# Bouba, Kiki, and the Movement Between: Toward A Sensorimotor Hypothesis of Sound-Shape Cross-modal Correspondences

Lou Couston, Mitsuko Aramaki, Sølvi Ystad, Richard Kronland-Martinet, Etienne Thoret

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Lou Couston, Mitsuko Aramaki, Sølvi Ystad, Richard Kronland-Martinet, Etienne Thoret. Bouba, Kiki, and the Movement Between: Toward A Sensorimotor Hypothesis of Sound-Shape Cross-modal Correspondences. 17th International Symposium on Computer Music Multidisciplinary Research, Nov 2025, Londres, United Kingdom. 10.5281/zenodo.17497860. hal-05352705

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# Bouba, Kiki, and the Movement Between: Toward A Sensorimotor Hypothesis of Sound-Shape Cross-modal Correspondences

Lou Couston<fnref n="1" />,<fnref n="2" />, Mitsuko Aramaki<fnref n="2" />, Solvi Ystad<fnref n="2" />, Richard Kronland-Martinet<fnref n="2" /> and Etienne Thoret<fnref n="1" />

**Abstract.** This study investigates the role of implicit motor dynamics in shaping cross-modal correspondences that underlie the Bouba/Kiki effect. Building on the hypothesis of a shared amodal code linking movement, sound, and shape, we conducted two behavioural experiments in which participants associated nonwords with either visual geometric shapes or friction sounds derived from hand-drawn movements. In both modalities, we observed systematic mappings consistent with the Bouba/Kiki pattern. Crucially, we manipulated the relative phase (RP) between the horizontal and vertical components of movement trajectories, a key parameter describing spatial coordination. Results reveal that RP significantly predicts participants’ judgments across modalities, forming a continuous perceptual gradient from "Kiki" (angular, phase ≈ 0/π) to "Bouba" (rounded, phase ≈ π/2). These findings suggest that perceptual judgments are modulated not solely by sensory features, but by an internal motor representation of the movement that could have generated the stimulus. These preliminary findings support the existence of a motor-based, amodal code that structures cross-modal correspondences.

**Keywords:** Cross-modal correspondences, sound synthesis, friction sounds, bouba/kiki effect.

## 1 Introduction

Sounds fill our daily lives like a soundtrack attached to our movements. Every movement, a step, a brush, an impact, is answered by a vibration, a resonance. Movement becomes a source of sound, and sound, a witness to movement.

This organic relationship between movement and sound spans ages and disciplines. Since Antiquity, Greek thinkers associated colors with sounds, perceiving in painting an echo of music. In the field of dance, this connection is even more obvious. The human body responds spontaneously to music, a reality observable from early childhood; babies themselves naturally sway to the rhythm of sounds. This

<footnote n="1">¹ Aix-Marseille University, CNRS, Institut de Neurosciences de la Timone (INT), UMR7289</footnote>

<footnote n="2">² Aix-Marseille University, CNRS, Perception Representation Image Sound Music (PRISM)
UMR7061, Marseille, France
lou.couston@grenoble-inp.org
etienne.thoret@univ-amu.fr</footnote>
phenomenon was formalized by Fraisse [1], who showed that music can have a dynamogenic effect, meaning it promotes spontaneous motor activation.

Although the separation between dance and music has been an area of experimentation in contemporary dance, notably with the exploration of silence by Merce Cunningham or Trisha Brown [2,3], it is also essential to recall that the relationship between body and sound can operate in the other direction. Indeed, if music can guide dance, the body can also be the driver of sound. Initially, in an intuitive way, a musician may adapt their playing to the impulses of a moving body. Then, more formally, thanks to technological tools. The advent of interactive devices opens new perspectives, where movement parameters, such as speed, direction, or curvature, can be translated into sound material, influencing timbre, intensity, or texture of the sound.

These correspondences become the core of numerous artistic and scientific research. Some choreographic works incorporate this real-time interaction between movement and sound, such as R-A-U-X-A by Aina Alegre, where the musician shapes a sound material in direct dialogue with the dancing body. This approach suggests that a better understanding of the links between movement and sound perception could open new paths for creation, but also, more broadly, for our understanding of the deep connections between sensory and motor modalities.

This exploration of the link between movement and sound led us to formulate a broader hypothesis: what if the perception of a visual shape or a sound was not solely a matter of vision or hearing, but also involved simulating the movement that might have produced them [4,5,6]?

This idea is supported by several works in cognitive psychology and neuroscience. Freyd [7] showed that memorizing an image involving movement, such as a jump, is accompanied by a mental anticipation of the movement, suggesting that the brain simulates the action beyond the observed scene. Other studies, such as Longchamp et al. [8], revealed that perceiving letters or known shapes activates motor areas involved in their production. These results converge toward the idea that visual perception of a shape engages a motor simulation of the movement that has drawn it.

We then draw a parallel with auditory perception: if we hear a sound resulting from friction, such as a pencil on paper or an object dragged on the ground, could we also simulate the movement that produced it? And could this mentally reconstructed movement evoke a shape?

The works of Thoret et al. [9, 10, 11] develop this idea within the framework of multimodal perception of movements and shapes. First, Thoret et al. [9] demonstrated that hearing a drawing sound allows perceiving the dynamic characteristics of the movement that produced it, notably the speed profile, that is, how the speed of the movement varies over time. This profile is essential to recognize and differentiate shapes by their dynamics, linking sound temporality and graphic trajectory. Then, Thoret et al. [10] showed that sound indeed influences the sensorimotor synchronization with a visual movement, for example, by biasing the reproduction of an elliptical trajectory toward a more circular shape depending on the sound properties and associated velocity profile. Finally, Thoret et al. [11] highlighted that these phenomena are not merely sensory, but reflect the imprint of an action
representation in prototype geometric shapes. They thus suggest that hearing an
elliptical movement activates a common prototypical representation integrating
sound, vision, speed profile, and motor simulation, which structures shape perception.

Based on these works, we formulated our hypothesis: there exists a shared amodal
coding between shape, sound, and movement, which underlies cross-modal
correspondences such as in the well-known Bouba-Kiki effect [12]. This effect
denotes a systematic association between nonsense words (such as "Bouba" and
"Kiki") and geometric shapes (rounded and angular, respectively), robustly observed
across cultures, languages, and ages [13]. A recent article by Winter and Perlman [14]
reviews the main hypotheses proposed to explain this phenomenon, distinguishing
two main directions: the articulatory explanation and the acoustic explanation. In this
perspective, Forte and Schwartz [15] propose an ecological approach that goes
beyond this opposition by rooting the Bouba-Kiki effect in the sensory experience of
the physical world: the sounds of "Bouba" and "Kiki" would respectively evoke the
sounds produced by round objects (rather low-pitched and continuous) or angular
objects (rather high-pitched and discontinuous), establishing an iconic association
between auditory perception and visual shape. Moreover, the articulatory hypothesis,
supported by Vainio et al.[16], posits that motor configurations involved in producing
the sounds (such as lip rounding for "Bouba" or tongue tension for "Kiki") also
contribute to visually evoking certain shapes.

Complementing these approaches, we formulated a motor hypothesis: these cross-modal correspondences could rely on a mental representation of the movement needed to produce the shape or sound, in other words, on a motor simulation. For example, "Kiki" could evoke an angular, fast, jerky movement, close to that needed to draw a pointed figure, while "Bouba" would correspond to a continuous, circular, fluid movement. In other words, beyond acoustic or visual features, it is the shared motor dynamics that could structure these associations.

To test this hypothesis, we conducted a first experiment in which participants associated stimuli, either visual shapes or their corresponding friction sounds, with the nonsense words Bouba or Kiki. The sounds were synthesized from hand-drawn trajectories, converting movement speed into timbre variations, following a model based on Conan et al. [17]. This allowed us to assess whether similar motor dynamics, perceived through different modalities, would yield comparable associations.

Results revealed a link between participants’ judgments and the relative phase (RP) of the underlying movement, a parameter describing the coordination between horizontal and vertical movement components. Lower phases produce more angular trajectories; higher phases result in rounder ones.

To investigate the effect of this parameter more systematically, we designed a
second experiment in which RP was progressively varied to generate a continuum
from angular to rounded shapes, with corresponding friction sounds. This approach,
inspired by the movement analyses made by Wamain et al. [18], allowed us to test the
impact of RP modulation on Bouba/Kiki judgments, and whether it generalizes across
modalities.
Together, these two experiments aimed to evaluate whether RP can structure cross-modal correspondences between shape, sound, and non-words, and to explore a possible shared, amodal, motor code underlying the Bouba-Kiki effect.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Historical shapes of the Bouba-Kiki effect [19]</figcaption></figure>

## 2 Experiment 1: Exploring the Bouba/Kiki Effect from the Visual Shapes and the Sound of Drawing Movements

This first experiment explored the existence of a Bouba-Kiki effect in the auditory modality, relying not on linguistic sounds but on friction sounds derived from the tracing of geometric shapes. The goal was to determine whether these sounds, carrying the dynamics of a graphic movement, could be spontaneously associated with the nonsense words “Bouba” and “Kiki,” similarly to what is classically observed with visual shapes.

To do so, we designed two association tasks: a visual task, in which geometric shapes were presented on screen, and an auditory task, in which participants had to judge sounds generated from these same tracings. This dual approach allowed us to compare the effects of modality on the observed associations while working from a common basis — that of the drawn movement and, more importantly, whether sounds alone, carrying only the dynamic of movement, were sufficient to convey such an association.

### 2.1 Methods

**Recording the Shapes.** We began by recording the drawing of geometric shapes on a touchpad. For this, we used the libinput tool under Ubuntu, which allows capturing input device events (such as pointer movements). We developed a script to capture POINTER_MOTION events and save them into a .csv file. This file contained, for each time step, the pointer's displacement in x and y relative to the previous instant. From these data, the trajectories of the movements performed on the touchpad were reconstructed. The recordings started when the finger touched the pad and ended when it left. No instructions were given regarding speed or duration, to preserve natural variability in movement execution. Twelve pseudo-random shapes were drawn by the first author, forming the basis of the stimuli. These shapes were characterized by the x and y positions over time. We are aware that the fact that all the shapes were drawn by a single person could introduce a bias in the data. However, we consider this as a first step to establish a baseline, and future work will aim to diversify the source of drawings to account for individual differences in gesture execution.
**Visual Stimuli.** Visual static stimuli were generated from these manual tracings. The 12 shapes were divided into two broad categories: rounded and angular. Each shape represented an extreme of the visual spectrum in terms of movement dynamics, allowing us to create a set of 24 visual stimuli.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Example of two visual stimuli (Bouba - Kiki)</figcaption></figure>

**Auditory Stimuli.** Each visual shape was associated with a synthesized friction sound designed to reflect the dynamics of the movement that produced that shape. We based our approach on the work of Conan et al. [17], which showed that the timbre of friction sounds, such as a pencil on paper, is related to the velocity of the movement, and the surface roughness. Their model considers sound as the result of interaction between a source (the pencil) and a resonator (the paper and table), within an ecological perception perspective. Following this approach, we generated auditory stimuli from the manually drawn trajectories. For each shape, we extracted the velocity profile, obtained as the derivative of the positions, and used it to dynamically filter a pink noise with a low pass filter whose cutoff frequency was linearly mapped to the frequency of the movement.

The resulting signal was then convolved with a wooden impact sound to introduce a resonant component, following the source-filter model proposed by Thoret [17,20,21]. This process allowed us to produce sounds in which the kinematics of the movement are encoded in the spectral content, simulating realistic friction. In total, 24 drawing sounds were generated, each corresponding to one of the 24 recorded visual shapes.

## 2.2 Participants

The study involved nine participants (mean age: 32.2 ± 18.6 years old), including five men and four women. All were right-handed and wore headphones during the experiment. Six were non-musicians or non-specialists in sound, two were musicians, and one was a sound professional.

## 2.3 Task and Procedure.

The experiment was conducted online via the Testable platform [22]. Participants were asked to be in a quiet, distraction-free environment and to wear headphones. Each performed two tasks, one visual and one auditory, presented in randomized
order. Each task was preceded by two familiarization examples to introduce the
Bouba/Kiki association principle in both visual and auditory modality.

**Visual Task.** A geometric shape appeared at the center of the screen. Participants had to indicate whether the shape evoked the nonsense word “Bouba” or “Kiki” by clicking on one of two buttons on the screen (“Bouba” on the left, “Kiki” on the right) or using keyboard arrows (left=Bouba, right=Kiki).

**Auditory Task.** A friction sound corresponding to a shape's tracing was played. With no image shown, participants were asked to imagine the underlying shape and then choose “Bouba” or “Kiki” based on what the sound evoked, using the same response method as in the visual task.

Stimuli were presented one by one, with a fixed 800 ms inter-stimulus interval after each response. The whole experiment lasted about 10 minutes.

## 2.4 Data Analysis

The main goal of this experiment was to assess whether Bouba/Kiki associations are consistent across visual and auditory modalities, and whether they can be explained by gestural parameters. The RP between the x(t) and y(t) (1) components of the movement was more specifically analyzed. This parameter captures the temporal relationship between horizontal and vertical movement components. RP was computed throughout the trajectory, and values at the time of response were used for analysis. RP(t)=φ<sub>y</sub>(t)-φ<sub>x</sub>(t) (1) (where φ<sub>x</sub>(t) et φ<sub>y</sub>(t) are the instantaneous phases of the x and y signals at time t, respectively.)

We first examined whether response patterns were consistent across modalities for the same stimulus. To this end, responses were binarized ("Bouba"=0, "Kiki"=1), and the mean responses per stimulus were compared between visual and auditory conditions.

To explore how RP relates to perceptual judgments, we visualized the distribution of RP using violin plots across the crossed conditions of Modality (visual vs. auditory) and perceived Shape (Bouba vs. Kiki). Finally, a two-way ANOVA was conducted to test the effects of RP and modality, as well as their interaction, on participants' responses.

## 2.5 Results

The average responses across modalities revealed consistent Bouba/Kiki associations (Fig. 1 (A)). In the auditory condition, sounds derived from round movements (category “b”) were more frequently judged as Bouba (M=.31, SD=.47), while those from angular movements (category “k”) were more often labeled as Kiki (M=.66, SD=.48). The analysis, which used Rationalized Arcsin Unit scores (Fig. 1 (A)), indicates that the stimuli were generally perceived as expected, although some stimuli were more ambiguous than others. This validates the possibility of recalling the Bouba/Kiki effect from the sounds evoking the motion of someone drawing geometric shapes.
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (A) Mean Rationalized Arcsine Unit (RAU) scores for each sensory modality across different stimuli - (B) Violin Plot Showing Average Density Distributions of RP by Modality (Visual, Audio) and Shape Label (Bouba, Kiki) — Experiment 1</figcaption></figure>

We then examined how RP contributes to these judgments. A violin plot of the phase distribution (Fig. 1 (B)) shows distinct phase patterns associated with Bouba and Kiki responses. Notably, the distribution reveals a peak around 0 radians for Kiki responses and around $\pi/2$ radians for Bouba responses, indicating distinct phase preferences. These patterns were consistent across modalities, suggesting that RP serves as a cross-modal perceptual cue.

A two-factor ANOVA confirmed a strong main effect of RP on categorization (F(7)=49.82, p < .001), with a smaller but significant effect of modality (F(1)=12.49, p=.0004). The significant interaction (F(7)=3.33, p=.0027) indicates that the influence of RP varies across sensory modalities.

## 2.6 Discussion

Experiment 1 confirms that the Bouba–Kiki effect extends to the auditory modality, with friction sounds from movements influencing shape judgments similarly to visual stimuli. This supports the idea of a shared gestural basis accessible across senses, possibly reflecting sensorimotor processes underlying perception.

The clear link between RP and judgments in both modalities reinforces this view. However, as this experiment was exploratory, further work is needed to identify the specific movement parameters involved, motivating the more targeted approach of Experiment 2.

## 3 Experiment 2: Effect of The Relative Phase on Bouba/Kiki Associations

The first experiment used freely drawn shapes, letting natural variations in tracing dynamics spontaneously influence the results. In contrast, the second experiment aimed to precisely control and manipulate the RP, a key dynamic parameter to test whether it alone governs the cross-modal correspondence.
We generated a continuum of stimuli from a single reference movement by gradually modifying the RP between the x(t) and y(t). This produced trajectories ranging from very circular to very angular shapes. We hereby sought to confirm that it is indeed the spatial structure of the movement, and not an isolated visual or acoustic property, that drives Bouba/Kiki judgments.

**3.1 Methods**

**Generation of the Stimulus Continuum.** We selected six geometric shapes from the stimuli of Experiment 1, that were not ambiguous in the results.

For each drawing, we applied a Hilbert transform separately to the x and y signals to extract their envelopes and instantaneous phases. RP was computed as the difference, at each time point, between the instantaneous phases of y and x. This measure reflects the cyclic coordination between the two spatial dimensions.

To create a continuum of 15 variations for each original shape (Fig. 1), we systematically modified the RP between the x and y components of the movement. Starting from an original trajectory, we added a constant phase offset ($\Delta\varphi$) to the instantaneous phase of the y-component, and then recomputed a modified y-signal ($y'(t)$). This process was repeated for 15 values of $\Delta\varphi$, linearly spaced between $-\pi$ and $\pi$, resulting in 15 distinct variations of the initial shape. This manipulation altered spatial coordination, thereby changing the position trajectories and their velocity profiles, which in turn affected both the perceived shape (rounder or more angular) and the dynamics encoded in the associated sounds.

<figure><img src="image_5.png" /><figcaption>Fig. 1. Example of a continuum made of 15 variations of a single shape</figcaption></figure>

**Synthesis of Auditory Stimuli.** The modified trajectories (x(t), y'(t)) were then used to synthesize sounds, following the same method as in Experiment 1.

**3.2 Participants**

The experiment involved 9 participants (mean age: 30.6 ± 12.6 years), including 5 men and 4 women, all right-handed. Among them, 4 were neither musicians nor sound specialists, 3 were musicians, and 2 were sound professionals. All participants reported using headphones during the experiment.

**3.3 Task and Procedure**

The experiment was conducted online using the Testable platform [23], under conditions similar to those of Experiment 1. Participants were asked to be in a quiet environment and to use headphones. Two tasks, one visual and one auditory, were presented in randomized order, each preceded by three familiarization trials. Each
task consisted of a continuum of 15 variations generated from 6 different shapes,
totaling 90 stimuli per modality. In the visual task, shapes were presented with no
time limit, and participants could respond freely. In the auditory task, each friction
sound was played in full (about 4 seconds), and again, responses were made without
time constraints. A fixed 800 ms interval separated each stimulus. Due to the larger
number of stimuli, the total experiment duration was estimated at around 20 minutes.

3.4 Data Analysis

The main objective of this experiment was to evaluate the influence of the RP
between the x(t) and y(t) components of the graphic movement on Bouba/Kiki
judgments, particularly in the auditory modality.

As in Experiment 1, we generated violin plots to visualize the distribution of RP
across the crossed Modality × Shape conditions. In addition, the mean response
(Bouba vs. Kiki rate) was plotted as a function of the dominant phase, defined as the
peak of the phase distribution, i.e., the most frequent bin, to better capture how
specific RP values drives perceptual judgments.

Responses were statistically analysed using a two-way ANOVA, with Modality
(visual vs. auditory) and RP (as a continuous variable, binned for analysis) as factors,
to assess main effects and their interaction.

3.5 Results

RP significantly influenced Bouba/Kiki judgments, although the effect appeared more
moderate than in Experiment 1. This trend was first observable in Fig. 1 (B), which
suggests a consistent pattern across both modalities: certain RP tended to be more fre-
quently associated with Bouba, and others with Kiki. Due to the shape continuum
used in this experiment, the key RP values observed in Experiment 1 are less clearly
defined here. Nevertheless, Fig. 1 (B) suggests distinct tendencies: Kiki responses
were generally more frequent around RP near 0 and π, whereas Bouba responses
tended to cluster around π/2.

<figure><img src="image_6.png" /><figcaption>**Fig. 1.** (A) Mean response by dominant RP for each modality - (B) Violin Plot Showing Average Density Distributions of RP by Modality (Visual, Audio) and Shape Label (Bouba, Kiki) — Experiment 2</figcaption></figure>
Plotting the mean response (coded as 0 for Bouba and 1 for Kiki) as a function of the dominant RP (Fig. 1 (A)) revealed a common trend across modalities. More importantly, it shows that RP influences participants' judgments in a continuous manner, rather than categorically or discretely.

A two-way ANOVA confirmed a strong main effect of RP on shape judgments (F(3)=1.17, p < .001), while modality showed no significant main effect (F(3)=.65, p=.057). However, a significant interaction (F(4)=3.34, p=.00038) suggests that the influence of RP is stronger and more consistent in the visual modality, in line with the observed behavioural patterns.

## 3.6 Discussion

This experiment confirms that RP continuously modulates Bouba-Kiki judgments across modalities rather than acting as a categorical cue. The stronger and more consistent effect in the visual modality may reflect differences in stimulus quality, as our stimuli's auditory rendering could introduce more variability. These findings highlight the importance of improving the generation model for the shape continuum and further investigating how stimulus properties affect perception.

Ambiguities observed at intermediate RP values emphasize the complexity of the underlying dynamics and the need for continued research to refine stimulus design and better understand cross-modal influences.

# 4 General Discussion

The two experiments reported here explored the role of implicit movement dynamics in the Bouba-Kiki effect, using the sounds produced by drawing movements to investigate non-word-shape associations. The findings suggest that beyond sensory features, participants may rely on internal representations of movement underlying the stimulus.

Experiment 1 showed that friction sounds derived from movement-based drawings tend to elicit Bouba/Kiki associations similar to those observed in the visual domain. This convergence supports the idea of a shared gestural substrate accessible across modalities, consistent with motor simulation theories. Experiment 2 examined this further by manipulating the RP between movement components. Results showed that this parameter continuously shaped judgments: RPs near 90° (π/2) were associated with Bouba, while values near 0 or 180° (π) aligned with Kiki. This suggests sensitivity to the temporal coordination of motion across both sensory modalities.

These findings, although very preliminary regarding the small participant sample size, expand traditional articulatory or acoustic accounts by introducing a sensorimotor dimension. Rather than being based solely on speech-related features, the observed associations may reflect an implicit reconstruction of the movement dynamics behind the stimulus. This motor perspective differs from linguistic interpretations, as it involves non-semantic, physical movement cues. It aligns with
embodied cognition theories, in which perception draws on internal simulations of
action.

In sum, implicit movement dynamics, such as RP, appear to shape cross-modal correspondences. The Bouba-Kiki effect may thus reflect a deeper motor principle, grounded in our embodied experience and shared across sensory systems.

5 Conclusion

This study offers new insights into the mechanisms underlying the Bouba-Kiki effect, demonstrating that implicit motor dynamics play a central role in associations between visual shapes, friction sounds, and non-words. Our results confirm that the RP of movements modulates participants' judgments continuously and consistently, across both visual and auditory modalities. This convergence supports the idea of a shared amodal coding system based on implicit motor simulation that links perception, sound, and movement.

More broadly, this work opens promising avenues for understanding how the brain integrates and links sensory modalities through dynamic movement representations. It suggests that perception is not merely the passive reception of information, but involves the active reconstruction of the movements behind perceived stimuli.

6 Acknowledgments

Research supported by grants ANR-16-CONV-0002 (ILCB), ANR-24-CE38-4175 (PSIND), and the Excellence Initiative of Aix-University (A*MIDEX). This work was carried out as part of the HARMONIE project of the PEPR ICCARE. It was supported by the French National Research Agency (ANR-23-PEIC-0002) under the France 2030 program. This project has received financial support from the CNRS through the MITI interdisciplinary programs.

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