# Flexible Reality: Can Pseudo-Haptics Enhance Vibration Intensity Perception in Co-Located AR Interaction?

Elodie Bouzbib, Gauthier Gendreau, Noé Guillaumin, Marc Teyssier, Claudio Pacchierotti, Anatole Lécuyer

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Elodie Bouzbib, Gauthier Gendreau, Noé Guillaumin, Marc Teyssier, Claudio Pacchierotti, et al. Flexible Reality: Can Pseudo-Haptics Enhance Vibration Intensity Perception in Co-Located AR Interaction?. IHM 2025 - 36e Conférence Internationale Francophone sur l'Interaction Humain-Machine, AFIHM; Université de Toulouse; ENAC, Nov 2025, Toulouse, France. 10.1145/3765712.3765724. hal-05294843v2

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# Flexible Reality: Can Pseudo-Haptics Enhance Vibration Intensity Perception in Co-Located AR Interaction?

## Peut-on augmenter des intensités de vibration perçues en réalité augmentée co-localisée grâce au retour pseudo-haptique ?

**Elodie Bouzbib**

elodie.bouzbib@unavarra.es
UpnaLab, Universidad Pública de Navarra
Pamplona, Spain

**Marc Teyssier**

marc.teyssier@devinci.fr
Research Center, Léonard de Vinci
Pôle Universitaire
Paris La Défense, France

**Gauthier Gendreau**

gauthier.gendreau@edu.devinci.fr
Research Center, Léonard de Vinci
Pôle Universitaire
Paris La Défense, France

**Claudio Pacchierotti**

claudio.pacchierotti@irisa.fr
IRISA, CNRS
Rennes, France

**Noé Guillaumin**

noe.guillaumin@edu.devinci.fr
Research Center, Léonard de Vinci
Pôle Universitaire
Paris La Défense, France

**Anatole Lécuyer**

anatole.lecuyer@inria.fr
Centre Inria de l'Université de Rennes
Rennes, France

<figure><img src="image_2.png" /></figure>

None

<figure><img src="image_3.png" /></figure>

Realistic

<figure><img src="image_4.png" /></figure>

Abstract

<figure><img src="image_5.png" /></figure>

Ripple

<caption>**Figure 1:** To illustrate *Flexible Reality*, we study whether pseudo-haptics stimuli can be induced in a co-located direct touch interaction in AR. We compare VISUAL, HAPTIC and VISUO-HAPTIC stimuli effects on users' perceived feedback intensity, in both VR and AR. We propose four VISUAL RENDERINGS to leverage pseudo-haptics: NONE, REALISTIC, ABSTRACT and RIPLE.</caption>

## Abstract

Pseudo-haptics can leverage and substitute haptic perception with visual or auditory cues. While largely studied in Virtual Reality (VR), pseudo-haptics are under-explored in Augmented Reality (AR), due to embodiment and immersion challenges in direct co-located interaction. In this paper, we propose *Flexible Reality*, a conceptual approach to induce and extend haptic feedback intensity perception in co-located AR through pseudo-haptics. We conducted a user study (n=22) to compare the effects of View (VR vs. AR), Haptic Intensity, Visual Rendering and Intensity on vibration intensity perception, and demonstrate that pseudo-haptics enhance vibration intensity perception with a significant result for equivalence between AR and VR. Qualitative feedback shows that agency and *self-causality* (e.g. believing to be the source of the vibration) increases the interaction believability. Overall, *Flexible Reality*'s concept demonstrates how reality can seamlessly be made *flexible* in co-located AR and

highlights the importance of ethical considerations in futuristic AR scenarios, especially involving sensory illusions.

## Résumé

Le retour pseudo-haptique influence la perception haptique via des signaux visuels ou auditifs. Largement étudié en Réalité Virtuelle (RV), le retour pseudo-haptique est sous-exploré en Réalité Augmentée (RA) pour des difficultés d'incarnation en interaction directe co-localisée. Nous proposons "Flexible Reality", une approche conceptuelle visant à induire et à étendre la perception d'intensité de retours haptiques en RA co-localisée via le retour pseudo-haptique. Nous avons mené une expérience utilisateur (n=22) comparant les effets de l'environnement (RV vs. RA), des rendus, et intensités haptique et visuelle sur la perception d'intensité de vibration. Nous démontrons que cette perception est accrue en pseudo-haptique, avec un résultat d'équivalence significatif entre RA et RV. Des retours qualitatifs montrent que l'agentivité et l'auto-causalité (se croire à l'origine de la vibration) augmente la crédibilité de l'interaction. Flexible Reality démontre comment la réalité peut être rendue flexible et souligne l'importance des considérations éthiques dans
des scénarios de RA futuristes, notamment impliquant des illusions sensorielles.

## Keywords

Pseudo-haptics, Virtual Reality, Augmented Reality, Empirical studies, Vibration, Vibrotactile, Tangibles, Sense of Agency

## Mots clés

Pseudo-haptique, Réalité virtuelle, Réalité augmentée, Études empiriques, Vibration, Vibrotactile, Tangibles, Sentiment d'agentivité

**ACM Reference Format:**

Elodie Bouzbib, Gauthier Gendreau, Noé Guillaumin, Marc Teyssier, Claudio Pacchierotti, and Anatole Lécuyer. 2025. *Flexible Reality: Can Pseudo-Haptics Enhance Vibration Intensity Perception in Co-Located AR Interaction?*. In *36eme Conférence Internationale Francophone sur Interaction Humain Machine (IHM '25)*, November 03-07, 2025, Toulouse, France. ACM, New York, NY, USA, 12 pages. https://doi.org/10.1145/3765712.3765724

# 1 Introduction

Visuo-haptic illusions leverage users' visual cues to make them perceive various haptic features, such as shapes [5], weight [50], size [7] or stiffness [11, 37]; or even to increase the number of manipulated items [3]. This perception is referred to as *Pseudo-haptics* – inferring a *pseudo-feedback* to their users. Apart from enabling various haptic properties, visuo-haptic illusions are considered in a growing number of use-cases, including training and education [56]; or to simplify hardware implementations workloads [11].

Pseudo-haptic illusions have been proven efficient in Virtual Reality (VR) for direct manipulation of objects [50], where users are fully immersed and embodied in virtual avatar hand representations; and with 2D displays [1] where the interaction is non-colocated (e.g., the real and digital interactions do not occur at the same location). However, their effectiveness is still unclear for Augmented Reality (AR) use-cases, where direct interaction techniques are co-located and users perceive their real hand and vicinity and the sense of agency and body ownership can be impeded.

In this paper, we propose *Flexible Reality*, a conceptual approach to extend overall feedback intensity in AR through visual vibration overlays (e.g. Ripple, Realistic etc). Our research questions for this work are three-fold:

1. Can pseudo-haptics be enhanced in AR similarly to VR despite its lack of immersion?

2. Can visual stimulation provide similar feedback intensity as real haptic stimulation?

3. Can visual stimuli enhance haptic feedback intensity?

In this work, we propose to focus on vibratory stimuli for rendering contact with tangibles. We investigate the effect of the visual supports over the users' haptic perception, and compare the effectiveness of visuo-haptic illusions involving vibratory stimuli both in AR and VR. Various design parameters are considered as "building blocks of the Haptic Experience" [33]: timeliness, intensity, density or timbre. We focus here on **intensity** - e.g., "the overall perceived strength of feedback" [33].

To demonstrate this technique, we conducted a user study (n = 22) where we controlled the real haptic vibration intensity (e.g., NONE, LOW, MEDIUM, HIGH), the visual renderings (realistic to

abstract visual representations, see Figure 1) and intensities; and their visual support (AR vs. VR).

Results show that (1) there is a significant result for equivalence between AR and VR regarding the perceived feedback intensity; (2) visual stimuli can induce similar overall feedback intensity as single haptic stimuli; (3) visual stimuli can be used to provide significantly stronger haptic intensities in AR and VR. Qualitative feedback also demonstrate how the visual contact rendering impacts the believability of the interaction: participants reported preferring and believing more the *Ripple effect* (see Figure 1), thanks to an increased *causality* [33] (e.g., they believed causing the effect themselves) and *agency* [32, 38]. As *presence* tends to be quantified in AR through *augmented object presence* [49], we believe this *self-causality* might relate to agency and embodiment in AR, as *being embodied within the interaction*.

These results indicate that vibratory perception can be induced using pseudo-haptics in a direct touch interaction in AR, even when no haptic hardware is available. *Flexible Reality*'s conceptual approach extends the capabilities of current AR, by enhancing immersion and embodiment through users' haptic perception and self-causality despite the usual lack of immersion. We believe *Flexible Reality* can be used in the future to modify the textures, stiffnesses and overall contact renderings in the users' vicinity by leveraging pseudo-haptics perception in pass-through AR.

The contributions of this paper are:

* *Flexible Reality*, a conceptual approach to extend overall perceived intensity in co-located AR through pseudo-haptics;

* Empirical results showing that pseudo-haptic illusions can be induced in AR as much as VR;

* Empirical results showing how visual stimuli can solely replicate **and** significantly enhance haptic vibratory perception;

* Qualitative user feedback indicating how agency and self-causality (e.g. embodying the source of the feedback) enhances the perception and global experience enjoyment.

# 2 Related Work

AR is considered as viable for many contexts-of-use and is becoming more prominent than VR outside the lab. In VR, it was proven how haptics can enhance the experience, however in AR, haptics is under-exploited. AR shows drastically different challenges regarding haptics. In this section, we define these challenges, and evoke why pseudo-haptics show advantages and could be leveraged in AR. We will then focus on vibratory feedback and explain its importance and relevance for virtual and augmented environments.

## 2.1 Challenges of Haptics in Augmented Reality

Augmented Reality and Virtual Reality differ in their level of immersion, as AR augments the users' real vicinity with visual virtual artifacts [4, 20]. The users can perceive their real hand and environment, and are therefore less subject to embodiment or presence, usually relying on sense of agency, body ownership and sense of self-location [32]; and could be distracted from seeing active haptic devices (such as wearables or encountered-type mobile robots [54]) actuate around them to provide them with physicality [10]. It is more adequate to evaluate an augmented object presence [14, 29]
and the adequacy of its alignment in the environment [49] (e.g. "it
is here" [39]).

When the object is fully visual, interaction techniques for ma-
nipulating in AR include pinching (e.g. such as with the Meta Quest
or [58]), pointing [31, 41], or even the use of self to add a more tan-
gible aspect [59]. When it includes tangibles, interaction techniques
consist in direct manipulation [13] or gesture-control [26].

When the interaction is direct, it is defined as *co-located* - as the
visual and haptic stimuli occur at the same (e.g. co-) location. Most
often, the visuals are superimposed [25] and the haptic fidelity
is limited to the tangible properties. We define in the following
how pseudo-haptics techniques can be used to alter this tangibles'
inherent properties.

## 2.2 Pseudo-Haptics Perception

Pseudo-haptics consist in leveraging users' more dominant cues
over haptics (e.g. visual, aural) to provide a haptic feedback per-
ception. Pseudo-haptics are currently mostly used in VR, where
the users are fully immersed and most of the time embodied in
virtual avatar representations, which can be easily altered to pro-
vide pseudo-sizes [7], pseudo-weight [50], pseudo-stiffness [11]
perceptions, or to increase the number of manipulated props [3].
Pseudo-haptics show a great benefit to reduce designers workloads
when designing interfaces [11], while providing consistent and
more especially various haptic properties [56].

While this embodiment in VR allows for co-located interaction
with tangible objects, in AR this modification of the hand represen-
tation cannot be performed. Pseudo-effects interactions therefore
cannot be co-located, and as they occur through screens [5, 6] or
proxies [40] (e.g. a mouse on the screen) or leveraged using other
sensory cues (e.g. auditory [30]). Pseudo-effects can instead be ren-
dered on the virtual objects' properties; for instance Issartel et al.
demonstrated that mass can be visually induced by adding a re-
action force to a virtual object [27]. These pseudo-haptic effects
were not studied with tangibles in pass-through AR with direct
interaction techniques [13]; nonetheless we do believe they can
enhance the augmented environments and provide richer haptic
experiences at a low development cost.

Apart from objects' haptic properties, contact rendering can be
intensified using visual cues in both AR and VR, to reinforce the
perception of objects' presence [15], to modify its stiffness per-
ception [47] or texture [1]. In this paper, we focus on co-located
contact rendering, and aim to induce and exacerbate haptic vibra-
tions through visual renderings.

## 2.3 Vibrations for Haptic Stimuli

Vibrations are important in the haptics community, as they allow
to simulate a large amount of haptic properties through low cost
implementations. Moreover, vibrations are involved in everyday's
interactions, as at contact rendering, small deformations of the skin
turn into small vibrations perceived by the skin's mechanoreceptors
(Meissner corpuscles for lower frequencies, and Pacinian corpuscles
for higher frequencies). Vibrations are also highly identifiable with
numerous frequencies and common for a user to perceive, as they
are used in our interactive technologies (such as our phones). They
are also quite common to use and replicate haptic perception, for

instance they can be enhanced through vocalization to simulate
either roughness or inertia when closing and opening drawers [18],
and usually improve the users' control perception and pleasure,
making the experience richer [53]. Vibrations can indeed be used
to induce textures [2, 28, 43], stiffness and compliance [17], or to
render contact [45].

Vibrations can also be coupled with pseudo-haptics, for instance
to render contact force and resistance [46] or texture roughness
[57], but can even be stimulated from scratch without physical
actuator, by leveraging the users' vision and auditory cues [36].

In the same line of observations, we aim to enhance vibratory
feedback using pseudo-haptics, to extend the haptic properties of a
simple tangible object (e.g. texture or deformation perception); and
quantify the pseudo-haptic effects over real physical vibrations.

## 3 Flexible Reality

We first describe the concept of *Flexible Reality* from its science-
fiction origins and present our hypotheses for this work. We then
describe how we instantiated *Flexible Reality*, by presenting the
hardware and software implementations and justifying our design
choices, notably regarding the stimuli choice and visual renderings.

## 3.1 Concept and Hypothesis

*Flexible Reality* consists in giving the illusion that **any solid object**
from the real environment can physically react to touch contact dur-
ing direct manipulation. This pseudo-haptics effect [48] is induced
when visual overlays in AR interfaces simulate tactile sensations
without any actual physical changes to the interacted object.

As the user touches a solid real object, the AR overlay displays
visual modifications such as ripples or vibrations on the object's
surface. This visual stimulus is designed to alter the user's sen-
sory perception, leading them to experience the illusion of tactile
changes in the object's texture or state. Such pseudo-haptic ef-
fects leverage the dominance of visual inputs over other sensory
modalities to modulate the perception of physical properties, thus
enhancing the immersive experience without the need for physical
actuators or modifications to the object itself.

This concept has been popularized and exemplified by Interfaces
and HMI (Human Machine Interfaces) present in the Science-Fiction
literature. Several movies, such as *The Matrix*, *Ready Player One*,
*The X-Men* or *Doctor Strange*, show a reality that can be altered
and where users cannot tell whether they are in an augmented
environment or not, and potentially cannot define its limits. The
environment is seen as somewhat tangible and deformable, to a
point where it can be malleable and flexible. With the current ex-
pansion of pass-through augmented environments - and where the
integration is more and more seamless - we aim to transpose this
concept in real-life and quantify whether 1. reality can be altered
in AR as much as in VR; 2. reality can be *flexible* using visual arte-
facts; 3. visual artefacts can enhance haptic vibratory feedback; in
**co-located interaction techniques**.

In *Flexible Reality*, we build upon the *Intensity* definition of the
*Haptic Experience* [33], as the “overall perceived strength of feed-
back”, and propose to enhance *Causality* (e.g. “how easily a user can
relate haptic feedback to the source of interaction”). Compared to
*Vibrollusion* [36], where the vibration is always occurring (e.g. even
when the user is not touching the object) and the visual is quite realistic [35], in this paper the visual effects are generated from the user direct manipulation, and we propose multiple visual effects with various levels of rendering fidelity [9] (from realistic to more abstract ones) (see Figure 1). We believe it is important to push beyond the "better-worse trap" [9] and realistic renderings, especially as vibrations are quite unusual to cause from tangible contact in the real environment; but can be used in extended environments to induce textures, slippage, deformations etc.

*Flexible Reality's* concept is a novel tangible augmented reality, where reality is flexible and textures malleable through visual renderings. We have three main hypotheses supporting this work:

H1. Pseudo-haptics can be enhanced in co-located AR direct interaction techniques; but VR-based pseudo-haptics intensities are significantly higher than in AR-based one;

H2. High haptic stimuli without visuals have significantly higher vibration intensity perception than high intensities visual stimuli without haptics;

H3. Visual and haptic stimulation coupled together significantly enhance users' global vibration intensity.

## 3.2 Hardware

We superimpose visual effects over a tangible object, actuated with a vibration motor. We describe here the implementation of the tangible and the tracking during the experiment.

### 3.2.1 Actuated Tangible.
The aim is to study and compare the visual renderings effects in conjunction with vibrations (visual vs. haptic vs. visuo-haptic) and thus need to provide a tangible interface to the users during the experiment. We chose to focus on vibrations as a haptic feedback for multiple reasons: (a) vibrations are easy to integrate and therefore show a low-development cost while (b) potentially enabling rich haptic feedback and numerous object properties, such as textures, deformation, compliance, slippage etc.

We first 3D-printed a cuboid using PLA, and attached a mini coin-motor (3-5V DC vibration motor) in it for it to vibrate. To limit the latency at interaction and avoid the implementation of prediction contact time models (e.g. [12, 22]), we chose to continuously turn on the vibrations (with each trial's respective intensity) during the experience. Though, vibrations tend to dissipate in the table and can therefore be felt prior to interaction and bias the users during the experience. We therefore decided to 3D-print a surface, and fixed it on top of custom-made supports, engineered to mitigate this vibratory transmission. We used a dual-layer damping strategy: the initial layer comprised springs, absorbing lower frequency vibrations, while the subsequent layer used foam to dissipate residual vibratory energy (see Figure 2 - B). We plugged the coin-motor beneath the surface, and powered it via 0-3.3V Pulse Width Modulation (PWM) signal from an ESP32 microcontroller. The microcontroller was connected to the entire application and XR headset through a private WiFi network.

### 3.2.2 Tracking and Coupling.
We aim to compare VR and AR pseudo-haptic effects, therefore we chose to perform our study using a Meta Quest 3. It allowed for both VR and passthrough-AR rendering, and an accurate hand tracking and avatar hand representation. We designed a VR/AR switch; and used it for calibrating

<figure><img src="image_6.png" /><figcaption>**Figure 2: A - Tangible cube texture: we chose a basic background basic UV texture to enhance the visibility of the visual effects. B - The actuated tangible mechanism with its dual-layer damping strategy (spring and foam layer to dissipate the coin-motor vibrations).**</figcaption></figure>

the experience. For both environments, we used the controllers to superimpose the virtual artefact to its real tangible counterpart. The experience was running through a Unity3D-designed APK, and the headset was connected to the previously mentioned private WiFi network - to send HTTP requests to the ESP32 microcontroller. The APK was thus sending the PWM parameters and adjusting the real physical vibratory intensity.

## 3.3 Software

### 3.3.1 Virtual and Augmented Scenes.
The virtual and augmented scenes were designed using Unity3D engine, coupled with the Oculus plugin. The virtual scene was replicating the real environment (a simple table, with the cube atop), such as in *Substitutional Reality* [51]; to remove as much bias from possible between the two environments (VR/AR). In VR, users were fully immersed in the environment and could only perceive the hand avatar representation; as in AR they could perceive the real vicinity, and their hand avatar representation was superimposed their real hands. As mentioned previously, we aimed for agency and *causality* in the interaction - where the users actually are causing the vibrations and deformations. To do so, we captured the collision between the users' avatar hand and cube representations, and used it as an event to initiate the visual effects.

### 3.3.2 Textures.
To facilitate user recognition of the effects, we added the Blender background basic UV texture (e.g. a black and white grid) (Figure 2 - A). This design choice drastically enhanced the visibility of the global effects and deformations; and avoided too much rendering differences between the real cube in AR and its virtual counterpart. The grid texture enhances the contrast caused by the virtual effects and their respective irregularities, thereby allowing users to easily discern and evaluate the manipulations occurring within the virtual/augmented environments.

### 3.3.3 Visual Effects.
In the literature [21, 24, 36], visual vibration effects were instantiated by blurring the virtual object boundaries, and/or using a cartoon-inspired model of vibration - shaking within various ranges. It is mentioned that moving below 0.2cm is not perceivable, and visual vibrations are considered as high after 1cm, with a frequency limited at 40 Hz. In this study, and to overcome the
aforementioned “realism trap” [9], we chose three types of visual effects over the textured cube (illustrated in Figure 1).

*Realistic.* Similarly to the blurring effects, this effect consists in vibrating the virtual cube using numerous small distances translations at high speeds. We used the Unity plugin DOTween<fnref n="1" /> for this purpose, and controlled the shake *Strength* and *Vibrato* parameters.

*Abstract.* The abstract effect is a vibration effect we designed by editing each of the object’s vertex height (see Figure 3). This was built upon the *Deform*<fnref n="2" /> Unity plugin - which enables the manipulation of objects’ vertices. This created a wavy effect propagation over the entire cube surface when the interaction occurred. We controlled the visual stimuli *Frequency* and *Amplitude*. The stronger the expected effect, the higher the amplitude and frequency.

<figure><img src="image_7.png" /><figcaption>**Figure 3: Abstract Effect, in AR (left).** Users can perceive a wavy propagation effect over the whole cube surface.</figcaption></figure>

*Ripple.* By definition, a *ripple effect* occurs when an initial disturbance to a system propagates outward to disturb an increasingly larger portion of the system; it is therefore intrinsically linked to the aforementioned *causality* [33]. This effect was designed from scratch, and relied on four main parameters (see Figure 4): *Amplitude* (e.g. the height of each wavelet); *Speed* (e.g. the propagation speed); *Density* (e.g. number of concentric wavelets) and *Radius* (e.g. maximum wavelet’s radius).

<figure><img src="image_8.png" /><figcaption>**Figure 4: Left: User hand and avatar representation after interacting in AR with the ripple effect. Right: Callout of the Ripple visual effect and visualization of two of its main parameters: Amplitude and Radius. Density represents the number of wavelets, and Speed is the propagation speed.**</figcaption></figure>

# 4 Experiment Design

We choose in *Flexible Reality* to study and compare multiple visual renderings and intensities to real authentic haptic vibrations and whether these visuals can enhance and extend real vibrations intensity perception<fnref n="3" />.

## 4.1 Participants

We recruited 22 participants (9F; 13M), aged 19 to 24 (average = 21.1, std = 1.4), all university students or authors' acquaintances. Two of them were left-handed, and 18% (4/22) of the participants had never tried VR or AR before; while 14% were considered as experts (using VR more than once a month). Another 32% only had tried VR or AR once in their life - we therefore had approximately 50% of novice users and 50% of more experienced ones.

## 4.2 Procedure

Participants entered the room and were informed about the aim of the study. They were told they would interact with a tangible cube, and receive different stimuli (visual, haptics, visuo-haptic) for contact rendering, and would have to rate the intensity of the vibration they perceived. They were then told how to interact within the augmented/virtual environment, i.e. to provide their ratings and validate them (through pinch gestures from the Meta Quest). They were then given the Meta Quest 3; and asked to wear noise-canceling headphones with white noise (to remove the bias from the coin motor vibrations (approximately 10dB above ambient), which could alter the results [36]). They then performed the experiment; each virtual/augmented experiment lasted approximately 10 minutes (std = 3 minutes). They could interact with both their hands, dominant or not. After the experiment, we conducted a semi-structured interview to gather qualitative feedback, gain some insights on their experience, their preferences, and their opinion about the effects.

## 4.3 Tasks and Stimuli

The augmented and virtual scenes were designed on Unity3D (Unity 2021.3.22f1 LTS). For each trial, participants wore the headset and were either in AR or in VR. They could see the textured cube over a table (virtual or actual table) and had to enter in contact with it using their fingers (e.g. not grasp the cube). Users were free to interact as many times as they wanted, with no time or touch limit. All along the experiment, they could see a panel with a sentence:

*My overall perception of the feedback was strong.*

Below it, they could perceive 7 interactable buttons, as a Likert-scale going from 1 to 7 ("Absolutely Disagree; Strongly Disagree; Disagree; Neutral; Agree, Strongly Agree; Absolutely Agree"); which they had to select using a raycast and pinching gesture (from the Oculus plugin). Once they selected their answer, they could press another button to go to the next interaction; and the same process would be iterated. Participants first had 4 random trials to get familiar with the augmented or virtual scene, and then they would start the real experiment.

<footnote n="1">¹www.dotween.demigiant.com</footnote>

<footnote n="2">²https://github.com/keenanwoodall/Deform</footnote>

<footnote n="3">³The git repository with the scripts, shaders and data analysis are available here.</footnote>
## 4.4 Conditions

To compare visual, haptic, and visuo-haptic renderings, we control four factors: VIEW, VISUAL RENDERING, VISUAL INTENSITY and HAPTIC INTENSITY.

- VIEW: We propose 2 views: VR or AR. Participants would first perform a block in one of these views; then follow up with a second block in the other view.

- VISUAL RENDERING: As described in Section 3.3.3, we propose 3 visual effects: REALISTIC, ABSTRACT, RIPPLE.

- VISUAL INTENSITY: We propose 4 levels of visual rendering intensity: WEAK, MEDIUM and STRONG; and (NONE), not linked to any visual rendering, and which could therefore provide a haptic-only stimuli.

- HAPTIC INTENSITY: Similarly as visual intensities, we propose 4 levels of haptic intensity: NONE (PWM: 0), WEAK (PWM: 68), MEDIUM (PWM: 98) and STRONG (PWM: 128), with a 9-bit resolution (up to 128 values).

Overall, we therefore propose 2 VIEW × (3 VISUAL RENDERING × 3 VISUAL INTENSITY + 1 VISUAL INTENSITY) × 4 HAPTIC INTENSITY = 40 CONDITIONS per VIEW = 80 CONDITIONS.

As a side note, to avoid latency when interacting with the vibrating cube, the coin motor was turned on at the beginning of the trial when the HAPTIC INTENSITY condition was different than NONE. Visually, the event was created when the avatar hand would collide with the cube on Unity.

## 4.5 Design

We used a within-design, and each participant performed 3 BLOCKS in each VIEW, in a pseudo-randomized order (e.g. randomized within the blocks). We also balanced the starting condition between users (VR or AR). The global experiment design was thus: 22 PARTICIPANTS × 80 CONDITIONS × 3 BLOCKS = 5280 TRIALS.

## 4.6 Measures

For each trial, we recorded each answer to the “perceived strength of feedback” question (extracted from the Haptic Experience [33] definition of **Intensity**), and the number of touch interactions. At the end of the experiment, we also asked the participants how many different visual stimuli and haptic stimuli they had perceived; and in what environment they felt more comfortable. We also gathered feedback from the semi-structured interview and when participants spontaneously commented on the effects during the experience.

# 5 Results

## 5.1 Quantitative Feedback

We defined the outliers by computing the z-score on perceived intensities data, and removed 27 data points over 5280 (0.5%). We analysed our results by conducting a 4-way RM-ANOVA, and verified normality. We aggregated block data as an average. Mauchly's test of sphericity was significant ($\chi^2(170) = 260.8, p < 0.001$, indicating a violation of the sphericity assumption. We thus applied a Greenhouse-Geisser correction to the repeated measures ANOVA results ($\epsilon = 0.46$). We then calculated pairwise comparisons using post-hocs with Holm-Bonferroni adjusted p-values.

### 5.1.1 Global Effects.
There was no significant effect on VIEW – against H1; but effects were noticed for all other factors. There was a strong effect for VISUAL RENDERING ($F_{(1.9,37.5)} = 26.6, p_{GG} < 0.001, \eta_p^2 = 0.58$), VISUAL INTENSITY ($F_{(1.1,21.9)} = 18.1, p_{GG} < 0.001, \eta_p^2 = 0.48$) and HAPTIC INTENSITY ($F_{(1.4,28.5)} = 51.4, p_{GG} < 0.001, \eta_p^2 = 0.72$). No block effect was noted, indicating neither fatigue or training effect.

Having four controlled factors, we also looked at cross-effects between them. There was a significant effect for VISUAL INTENSITY × VISUAL RENDERING ($F_{(2.7,54.2)} = 6.8, p_{GG} < 0.001, \eta_p^2 = 0.25$).

Finally, we also noted a small (but non-significant) effect for VISUAL RENDERING × VISUAL INTENSITY × HAPTIC INTENSITY ($F_{(8.2,163.8)} = 1.78, p_{GG} > 0.05, \eta_p^2 = 0.08$).

### 5.1.2 AR vs. VR (H1).
Beyond finding no significant effect on VIEW, we decided to run a Two One-Sided Test (TOST) to evaluate whether results are equivalent for both views (non-significance means the effect is not big enough to be anything other than a chance finding; TOST means the effect is zero and samples are similar and equivalent). A significant result was obtained with $t_{(5270.1)} = -2.041, p < 0.05, 90\% CI = -0.17 to -0.018$ with an interval of [-0.5, 0] in raw values and of [-0.3, 0] in Cohen's *d*, with an alpha of 0.05. An equivalence can thus be made between AR and VR, which average scores were respectively 4.1/7 (std = 1.7) and 4.2/7 (std = 1.6).

### 5.1.3 Visual Rendering.
There was a strong effect between NONE and the other renderings ($p_{holm} < 0.001$), but otherwise no post-hocs or contrasts were found between the visual types per se (see Figure 5 - Right). More globally, the average without visual rendering was quite low (average 2.8/7, std = 1.4), ABSTRACT and REALISTIC had the same average (= 4.2/7); and the ripple effect was largely better in terms of feedback strength perception, with an average of 4.4/7 (std = 1.6).

<figure><img src="image_9.png" /><figcaption>**Figure 5:** Boxplot of VISUAL INTENSITY (left) and VISUAL RENDERING × VISUAL INTENSITY (right) effects on the users' perceived feedback strength. Dotted lines indicate Global VISUAL RENDERING EFFECTS; Filled lines indicate effects of INTENSITY within and cross-visual types.</figcaption></figure>

### 5.1.4 Visual Intensity.
All visual intensities show significant effects between each other. As expected and indicated in Figure 5 - Right, the averages scores are proportional to the provided visual intensity. Post-hocs indicate strong effects between all intensities ($p_{holm} < 0.001$ between WEAK – average = 3.9/7, std = 1.5 – and MEDIUM,
and **WEAK** and **STRONG**, and $p_{holm} < 0.005$ between **MEDIUM** and **STRONG**, resp. average = 4.3/7 and 4.5/7, std = 1.6).

5.1.5 **Haptic Intensity.** Similarly, the effects for haptic intensities are all significant with **NONE** ($p_{holm} < 0.001$); and between **WEAK** (average = 4.1/7, std = 1.5) and both **MEDIUM** and **STRONG** ($p_{holm} < 0.001$). However, we did not note an effect between **MEDIUM** and **STRONG** intensities (resp. average = 4.7/7 and 4.8/7, std = 1.3).

5.1.6 **Visual Rendering × Visual Intensity.** First, and as expected, we can note on Figure 5 that the global average (green triangle) is increasing proportionally with the provided visual intensity. There are notable significant effects within VISUAL INTENSITY, especially from WEAK to MEDIUM.

<figure><img src="image_10.png" /><figcaption>**Figure 6:** Boxplot of feedback strength perception. A. Results as a function of VISUAL INTENSITY when there is no haptic feedback. Hue indicate VISUAL RENDERING. B. Results as a function of HAPTIC INTENSITY when there is no visual effect. The dotted line indicates that medium and strong visuals (especially RIPPLE) are perceived as strong (e.g. intense) as strong real haptic vibrations without visuals, illustrating H2.</figcaption></figure>

5.1.7 **Visual Rendering × Visual Intensity × Haptic Intensity.**

*Visual without Haptics vs. Haptics without Visuals (H2).* We compared the users' perceived feedback strength as a function of VISUAL RENDERING and VISUAL INTENSITY **when there is no Haptic feedback** (see Figure 6 - A); and as a function of HAPTIC INTENSITY **when there is no visual rendering intensity** (see Figure 6 - B) and a TOST showed equivalence ($t_{(526)} = -1.042, p < 0.05, 90\%$ CI = -0.263 to 0.1 with an interval of [-0.3, 0.3] in Cohen's *d*, with an alpha of 0.05). The grey dotted line Figure 6 shows the average perception with strong haptics; we see that strong visuals without haptics thus perform similarly. When there is no haptic feedback, the average reported score is 3.3/7 for strong visuals (std = 1.6) against 3.5/7 (std = 1.4) for strong haptics without visuals. More specifically, results for the RIPPLE effect show an average of 3.6/7 (std = 1.8) (e.g. slightly higher than strong haptics without visuals).

*Visual and Haptics (H3).* As shown in the Figure 7, many significant effects are also to be noted per visual rendering, e.g. between haptic intensities; and between visual intensities. We note that *within similar visual intensities* (dotted lines), the perception of

feedback strength is significantly enhanced by haptics, between **NONE** and **WEAK** to **MEDIUM** and **STRONG**. We also note that within similar haptics intensities (plain lines), the perception of feedback strength is significantly enhanced by visual intensities, between all the **WEAK** and **MEDIUM** visual intensities, **WEAK** and **STRONG** and **MEDIUM** and **STRONG**. We also demonstrate that the average perceived intensity (green triangle) increases both as a function of haptic intensity (e.g., purple hue) **and** as a function of visual intensity (e.g., x-axis).

We also note that the RIPPLE effect is performing better than the others for all intensities; but that globally, as expected, the perceived strength of the feedback increases proportionally with the visual intensity.

5.1.8 **Number of Touch Interactions.** We did not find an effect from VIEW on the number of touch interactions; however in average, users tended to interact slightly more with the tangible actuated cube in AR (average 5.9 touch interactions, std = 6.7) than in VR (average 5.5 touch interactions, std = 5.9).

5.1.9 **Number of Perceived Stimuli.** When asked about the variety of feedback they had perceived throughout the whole experiment, participants answered that they could distinguish an average of four (3.8) different visuals (std = 0.7), and five (4.9) different physical vibrations (std = 1.2).

5.1.10 **Preference between AR and VR.** We also gathered feedback regarding which environment felt the most comfortable; and once again there was no effect on VIEW. Over 22 participants, 11 preferred the VR environment and 11 preferred the AR one (50%). There was no correlation between this preference and the starting condition.

## 5.2 Summary per Hypothesis

5.2.1 **H1: VR pseudo-haptics > AR-pseudo-haptics?** We showed a significant result for equivalence between AR and VR, **against H1**; showing that 1. pseudo-haptics can be enhanced in co-located AR direct interaction techniques; 2. with similar results than in VR (average perception resp. 4.1/7, std = 1.7, and 4.2/7, std = 1.6).

5.2.2 **H2: High haptic intensities > High visual intensities?** When no haptics is provided, the perceived feedback strength is rated of 2.7/7 with **WEAK** visuals, 3.0/7 with **MEDIUM** visuals and 3.3/7 with **STRONG** visuals, with a maximum average of 3.6/7 for the *Ripple* effect. When no visual is provided, the perceived feedback strength is rated of 2.6/7 with **WEAK** haptic stimulus, 3.3/7 with **MEDIUM** haptic stimulus and 3.5/7 with **STRONG** haptic stimulus. We thus show that users' perceived feedback with high intensity visuals and no haptics is as strong as their perceived feedback with high intensity haptics and no visuals, **against H2**.

5.2.3 **H3: Visual AND Haptic > (Visual OR Haptic)?** Apart from having similar results independently from each other, when coupling haptics and visual intensities, results are significantly enhanced. All visual intensities with **STRONG** haptics are between 4.5/7 and 5.2/7; and so are all haptic intensities with **STRONG** visuals. We thus show that VISUAL and HAPTIC intensities enhance each other, **validating H3**. We note in Figure 7 that for each rendering, the relationship between INTENSITIES and the users' perceived strength of feedback is clear. Results reach 5.2/7 (std = 1.2) for ABSTRACT
<figure><img src="image_11.png" /><figcaption>Figure 7: Boxplots of the users' feedback strength perception, as a function of VISUAL RENDERING and VISUAL INTENSITY; hue within boxplots indicates HAPTIC INTENSITY. We note that the perceived strength increases as a function of both HAPTIC and VISUAL INTENSITIES, illustrating H3.</figcaption></figure>

rendering, 5.0/7 (std = 1.5) for REALISTIC rendering and finally 5.3/7 (std = 1.3) for RIPPLE rendering.

## 5.3 Qualitative Feedback

5.3.1 *Enjoyment.* Participants globally enjoyed both AR and VR environments and reported a general comfort with both modalities. They appreciated the visual rendering effects. Opinions however diverged based on the simplicity of the environment, or the believability of the effects.

*View.* To justify their preferred view modality in terms of enjoyment, participants argued around the simplicity of the environments; P2 or P12 preferred the AR environment, stating that "VR felt overly simplistic" or too empty; while P1 and P9 favored VR environment for the same reason — as "its simplicity made it more appealing" (no distraction for the real vicinity). Similarly, some participants preferred AR as they enjoyed seeing their real hands and causing the visual effects; while others preferred VR for the exact opposite reason (e.g. would rather only see avatar hands in a full immersive environment).

*Visual Renderings.* Regarding visual effects, numerous spontaneous comments were provided regarding the RIPPLE effect (P1: "This effect is bonkers! It is incredible!", P22: "The watery effect is really satisfying!"). Surprisingly, multiple participants also mentioned that they enjoyed more the RIPPLE wavelet effect as it felt closely aligned with real-world interactions (P20, P22). P5 mentioned that the RIPPLE wavelet effect felt more enjoyable as the feedback was more localized - therefore making his touch interaction a direct cause for the feedback and enhancing its likeliness. In opposition,

P11 preferred the ABSTRACT effect, appreciating the congruence between visual and haptic feedback, which appeared to synchronize well in terms of frequency. This participant noted that the RIPPLE effect felt more localized compared to the more uniform global vibration. P18 also preferred the ABSTRACT effect as they believe it was more believable, and P16 referred to it as a "piece of paper being shaken".

5.3.2 *Confusion due to Flexible Reality's Concept.* Some participants experienced a mismatch between expectation and reality, which proved somewhat disorienting. P5 mentioned that they "felt disturbed when the events did not align with [their] expectations", leading them to physically verify the reality by touching the table to reaffirm its solidity, and to "remember what is the real reality". This comment opens up questions regarding seamless illusion integration and ethical considerations to take into account when instantiating Flexible Reality's concept (see Sec. 6).

5.3.3 *Agency & Realism.* When asked the reasons for preferring RIPPLE or ABSTRACT visuals, all participants mentioned either the interaction realism and/or believing to cause the feedback (e.g. for both local or global effects). While Causality is one of the Haptic Experience [33] usability requirement (e.g. the understanding of the source of feedback), in this experience we define self-causality as "embodying the cause of the effect" (e.g. believing to be its source). This concept is highly related to the sense of agency in VR, the subjective experience of action and control, expecting visuo-motor synchronicity and a visual consequence to a motor task [32]. Based on the gathered qualitative feedback, we believe
this self-causality could be the source for interaction likeliness and enjoyment, similarly to [38].

## 5.4 Findings & Discussion

5.4.1 **H1: VR pseudo-haptics higher than AR-pseudo-haptics?** First, quantitative results showed that pseudo-haptics can indeed be induced in AR - which verifies our main hypothesis - but surprisingly, there was a **significant result for equivalence** between the users' feedback strength perception in AR and VR - which denies our first assumption. This indicates that pseudo-haptics can therefore be leveraged for co-located direct interaction in AR.

5.4.2 **H2: Haptics with no visuals higher than Visuals with no haptics?** We hypothesized that high intensities haptic stimuli would perform significantly higher than visual ones, and it showed that users reported equivalent feedback intensities whenever these two cues were solely provided. We also can correlate this finding with the number of different perceived "haptic stimuli": users reported in average 5 stimuli while there were only three different ones (Low, MEDIUM, STRONG). We thus believe that through pseudo-haptics, visual stimuli were perceived as haptic ones. This means that we can potentially rely on visuals to provide haptic perception in both AR and VR. For instance, from both qualitative and quantitative feedback, the RIPPLE effect was the highlight of our experiment and perceived as the highest feedback intensity, with an average higher than Strong haptics without visuals.

5.4.3 **H3: Visual & Haptic enhance each other?** Regarding cross-modal interaction efficiency, and as expected, visual renderings and intensities significantly enhanced physical haptic ones; and vice-versa. P5 also mentioned that sometimes, having the physical feedback without visuals and vice-versa was a bit "frustrating"; and P11 mentioned how he preferred when both stimuli were coupled together, especially when it seems that "*the cube is vibrating at the same frequency visually and physically*". We noted that the perceived strength of the feedback was increasing as a function of both visual and haptic intensities. The results are also highly significant compared to other visual or haptic intensities; even though we do note a plateau whether the visuals or the haptic intensity are MEDIUM (see Figure 7, between light and dark purple for all the proposed visual renderings).

5.4.4 **Other Findings.** Participants were globally quite excited about the visual rendering effects that were proposed. The RIPPLE rendering effect was often compared to wavelets, or water effect. While this can - at first - not be perceived as congruent with physical vibrations, this effect received the highest ratings. We believe the reasons to be three-fold: (1) participants related to this effect in the real-life interactions; (2) vibrations can be used to induce various haptic features - including local deformation - that was perceived there; (3) the local deformation was perceived as enhancing the **self-causality** for most users - they truly embodied the source of the visual rendering, even in AR, where they could perceive their real vicinity (and the real tangible actuated cube). P14 for instance mentioned that the "waves effect made believe that the cube was interacting with the user" and that the interaction was somewhat perceived as reciprocal.

More importantly, while pseudo-haptics in AR are perceived as a challenge because of the lack of immersion and embodiment: it is harder to feel a sense of ownership, of agency and self-location in an avatar hand when our physical hand is visible. However, we do believe that this **self-causality** enhanced this embodiment in the interaction, mostly through a augmented sense of agency in the interaction. Similarly as per the definition of *presence* in AR - which qualifies more the object as "being here" rather than the user "being elsewhere than the real environment" [39], here we believe the **embodiment to be felt throughout the interaction with the object** rather than in the avatar hand representation.

## 6 Discussion and Future Work

AR using pass-through has made global breakthroughs recently, with more and more convincing technology and seamless integration with the real environments (as per the Apple Vision Pro or the Meta Quest 3); and AR seems to be considered for a growing number of contexts-of-use, despite its lack of haptic feedback.

Our findings demonstrate that the concept of *Flexible Reality* can effectively enhance the perception of haptic feedback strength and intensity in AR and VR. This challenges the prevailing assumption that AR, due to its less immersive nature compared to VR, may be less effective for pseudo-haptics applications. Notably, a significant equivalence was reported in the perceived intensity of feedback between AR and VR settings. This suggests that the tactile illusion induced by visual alterations (e.g., ripples or vibrations on a touched object's surface) can be just as compelling in AR. Our results align with previous research indicating that visual dominance can significantly modulate sensory perceptions, thereby compensating for the lack of physical haptic feedback in AR systems.

### 6.1 Importance of Self-Causality

An unexpected aspect of our study was examining how the perception of self-causality — the user's belief that they are causing the visual effect — influences the interaction's believability and overall enjoyment, similarly to Limerick et al. causal relationship to satisfaction [38]. Participants consistently rated interactions where they felt a direct causality (i.e., their actions leading to visible changes in object texture) as more engaging and realistic. This finding emphasize the importance of designing AR experiences that reinforce the user's role as a direct agent in the interaction, which can significantly enhance the immersive quality and satisfaction of the experience, even in the absence of genuine haptic feedback. While no effect was perceived on feedback intensity between AR and VR, future work includes a study comparing on the sense of agency, sense of body ownership and self-location (e.g., virtual embodiment) [32] between AR (avatar hand superimposed on the real hand) and VR (only avatar hand) to verify this assumption.

From our experiment, we hypothesize that this self-causality is highly correlated to a feeling of **embodiment** within the interaction. As presence in AR tends to be more quantifiable through *object presence* [29, 49] then *user presence*, we believe **self-causality** to respectively relate to *agency* [42] and virtual *embodiment*. This raises questions on the extent of this embodiment in AR, and whether it can be combined with more usual embodiment in avatar representations [23, 52]. Future work includes multiple questions: First, can
we extend this self-causality over the users' own body perception? Second, and reciprocally, what would happen if an object came in contact with the user? Can it externally alter one's body perception and sense of ownership?

## 6.2 More Haptic Features, Objects and Contexts-of-Use

We focused in this paper on vibrations and how they could be enhanced through visuals as much as haptics. We justified this focus as vibrations are the most common stimuli in haptics, and can provide a whole set of object properties (local deformation, slippage, texture etc.). Future work includes the investigation of more haptic properties, for instance thermal properties [8, 19] (see Figure 8). This effect could be studied as coupled with Peltier cells. We believe that studying and enabling more pseudo-haptic features will enable more realistic scenarios in the future of AR. In a training scenario for instance, being able to perceive local conditions (weather etc.) can potentially contribute to a better skill transfer in the real-life.

<figure><img src="image_12.png" /><figcaption>**Figure 8:** Visual representation of a custom-made COLD EFFECT Visual Rendering. Various cold pseudo-haptic perception could be provided (from left to right: really cold to warmer visual rendering).</figcaption></figure>

Our initial experiment utilized a solid surface with a minimal UV texture to assess the efficacy of pseudo-haptic effects in AR. Future research could explore how these illusions perform with different materials, such as soft object or ones made with fabric. Moreover, we did not tackle the Geometric Complexity and intrinsic properties of target objects in AR, though shapes [55], sizes [7] or stiffnesses [11] are already enhanced through pseudo-haptics in VR.

Our implementation was limited to a simple, small object, restricting the applicability of results to more complex scenarios. Further developments involve applying visual deformations selectively on more complex objects, simulating different material properties across distinct regions. This approach could lead to more realistic simulations and studies on user perception and responses to varied stimuli in augmented environments. Such advancements could pave the way for more intuitive and immersive user interfaces in fields like teleoperated robotics or AR-assisted surgery, where precise haptic perception is critical.

Future research will also include studying the integration of direct manipulation with interactive objects. We identified opportunities when combining tangible actuators which haptic output could change shape in response to user interactions.

## 6.3 Illusion Limits and Ethical Considerations

Defining the precise moment when a pseudo-haptic illusion “breaks” the user’s perception is crucial for refining VR/AR experiences. Future research should fine-tune the parameters of these illusions to identify the limits of user perception under varying conditions. This involves further experimentation with the intensity, frequency, and type of provided visual cues. Compared to VR, where the environment is fully controlled, the limits in AR might end-up different, where external distractions can break the occurring illusion.

The manipulation of sensory perception in AR, while offering extensive possibilities for enhancing user experience, also raises ethical questions similar to those presented in science-fiction. *Flexible Reality* can be instantiated in seamless AR environments, where the real vicinity can literally be augmented through our VISUAL RENDERING effects and where objects can be made visible/disappearable or available on the designer’s demand [16]; or, as mentioned previously, potentially change the perception of the users’ self. While these can be considered as fun experiments for gaming, where the users are aware and conscious about the augmented environments alterations, there is a need to consider the implications of altering a user’s sensory inputs - especially in contexts where such changes may affect decision-making or cause psychological impacts [34, 44].

## 7 Conclusion

In this paper, we presented *Flexible Reality*, a conceptual approach to extend feedback intensity perception in co-located AR using pseudo-haptics. We studied (n = 22) whether pseudo-haptics could be induced in a co-located direct interaction in pass-through AR, despite its lack of embodiment and immersion. Our results indicate that pseudo-haptics can be indeed be induced in AR, as much as in VR; and also demonstrate how visual renderings and intensity can induce the same feedback intensity as real haptic implementations. We also show how visuals and haptics intensities enhance each other to provide users with higher feedback strength perception. Based on qualitative feedback, we demonstrate the importance of self-causality - e.g. believing to be the source of the interaction - for interaction believability and enjoyment. Similarly to presence in AR, related to the presence of the augmented object, we relate this sense of agency and effect of embodiment in AR to an embodiment within the interaction.

*Flexible Reality* demonstrates how reality can seamlessly be made *flexible* in AR and alter haptic properties of a real user vicinity (through deformations, textures, vibrations, compliance) in a direct co-located manipulation at a low hardware development cost. However, and as *Flexible Reality* is originally inspired from science fiction, we finally use *Flexible Reality*'s concept to highlight the importance of ethical considerations in the future of AR scenarios, especially involving sensory illusions.

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Received 20 February 2007; revised 12 March 2009; accepted 5 June 2009