has been used for muscle
rehabilitation [8], gaiting [82], and even swallowing [56]. Moreover,recently, it has also been used for interactive force-feedback [87].
It is worth noting that, unlike TMS, these magnetic skin/muscle
actuation systems create sensations by stimulating the periphery—these are still midpoint approaches.
TMS in interactive systems. Although it is known that TMS
applied to the motor cortex induces limb movement [95] and/or
touch sensations in the hand [25], its presence in human-computerinteraction remains limited. Recently, Bassolino et al. demonstrated
that TMS to the motor cortex improves the sense of embodimentin a VR rubber hand illusion [6]. While this implies the utility ofhaptic feedback via TMS, to our knowledge, there have been no

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Yudai Tanaka et al.

Figure 2: Overview of our mechanical actuator to interactively displace the TMS coil over key areas of the user’s brain.

interactive devices that leverage TMS to extend haptics to the user’s
full body.

3

IMPLEMENTING SOURCE-EFFECTOR VIATMSTo help readers replicate our design, we now provide the necessary
technical details (hardware & software), alongside a discussion ofimportant safety considerations for any future interactive system
based on TMS. Furthermore, to accelerate replication, we provide
all the source code & hardware of our implementation1
Design rationale. As the first instantiation of the concept of ahead-worn centralized haptic device, we focused on understanding
the range of sensations & locations that can be created using TMS
rather than optimizing for form-factor or ergonomics. As such, we
opted to implement a versatile design that could move a powerful
medical-grade TMS coil. This gave us maximum flexibility in ourfindings, as the coil has excellent stimulation depth; in other words,
using smaller coils would result in a smaller device, but would notallow us to learn the full range of haptics that TMS can unleash.
Now that we know the range of haptics that can be generated via
TMS (Study#1) and the afforded user experiences (Study#2), weare poised to also explore new design trade-offs. These include thepotential use of multiple smaller coils rather than moving a single
powerful coil, which we will discuss later.

3.1

Hardware Implementation

We engineered the actuation system shown in Figure 2, capable ofmoving a TMS coil (Magstim D702 butterfly coil) around the user’s
head, which is connected to a medical-grade magnetic stimulator(Magstim Super Rapid2).
1 https://lab.plopes.org/#source-effector

Robotic platform. The key component in our hardware implementation is a robotic gantry that mechanically moves the TMS
coil across key areas of the user’s scalp. Our design is inspired bya traditional X-Y-Z gantry system commonly found in CNC machines [51], but with key modifications that allow it to: (1) mostimportantly, conform to the curvature of the scalp around the pitch
axis (i.e., front/back), which is estimated to be ∼18 degrees forthe sensorimotor cortex area, based on our measurement from astandard head shape dataset [46]; (2) accommodate different heads,
including different curvatures and sizes; (3) actuate with sufficient
force to move a medical-grade TMS coil (∼1 kg); (4) actuate withsteps smaller than 8.5 mm, as determined by our Study#1; and, finally, (5) provide a structure that can be either directly mounted to a
VR headset or suspended from the ceiling (as in [52, 78]). To satisfy
these requirements we engineered the actuation system shown in
Figure 2.
Actuators. We feature three actuators, respectively, to move thecoil in the X- (ear-to-ear translation), Y- (nose-to-back translation),and Z- (height away from the scalp) axes. Since the X-axis exhibits
most curvature as the coil moves towards the ear, we actuate it
via a servo motor (Feetech FT5335M; maximum torque=392N·cm)
with a built-in encoder, this offers a reliable, fairly compact, strong
way to actuate the coil—as the coil moves closer to the ear it exerts
more torque force to the servo lever. As we found in our technical evaluation, we can move the servo reliably & repeatably at
4.2 mm steps. For the Z-axis (height away from the scalp) we employ a linear actuator (Actuonix L16-P; gear ratio=63:1; maximum
load=100N) with a built-in encoder, mounted directly to the lever of
the aforementioned servo. Finally, for the Y-axis (as the coil moves
from nose to back) we also use a linear motor (Actuonix P8-P; gear
ratio=165:1; maximum load=110N) with a built-in encoder. The
Y-axis is mounted to the endpoint of the Z-axis actuator and, ultimately, holds the coil via a custom metallic adaptor. As we foundHaptic Source-effector: Full-body Haptics via Non-invasive Brain Stimulationin our technical evaluation, we can move the Y- and Z- actuators
reliably & repeatably at 1.9 mm and 2.6 mm steps respectively. Sofar, what we described is a typical gantry (with carefully chosen
actuators that are strong enough to move the TMS coil), however,with the standard design with fixed perpendicular axes, we would
not be able to account for the head’s curvature. This would requireat least adding more actuators to tilt the coil. Instead, to avoid moreactuators, we opted to build a structure with passive mechanisms.
In the following, we describe the structural elements that make thispossible.
Structural elements. To account for curvature, the Z-axis
needs to be lifted as the coil traverses the head. One option, that
we explored in the early phase of the engineering, was to let thecoil hang entirely at the end of the Y-axis, with no further structure
to support it. This is similar to some CNC machines that optimize
for space and do not feature two rails per degree of freedom (e.g.,
EasyThreed X1). However, we noted this increased the slack andbacklash due to the coil’s weight. As such, we added a mount on
the faceplate of the VR headset (Meta Quest 2), which is depicted
in Figure 2. The faceplate was drilled and secured to our custom
mount using M4 screws and a ball bearing. To enable movementwith the new mount point, we incorporated a passive mechanism
comprised of linear rails and bearings (Figure 2). This design allows
the Z-axis linear actuation to tilt the coil around the pitch axis,
adapting to the nose-to-back curvature of the scalp. Lastly, thesecondary mount attaches to the back of the VR headset’s strap
using a custom 3D-printed bracket.
Alternative designs. Many other designs are certainly possible
and ours depicts one possibility. For instance, one could add ahemispheric rail connecting the sides of the VR headset’s strap to apassive rail, thus guiding the coil with greater precision. However,after experimenting with this, we noted that this also has drawbacks:(1) it prevents the headset’s strap-size adjustment from working,rendering our system fixed in size unlike our current design (i.e., therails properly slide as the user adjusts the headset for their size); and(2) It restricts the coil’s movement to a single line over the cortex,
eliminating the possibility of diagonal movement. Additionally, it
is important to note that our evaluation showed our system has
already achieved sufficient accuracy to stimulate the sensations
found in Study#1.

3.2

Software Implementation

To control our medical-grade TMS stimulator, we implemented acustom middleware via MagPy Python Toolbox [54], which routed
serial communication to the stimulator and received Open Sound
Control (OSC) commands. Our main applications for both studies
ran on Unity3D, which dispatched OSC commands to our custom
middleware for TMS, and serial commands to a SAMD21-XIAO
microcontroller connected to the motors and encoders. For tracking
the hands and feet, we used HTC VIVE 3.0 Trackers attached to theuser’s extremities, and integrated their tracking information into
Quest 2’s headset tracking using Open VR Space Calibrator [45].
Stimulation flow. When a VR experience requests feedback,the following process triggers the stimulation: the Unity3D application communicates the required stimulation intensity to the Python
middleware via OSC, which is then conveyed to the stimulator as a

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

serial command using MagPy. As a result, the stimulator charges its
capacitor arrays at the requested intensity. In parallel, the Unity3D
application also sends serial commands to the motor controller tomove the coil to a target area. Once the coil has reached the position
and five seconds have elapsed since the previous stimulation (see3.3 Safety), the Unity3D application sends another OSC command
to our middleware to trigger the TMS stimulation using three consecutive 300-𝜇s biphasic cosine cycle pulses [50], each separated
by a 50-millisecond interval. Finally, the resulting current flow
through the coil creates the oscillating magnetic field, delivering
the stimulation to the target brain region.

3.3

SafetyRisks of TMS. Like any other electrical or brain stimulation technique, TMS is not without its associated risks. These are primarily
local skin-tingling at the scalp or magnetic interference with implanted devices, which can be avoided through user screening andthe proper selection of stimulation parameters [71]. Moreover, according to a set of safety & ethics guidelines for TMS released in
2021 [70], as long as TMS is applied to subjects without epilepsy,
no lasting adverse events (e.g., seizures) have been reported. This
claim is further supported by a comprehensive review of TMS, including long-term use of up to 26 months [22]. TMS is also an
FDA (Food and Drug Administration) approved technique for therapeutic purposes [18]. The minimal risk of TMS is ensured byproperly screening the population and limiting stimulation within
the known safety range.
User screening. In adherence to the safety guidelines [70, 71],
we strictly applied the following user eligibility criteria. First, we
did not recruit participants with medical conditions that could increase the risks associated with TMS (i.e., epilepsy, or history of
seizures). Second, in line with the safety guidelines for fMRI [20],we did not recruit participants with implanted devices (e.g., pacemakers, metal implants) to prevent unintended interactions withthe electromagnetic field generated by the coil. We also ensured
ethical compliance by obtaining informed consent from participants prior to using our device—overall, these screening criteria
are mostly analogous to those for EMS [41].
Stimulation parameters. Rossi et al.’s guidelines provide a
range of stimulation parameters that ensure minimal risks during
TMS [71]. These specify the maximum stimulation duration andthe necessary break period between stimulations as a function of
the stimulation frequency. For instance, for a 20 Hz stimulation,
the guidelines prescribe a safe continuous stimulation up to 400
ms (i.e., 8 consecutive pulses). Following this guideline, we use
the stimulation parameter of three consecutive pulses with 50 ms
intervals—a conservative & safe approach using less than half ofthe maximum pulse count that is deemed safe. Moreover, for thesestimulation parameters, Rossi et al. further recommend adding afive-second break between stimulation sequences. As such, oursoftware apparatus does not allow the TMS stimulation to be reactivated unless five seconds have elapsed prior to the preceding
stimulation. This additional safety was also incorporated into our
VR experience used in Study#2, by adjusting aspects such as thetiming of the enemy’s attacks or charge-up duration for the user’s
projectile launcher.

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Yudai Tanaka et al.

Figure 3: (a) Measurement setup for the actuation characterization. (b) Mean errors and standard deviations (SD) for all targets.

Runtime safety. For runtime safety, we implemented an emergency stop function in our Unity application, which immediately
shuts off both the TMS stimulation (hardware switch) and motor
power (hardware switch).

4

TECHNICAL EVALUATION

We conducted two technical characterizations: (1) device characterization (i.e., end-to-end latency, noise, weight), and (2) actuation characterization (i.e., accuracy in moving the coil on theuser’s scalp).

4.1

Device Characterization

End-to-end latency. We measured an end-to-end latency of 16 ms
using a 240fps camera that also recorded sound—this is a complete
round-trip, from an event in Unity3D to the audible sound of a TMS
pulse. For simplification, the coil was already at the target, we will
later characterize the actuation speed delay while moving the coil.
Operating noise. We measured the sound generated by our
TMS using a decibel meter (AS824) placed at the height of a user’sear (ten repetitions). We measured an average peak of 79.8 dB at the
average TMS intensity used in Study#2 (actuating the leg, which
requires the most amplitude). In comparison, haptic devices such as
propeller-based tend to be louder (e.g., [72] produces up to 95.3dB).
Also, propeller-based devices actuate for several seconds or evenstay on for minutes, instead, our TMS produces sounds for only
150 ms.
Device weight. The total weight of the device is 3.1 kg (measured with a Camry Digital Scale, 50g resolution), which consistsof 1.1 kg from the effective weight of the TMS coil when wornand connected to the stimulator and 2.0 kg from our mechanicalactuators, rails, and attachments.

4.2

Coil Actuation Characterization

As we will see in our Study#1, we observed that the shortest distance
between adjacent locations on participants’ scalps that correspondto haptics on different limbs, was approximately 17 mm. Consequently, we required an actuator that meets this level of precision,
ideally with a resolution of at least half that distance, i.e., 8.5 mm.
Measurement setup. All the evaluation in this section was
conducted with a HTC VIVE 3.0 tracker (with a tracking accuracyof 0.3 mm [7]) attached to the coil of our complete system worn bya mannequin head (Figure 3a). We used our Unity3D application
to control the motors while recording the tracker’s position. Notethat the positional difference between the tracker and the centerof the coil was compensated via an offset applied in the Unity3D
spatial coordinates.

Range of motion. We characterized the range of motion foreach motor axis. We found that the rotation of the servo motor(X-axis) spans 24 cm across the head. The actuation range of the
Y-axis linear actuator equates to 7.4 cm across the head, and the
Z-axis linear actuator’s range corresponds to a 24-degree tilt of thecoil in the pitch axis.
Minimum step-size. We characterized the step-sizes that thecoil can robustly move on the scalp using its servos and linear
actuators, which we found to be: 4.2 mm, 1.9 mm, and 2.6 mm for
the X-, Y-, and Z-axes respectively.
Actuation consistency. The most important factor is the ability
to consistently replicate target coil positions multiple times after
calibration, i.e., measuring the motors’ drift. For this test, we instructed the system to actuate to eight known targets in the four
cardinal directions at two distances that were pre-defined. In thesemeasurements, the coil was first set to the initial position where thecenter of the coil coincided with the center of the mannequin’s scalp.
Then, the coil was actuated at the speed employed in our Study#2(50mm/s on X-axis, 8mm/s on Y-axis, and 18mm/s on Z-axis), After
stopping, we calculated the error. We repeated this 10 times foreach target to obtain a variability, as a metric of robustness. Figure
3 (b) shows our results. We found that the average error across thetargets was 1.4 mm (SD=0.7) on the X-axis (left/right) and 1.5 mm(SD=0.4) on the Y-axis (front/back). Consequently, this actuationconsistency, along with the minimum step sizes, confirmed that
our system surpasses the required precision of 8.5 mm.
Actuation latency. We measured the time that our actuation
system would require in a worst-case scenario, where the coil needs
to travel from the jaw location on the left side to its counterpart on
the right. Drawing from the average unit distance between different
stimulation targets on the scalp as determined in our Study#1, we
set the distance for this case to be 13.6 cm. We found it takes 3.2
seconds for our system to cover this furthest distance.

5

CONTRIBUTION, BENEFITS & LIMITATIONS

Our key contribution is a novel interface concept, haptic sourceeffector, which we explore as an alternative direction to scaling
haptic interfaces to be able to render full-body sensations from acentralized on-body device.
Our approach has four key benefits: (1) One actuator for fullbody haptics: unlike haptic systems created by a constellation ofmany devices, our implementation has only one centralized pointof contact with the user’s brain; this is beneficial in that it demonstrates a path forward to scaling haptics without the complexities
associated with wearing many actuators. (2) Easily reconfigurable: as our TMS coil moves over the user’s scalp, it gains accessto eliciting tactile/force sensations across many parts of the user’s

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body, thus the device can reconfigure by moving to a new stimulation location of the user’s brain—contrast the distance that an
actuator would have to be moved if swapped from hands to feet(source-effector’s movement on the scalp is 50-times smaller thanthat). (3) Freeing up the body: because source-effector only sitson the head, it frees up the rest of the body from hardware—noneed to wear end-effectors which would otherwise limit the user’s
ability to feel sensations in their hands or feet; (4) Force & touch:since any sensory information takes the form of electrical currents
passing through neurons in the brain, our approach renders bothtouch & forces—two different modalities that conventionally require two different types of actuators (e.g., vibrotactile motors andforce-based exoskeletons).
Limitations. We acknowledge that our current implementationof source-effector based on TMS has its limitations and every benefit has associated caveats. First, while our concept is novel in that asingle point of instrumentation can realize haptics in many body
locations, this instrumentation is currently heavy. Second, thispoint of instrumentation leaves plenty to be optimized for in termsof comfort (not only total weight but distribution of weight too).
Third, while this haptic actuator is unique in its ability to reconfigure to stimulate new body locations, the fundamental limits of TMS
dictate the expressivity of this approach—like any other techniquebased on stimulating neurons, TMS is also non-selective, i.e., electrical eddy currents traverse through many neurons at once, which
limits induced sensations (i.e., touch & forces, but not thermal),limits spatial fidelity (i.e., we cannot stimulate small brain areas
and those corresponding to high thresholds such as the torso), andmodality isolation (e.g., force-feedback occurs with touch). Moreover, we discuss more detailed limitations in our roadmap as we
believe that these point to a set of challenges worth pursuing tofurther unlock TMS’ interactive potential.
Finally, we reflect on the nature of our proposal: the TMS technique we employed is not new—it originates from neuroscience andmedicine as a method to understand our brain function. However,the value of our proposal also lies in the translation of this medical
tool as a new way for full-body haptics. In fact, similar translational
efforts have previously advanced our field, e.g., the PossessedHand[69] translated a 50-year-old medical tool (i.e., EMS) into our field,
leading to a growth in force-feedback research. Likewise, our effort
is the first to translate transcranial magnetic stimulation from therealm of neuroscience into new knowledge for designing brainhaptic systems.

6

STUDY#1: GENERATING HAPTICSENSATIONS ACROSS THE BODY WITH
BRAIN STIMULATIONIn our first study, we focused on characterizing the range of oursource-effector concept when implemented by means of medicalgrade transcranial magnetic stimulation (TMS), by measuring: (1)in what locations can haptic sensations be created using TMS? and,(2) what types of haptic sensations can TMS create?
As such, a TMS-trained experimenter stimulated the nine predefined locations on the right hemisphere of participants’ sensorimotor cortex using TMS, while participants reported where theyfelt sensations and the strongest point of tactile sensation, as well

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

as observing any resulting limb movements—this study design was
based on traditional neuroscientific methods used by prior work toinvestigate motor responses to TMS [13, 95], as well as perceived
locations [74, 94] or qualities [2, 76] of electro-tactile stimulation.
The study was approved by our Institutional Review Board (IRB210055).

6.1

Study Design

Participants. We recruited 12 participants from our institution(7 identified as male, 5 as female, average age = 23.2 years, SD =2.2). All participants were right-handed. The participants werecompensated with $30 USD.
Apparatus. We used a medically compliant magnetic stimulator(Magstim Super Rapid2 ) with a butterfly coil (Magstim D702 ). The
participant sat in a reclined chair, wearing earplugs and a whitefabric cap on their scalp, which allowed the experimenter to markthe stimulation locations prior to actual trials. For reporting elicited
touch sensations, we provided participants with an iPad and Apple
pencil running our GUI application. This enabled them to draw theperceived sensation area on front/back views of a human model.
They reported the quality of the sensation by selecting from thefollowing six keywords: “tapping”, “vibrating”, “tingling”, “pressing”, “skin-stretching”, and “thermal”. For evaluating force feedback
from TMS, we video-recorded participants throughout the study
and post-annotated any involuntary joint movements via TMS that
occurred during each trial.
Stimulation. We stimulated the right hemisphere of the sensorimotor cortex (corresponding to the left side of the body [35]) withthree consecutive 320 𝜇s TMS pulses separated by 50 ms, resulting
in a stimulation of ∼150 ms. While we opted to only stimulate theright side of the brain to avoid fatigue, the results will be generalizable to the left side. In fact, we purposefully picked this side
because it allowed us to measure the worst-case scenario since theother side (dominant side) of the brain is known more sensitive to
TMS [92].
Defining the stimulation locations. We identified two locations on the participant’s scalp that yielded minimum stimulation
intensities to elicit observable limb movement (i.e., motor threshold)for the hand and foot. For this, the experimenter moved the coilfollowing the grid-search method used by Franza et al. [25], while
adjusting the intensity based on Awiszus’ method [3]. After setting
the hand and foot locations, we defined the final nine locations on
a line between them. These were equidistant at one-sixth of the
distance between the hand and foot locations.
Procedure. We started trials at the foot location. For each location, the intensity was set to 10% below the hand’s motor threshold.
The amplitude of TMS stimulation was reported in percentage (100%is the stimulator’s maximum). During a trial, the experimenter stimulated the target location. Afterward, the participant reported thestrongest point and area of a perceived touch as well as a keyword(or if nothing was felt). Then, the experimenter increased the intensity by 5% while ensuring the participant’s comfort & consent, andmoved to the next trial. This process continued until the participant
reported the same location and same quality of sensations for two
consecutive trials, or the intensity reached the maximum (i.e., 100%).
At that point, the procedure advanced to the next location.

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Figure 4: Overview of results depicting the locations where we were able to induce touch and/or movement in participants.
Analysis. After each study session, we organized the participants’ responses regarding touch sensations based on where thestrongest point of the sensation was. We also annotated each trial
to indicate which of the following body parts moved: “none”, “jaw”,“upper arm”, “forearm”, “hand” (i.e., the palm), “fingers”, “upperleg” (i.e., the thigh), “lower leg”, and “foot”. To focus exclusively on
clear and meaningful force feedback situations, we adhered strictly
to the following two rather conservative criteria: (1) when multiple
parts of the same limb moved, it was counted as the movementof the part closer to the torso, which is typically the largest/coarsest (e.g., if both forearm and fingers moved, it was annotated as
forearm)—thus, in our analysis, ”foot movement” means only thefoot moved, and so forth; (2) movements that occurred when thestrongest touch sensations were felt outside the relevant body part(e.g., a trial with hand movement, but strongest sensation in theforearm) were counted as “none”.

6.2

Results

Overview. Figure 4 depicts an overview summarizing the body
locations where we were able to induce touch and/or movementin participants. Overall, these results suggest that we were able
to induce touch sensations without body movement in two
locations (hand & foot), and involuntary body movement accompanied with or without touch in six locations (fingers,
hand, forearm, lower leg, foot, and jaw). Moreover, it is worth
noting that we only actuated the right hemisphere of their cortex
corresponding to the non-dominant hand side of the body for thesake of participants’ time (study durations of up to 1.5 hours). On

Figure 5: (a, b) Extracted raw data of where the participants
felt touch sensations in the hand and foot. (c) Quality of thesensations.

this note, neuroscience research suggests that it is more challenging to induce somatosensory responses in the side we actuated(non-dominant) via TMS [92]—thus, these results are expected togeneralize even better to the other side. When accounting for bothsides of the user’s body, this brings the haptic sensations that can be
induced by TMS to a total of 15 sensations with a single actuator:
four touch sensations and eleven force-feedback sensations. Notethat we regarded jaw movement as one sensation. In the following,
we analyze each modality (i.e., touch / force-feedback) in moredetail.
Touch. Results suggest that we were able to induce, by means of
TMS, touch sensations (i.e., only tactile in isolation of any noticeable
movements) in two unique locations: hand and foot, which wereboth experienced by 75% of the participants (i.e., nine participants).
The next most promising area was found to be touch on any of thefour fingers, experienced by 67% of participants, which we did not
find to meet our criteria. Now, we turn to where in the hand or footthese touch sensations occurred. For this purpose, for each of thenine participants who reported sensations in these areas, we plotted
raw data from one trial where they felt the strongest touch sensationin these areas. As shown in Figure 5 (a, b), TMS could induce thesensations on the front side of the hand or the either side of the foot,meaning no simultaneous sensations were observed in other areas.
Moreover, we found that these tactile sensations had spectrum ofquality with the majority being described as “tapping” (33%) or“vibrating” (33%), and less often as “tingling” (11%), “pressing” (11%,22%) or stretching (11%, 0%)—as depicted in Figure 5 (c).
Force-feedback. As shown in Figure 4, our results suggest that
we were able to induce, by means of TMS, force-feedback sensations(i.e., noticeable involuntary movements) in six unique locations:jaw (75%), forearm (100%), hand (92%), fingers (83%), lower-leg(92%), and foot (92%), which were all experienced by >75% of theparticipants. In fact, most of the actuated limbs were observed inalmost all participants (>90%) except for the jaw (75%) and fingers(83%). The next most promising candidate would be the upper leg(58%), which we did not find to meet our criteria. Furthermore,
all these force-feedback sensations were associated with touchat the same location (e.g., if the hand was actuated involuntarily,
there was touch felt & reported on the hand), except for the jaw
movements. Jaw movements were observed to be isolated fromtouch sensations (i.e., only movement was observed). Moreover,while the jaw movement was likely also unilateral (i.e., it moved
the left side of the jaw more than the right) its effect was perceived
by participants as one single movement, i.e., jaw clenching on bothsides; hence, we only consider this as a single haptic sensation.
Correspondence to the brain areas. Next, we analyze which
locations of the sensorimotor cortex were more effective in creating
haptic sensations. Figure 6 depicts the brain region that we targeted,with a line emphasizing the principal axis of the sensorimotor
cortex, which was defined for each participant prior to trials. From

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CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Figure 6: A heatmap of stimulation locations of the sensorimotor cortex corresponding to evoked touch sensations & movements.

here, we depict the effectiveness of each stimulation location increating touch or movement at nine equally separated positions
on the main axis. In defining these positions, we found that the
average unit distance between each location was 8.3 mm (SD=0.9;
min=7; max=10). This suggest that only position 2 was redundantwith 1, and the remainder were useful for haptics. Moreover, thetwo nearest adjacent locations that successfully stimulated limbs
were separated by ∼17 mm (x2 average unit distance). This indicatesthat interactive systems operating a TMS coil should maintain apositional precision of at least ∼8.5 mm, which we used as ourdesign specification.
Stimulation thresholds. Finally, with respect to inducing haptic sensations by means of TMS, we investigated at what intensity
haptic effects occurred. Figure 7 shows a breakdown of the intensity
at which touch sensations and/or movement were first observed
on average. Overall, haptic effects occurred within the range of60∼85%. Results also suggest that body parts closer to the torso consistently required higher stimulation intensities (e.g., the forearm
required more than the hand), indicating that the extremities areeasier to stimulate. Additionally, touch sensations always occurredat lower intensities than movements, which aligns with Franza etal’s findings on TMS studies of the hand [25].

Figure 7: A summary of the minimum stimulation intensities
observed to induce touch sensations & movements.
Study conclusions. Overall, our results imply that, when considering both sides of a user’s body, TMS can produce a total of
15 haptic sensations using just one actuator: touch sensations at
each of the four extremities and force feedback at eleven locations
spanning across the jaw and the four limbs. Finally, building on
our findings, we designed an interactive experience that can render
force feedback to the user’s hand, forearm, foot, and lower leg on
both sides of the body and used it to probe into experiential aspectsof our approach.

7

STUDY#2: USER’S EXPERIENCE WITH
SOURCE-EFFECTORThis study was designed so that participants could provide qualitative feedback regarding their experience with this on-of-a-kinddevice during an actual interactive use. To this end, participants
experienced a custom-design VR game that featured several sensations that our device is capable of (from Study#1). VR experience
was followed by a semi-structured interview to let participants voice
their experiences. This study was approved by our Institutional
Review Board (IRB21-0055).

7.1

Apparatus, VR & Haptic Design

Apparatus. Participants wore our complete device and a VR headset (Meta Quest 2), as described in Implementation. Their hands andfeet were tracked via four HTC VIVE 3.0 Trackers attached with
Velcro-straps. Participants wore headphones (Apple Airpods Pro)to hear the VR experience.
VR experience. Participants embodied the avatar of a cyborg
trying to escape a robotics factory that has malfunctioned, as depicted in Figure 8 (a). However, when they find the escape route
blocked by malfunctioning robots that fire at them, —as depicted
in Figure 8 (b)— the VR experience commands our haptic device
to render tactile sensation on the affected area (e.g., the left hand in
this case, but both hands and feet are possible). To advance, participants can counteract by charging up their plasma-hand and firing
plasma-projectiles to deactivate the robots—as depicted in Figure 8(c)—when they open the palm of their hands in a firing gesture, theVR experience detects this gesture and prompts our haptic device
to render force-feedback and tactile sensations on the firing hand(e.g., the right hand in the case, but both hands can fire plasma
shots). After this, the user continues to counteract any robots thatappear, which can fire shots against any of the user’s VR limbs (i.e.,the hands or feet). Figure 8 (d) shows the user being shot in theirright foot—just before this happens, the VR prompts our haptic
device to render tactile sensation on the right foot. After a while,the user’s plasma-hand stops working, and they need to recharge
the energy. They locate a crate on the floor and stomp it with theirfeet to release its charging energy—as depicted in Figure 8 (e)—justbefore the stomping releases the energy, the VR commands our
haptic device to render force-feedback and tactile sensation on the

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Yudai Tanaka et al.

Figure 8: Full-body interactions in our Study#2, including: force-feedback and tactile sensations on both hands and both feet.

left leg. The user keeps fighting until they eventually find a button
that opens the exit door—as depicted in Figure 8 (f)—as they areabout to press it, the VR requests that our haptic device rendertactile sensation on the right hand. The user has escaped the factory.
Haptic design. Although our system has some mechanicallatency and added safety pauses (see 3.3 Safety), we tailored ourgame design to overcome it. For instance, to give the coil time toarrive at the hand (∼3.2 seconds if coming from the furthest away),
the participant hears a charging sound and sees the hand charging
before the plasma shot is fired. This is an example of how haptic
designers create VR situations to set up the user’s mental modelto handle haptic devices that include some latency (e.g., shapechanging devices [34, 90]). Another way to handle latency is topreemptively schedule an event just before it happens, which weleverage when the user is hit by the robots (a popular design usedin prior work, e.g., [89] or [17]). Finally, we also took advantageof events not initiated by the user’s voluntary actions (e.g., theappearance of the box, the start of enemy firing, spawning newenemies, and so forth). Thus, we only triggered interactive eventswith haptics after the coil was set atop the target position and/or
the completion of the five-second safety interval—a strategy known
as queueing.

7.2

Study Design

Participants. We recruited eight participants (five identified as
male, three as female, average age = 25.3 years, SD = 3.0) from
our institution; three had partaken in our first user study. All
participants were right-handed. Moreover, with the participants’consent, we videotaped the study. Participants received $30 USD
as compensation.
Procedure. Prior to the VR trial, we calibrated the coil’s position
and the stimulation intensity to induce the visible movements of
the hand, forearm, foot, and lower leg for both sides of the body. We
adjusted the coil position with our motor structure while sending
commands to motors via GUI sliders on our Unity application. Afterthe calibration, the participant experienced the VR scene. Then we
conducted a semi-structured exit interview.
Interview structure. Our semi-structured interview included
four phases: (1) Overall experience: opener questions about theoverall experience with the device; (2) Open-ended feedback: weinvited participants to provide any comments regarding any aspectof their experience in the entire study; (3) VR experience: onequestion per VR interaction; (4) Future: we invited participants
to share use-case/features they would like to see. In total thiscomprised eight questions. Finally, interviews were recorded inaudio (with consent) for transcription.

7.3

Results

We analyzed video-recordings from the VR experiences, depicted in
Figure 9, as well as responses to our interviews. We identified eight
unique topics: (1) surprise, (2) coil, (3) ergonomics, (4) full-bodysensations, (5) force feedback, (6) tactile feedback, (7) unexpected
sensations, and (8) future form-factors & applications.
Surprise. Six participants (out of eight) expressed surprise:”felt like it’s actually happening. Was impressed[!]” (P5), ”verysurprising” (P2), ”surprise that it works, the location of the sensationand motion was correct despite the device being on the head (. . .)hard to imagine that it can work but it does[!]” (P4), and ”is on topof your head which you wouldn’t expect to be able to move yourhands and feet. It was a little counterintuitive[!]” (P7), ” it is coolthat it is coming directly from your brain. (. . .) [no need to] attach
stuff to other parts of your body” (P6), and ”It felt natural because
there was nothing there, like using my natural hand [versus] using
a glove (. . .) bright future! (. . .) a natural feeling and doesn’t feellike needles and there is no glue to remove at the end ” (P1).
Feeling the coil movement or coil action. Three participants(out of eight) recalled feeling the coil’s movement: ”you can kindof just ignore the movement, all of the feedback was unexpected.
You can’t anticipate the sensation in advance” (P4) or ”[the coilmoving] feels like it’s massaging your head, which is good!” (P6)and ”the movement of the device could be distracting but the actual
sensations were realistic” (P8). Finally, two participants (out ofeight) commented on feeling the coil’s stimulation ”everything else
[in VR] distracted from it” (P6) and ”felt [haptics] in a specific bodyarea, sometimes it was covered by the sensation on the skull” (P3).
Device’s ergonomics. Three participants (out of eight) commented on ergonomics: ”a little heavy, but everything else wasfine. It was comfortable aside from the weight. It didn’t feel uncomfortable when was moving around” (P7), ”not really aware of
the moving part but was aware of the heaviness (. . .) but it is a lot
more adaptable, better than putting 10 different things on body”(P5), ”If there is more weight on the front, it would be better” (P6).
Full-body. All eight participants recalled how they felt all sensations successfully, including tactile and forces on their hands,
feet, and arms during interactions with firing a projectile, stomping
on a box, getting shots on the hands/feet, and pressing the escapebutton. For instance, ”felt sensations on the whole body, which
is very convenient. Liked how you only need to calibrate once atHaptic Source-effector: Full-body Haptics via Non-invasive Brain Stimulation

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

Figure 9: Photos of participants’ experiences (reproduced with their consent): feeling the recoil of throwing a projectile (a, b);
stomping on a virtual box (c, d); feeling the impact of a projectile on the hand (e); pressing a virtual button with haptics (f).
the beginning and do not need to calibrate again” (P2; note thatrecalibration could be needed depending on the situation).
Strong force feedback. Seven (out of eight) participants commented on the strength of the force feedback, for instance, ”kneejoint bounce [proceeds to make ”boom” sound], it felt strong (…)
like stomping on a rubber ball” (P2, stimulated on leg as they
stomped on the box), ”felt [a] pushback, cool!” (P1, stimulatedon hand as they fired the laser), or ”Felt very strong, maybe because it suddenly moves your foot and you need to balance back.
surprised by the attack [of the robot].” (P4, stimulated on foot), ”
really strong, like a kickback (…) kind of like a recoil”. (P7)— P1,
P4, and P8 went as far as to call the effects felt ”very natural”. The
participant who did not enjoy the haptics mentioned: ”All of thehaptic sensations made sense (. . .) [I] just didn’t like them (. . .) [andthen proceeded to explain that they felt] scared of losing balance
initially [when feeling foot force feedback]” (P6).
Tactile feedback. All eight participants reported feeling tactile
sensations during the VR experience. Two participants (out ofeight) commented that these felt less expressive than the forcebased sensations, ”movement is stronger than tactile” (P2), and”[feeling shot on hand was] less realistic than laser or stomp [bothforce]” (P3).
Unexpected sensations. One participant reported a sensationthat was not force or touch, ”the [foot] sensation was very sudden(. . .) may have contributed to the burning sensation, made me
realize I got shot” (P2). Also, one participant was surprised by the
involuntary actuation, ”kind of scary. Interesting because I am notin control” (P1).
Future form-factors & applications. Regarding future applications beyond the VR they experienced, three participants stated,”Learn to play instruments since hands are free (. . .) or handle
emergency situations [it] can get your attention very quickly” (P4),”stationary mouse + keyboard games” (P2), and ”use with constantforces, liquid, wind, etcetera” (P3). Regarding future form factors,
two participants added other ways they would like to experience
our device: ”many coil arrays so no [need for] motor movement(. . .) [and later added] this could be a structure that carries the coilwithout placing the weight on the user” (P2) and ”maybe move thecoil up a couple of centimeters when it is not stimulating” (P1).

8DISCUSSION & ROADMAPIn this section, we present our reflections and a potential roadmap
to further exploring this novel idea.

8.1

Roadmap for Technical Limitations

As one of the very first haptic systems making use of TMS as a centralized haptic actuator, our exploration let us engage & measureseveral of its current limitations. We believe these current limitations point to significant areas for future research. It is important
to note that many in neuroscience/medicine do not push past theselimitations because, for most of their practical purposes, these areminor issues (e.g., TMS’ audible noises are a minor nuisance during
TMS-depression therapy). We argue that incentives to push TMS
past its current form factor might have to come from outside of the
medical space. This is perhaps akin to how researchers & industry
in HCI have engineered compact & easy-to-wear EMS stimulators
when compared with medical-grade EMS devices. We argue thatthe translation & exploration of TMS in interactive systems might
accelerate & motivate technical advances.
Acoustic noise. Rapidly oscillating magnetic fields create audible coil vibrations. Some advances in noise reduction are possible.
For instance, Peterchev et al. demonstrated a TMS coil that effectively reduces its noise by 19 dB—achieved by altering the coil’s
casing to shift sounds outside the user’s hearing range [67].
Coil ergonomics. TMS coils are larger than most types of nerve
stimulation (e.g., EMS). While participants in our Study#2 wore ourcomplete system, we believe that this heavyweight may negatively
impact experiences of prolonged use and do not recommend these.
Improving the coil’s form factor is paramount for better wearability.
One approach is altering coil geometries (e.g., single-loop coils
reduce the size by half [21]). Another approach is using smaller
coils designed for small mammals [88], which might still be enoughfor applications with weak haptics. With all these, there is a tradeoff
between size/weight and depth of the stimulation [21].
Stimulator size. Current TMS stimulators are still large, e.g.,ours measures 46×38×31 cm. This is because, to produce strong
magnetic fields, the capacitor arrays require large power (4600 W
at peak—approximately the energy intake of two hairdryers). At
the current stage, we recommend TMS tethered to the stimulator,
where the user might be free to move around, but limited to therange of the cable. Despite this, advances in engineering are likely

CHI ’24, May 11–16, 2024, Honolulu, HI, USA

to improve this. For instance, Sauvage et al. designed a compact
TMS (11×27×29 cm) at 9 kg, which operates on a small 24 V DC
power supply [16] and exhibits an output of 700 mT at 1 Hz (i.e.,
70% of our stimulator’s maximum output).
Alternative mechanical designs. We do not think of our headworn implementation as the only way to realize source-haptics
using TMS. To minimize weight, researchers might explore mounting the coil and the actuation system to the user’s chair (e.g., desktop/sited applications) or suspending them via the ceiling (e.g.,similar to early VR setups that were heavy, or large free-moving
VR arenas). Alternatively, if the stimulation points are predefinedfor a given application, one could replace the robotic actuators witha fixed mount. Particularly, for single-point stimulation, a coil on ahelmet could reduce weight [4]. For multi-point stimulation, a helmet with multiple fixed coils might also be beneficial [59]. All thesenon-motorized approaches also eliminate the latency associatedwith our approach’s mechanical actuation of the coil. However,this comes at the cost of stimulation resolution, which is limited by
the coil’s size (e.g., 5 cm steps in [59] vs. our <8.5 mm steps via ouractuators).
Limited sensations. While we confirmed that our approach
can create a total of 15 haptic sensations including both force &touch, it still has limitations. For instance, the spatial resolutionof haptic feedback via TMS is influenced by the size of the coil’s
focal point (e.g., ∼1 cm radius with the butterfly coil we used [91]).
While this can be smaller than ∼5 mm by optimizing the coil geometry [33], one has to note that a smaller focal point is a tradeoff
to the stimulation depth [21]. This poses challenges in rendering focused tactile cues in small skin areas, e.g., multiple tactile
points on the fingerpad. At this stage, this is not possible withexisting non-invasive stimulation techniques such as TMS (and,instead, necessitates turning to invasive intracortical stimulation[24]). However, it is important to note that the upper bound of
TMS’ spatial resolution is yet to be fully understood (see 8.2 Stimuli
design and understanding). Additionally, by limiting the stimulation
duration (see 3.3 Safety), our force sensations are impact-like (i.e.,
applied for a short duration) and not sustained forces. Also, as
previously discussed, our touch sensations are less prominent thanforces due to the difficulties in stimulating somatosensory neurons,compared to motor neurons.

Yudai Tanaka et al.

of regions that should be stimulated for this particular participant[69].
Stimuli design and understanding. While our system cancreate 15 distinct haptic sensations, this observation is specificto our current actuation resolution (8.5 mm). We hope that ourfindings facilitate exploration of the upper bound of how manydifferent points across the body TMS can deliver haptics. Possible
strategies for this include defining finer grids for coil positioning
or implementing the aforementioned MRI-based approach to accurately identify cortical regions. Moreover, broadening the range of
haptic sensations—such as different textures or even pressure—isalso a promising avenue for future research. One way to approach
this is to explore different TMS pulse parameters (e.g., pulse width,
frequency, and waveform [58]) and evaluate elicited sensations
through psychophysics studies, akin to methods employed in thedomain of surface electrical stimulation [2].
Beyond TMS. Finally, we hope our work inspires new implementations of this concept of a source-effector. One promising avenue
is via the aforementioned transcranial ultrasound stimulation—
despite its still nascent status, it might offer some avenue to mitigate the size of the TMS coils. However, its accuracy & limitations
are still being understood [44, 98].

9

CONCLUSION

We proposed, implemented & evaluated a novel concept for haptics
in which one centralized on-body haptic actuator renders haptic effects on multiple body parts—we called this a haptic source-effector.
We implement it by leveraging transcranial magnetic stimulation(TMS). We can render touch/force-feedback in hands, arms, legs,feet, and jaw—without needing to instrument these with individual
actuators—which we validated in our first user study. In the sourceeffector, a single magnetic coil moves mechanically across the scalp.
For instance, if the user is meant to feel haptics in their hand, thecoil is moved and stimulates the area of their sensorimotor cortex
where hand sensations are processed. In our second user study,
we probed into participants’ experiences while using our haptic
display in an interactive VR experience.
Finally, as a first implementation of TMS-based haptics, we thoroughly discussed its limitations and proposed a roadmap to further
its use in interactive contexts.

ACKNOWLEDGMENTS
8.2

Roadmap for the Future of Interactive TMS

Now that we have discussed the challenges brought by our specificimplementation using TMS, we turn our attention back to the wider
concept of a centralized haptic actuator worn by the user (i.e., asource-effector).
Closing the loop. One possible reason why we observed someof our limitations (e.g., fewer points for touch sensations, or evenless strength for touch than forces) might be related to the lackof a closed-loop system. In other words, during calibration, it iseasy to accurately determine if a force occurred (i.e., it causes avisible contraction of a muscle) but the same cannot be said fortouch. One way that future researchers might tackle this is byattempting to close the loop on the stimulation. This might be done
by first scanning participants via MRI to obtain a more precise map

We would like to express our sincere gratitude to Prof. Howard
Nusbaum and John Veillette for their generous support and training with transcranial magnetic stimulation. We would also like tothank our colleague Romain Nith for his support in mechanicaldesign, and Alan Shen for his assistance in audiovisual production.
Moreover, we are grateful for the constructive feedback from thereviewers. This work was supported by NSF grant 2047189 and2024923. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do notnecessarily reflect the views of any funding agencies.

SPECIAL ACKNOWLEDGMENTS
We would like to acknowledge that the ACM CHI 2024 conference
has decided to take place in Hawaii, offering extremely limited

Haptic Source-effector: Full-body Haptics via Non-invasive Brain Stimulation

options for remote participation. While some authors have chosen
to present their work in person or bring its physical prototype to
the CHI community, they have done so with compassion towards
Native Hawaiians and with a heavy heart. We urge that the ACM’s
process for conference selection should more carefully consider the
impact our conferences have on local communities. We especiallywant to acknowledge Prof. Josiah Hester, a Kānaka Maoli Professor
in Computing, who organized comprehensive resources that educated us about the negative impacts of over-tourism and climate
degradation in Hawaii2 . Finally, the decision of some authors toparticipate in this conference does not represent the views of othermembers of our lab, who have chosen not to engage with this edition of the CHI conference due to its impact on Native Hawaiians.

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