Animals Make Music: A Look at Non-Human Musical Expression

Abstract

The use of musical instruments and interfaces that involve animals in the interaction process is an emerging, yet not widespread practice. The projects that have been implemented in this unusual field are raising questions concerning ethical principles, animal-centered design processes, and the possible benefits and risks for the animals involved. Animal–Computer Interaction is a novel field of research that offers a framework (ACI manifesto) for implementing interactive technology for animals. Based on this framework, we have examined several projects focusing on the interplay between animals and music technology in order to arrive at a better understanding of animal-based musical projects. Building on this, we will discuss how the implementation of new musical instruments and interfaces could provide new opportunities for improving the quality of life for grey parrots living in captivity.

Full text

Multimodal Technologies
and Interaction
Review
Animals Make Music: A Look at Non-Human
Musical Expression
Reinhard Gupfinger * and Martin Kaltenbrunner
Institute of Media Studies, University of Art and Design Linz, 4020 Linz, Austria; martin.kaltenbrunner@ufg.at
*Correspondence: reinhard.gupfinger@ufg.at
Received: 20 April 2018; Accepted: 28 August 2018; Published: 2 September 2018


Abstract:
The use of musical instruments and interfaces that involve animals in the interaction process
is an emerging, yet not widespread practice. The projects that have been implemented in this unusual
field are raising questions concerning ethical principles, animal-centered design processes, and the
possible benefits and risks for the animals involved. Animal–Computer Interaction is a novel field of
research that offers a framework (ACI manifesto) for implementing interactive technology for animals.
Based on this framework, we have examined several projects focusing on the interplay between
animals and music technology in order to arrive at a better understanding of animal-based musical
projects. Building on this, we will discuss how the implementation of new musical instruments and
interfaces could provide new opportunities for improving the quality of life for grey parrots living
in captivity.
Keywords: ACI; animal music; animal-centered design; metamusic
1. Introduction
The number of physical products and software applications developed specifically for animals
and the commercial interest in technological mediators for human–animal interactions is growing [
1
–
4
].
In addition to the physical and graphical interaction design aspects of such technology, sound and
music can also play an important role in the design process [
5
]. Despite the increasing amount
of hardware and software for animals, there are only a few studies devoted to the use of audio
technologies to enrich the living environment of animals in captivity [6–8].
In 2016, a research project was started by the authors of this paper in collaboration with the artist
group alien productions and the zoologists and animal keepers of ARGE Papageienschutz. The principal
goal of this project is to design and develop musical instruments and interfaces for a group of grey
parrots held in captivity at an animal shelter near Vienna. This research is based on an ongoing art
project named metamusic [
9
], which was initiated in 2012 by alien productions. Adopting an artistic
approach, the collective has been investigating how grey parrots react to musical stimuli, exploring
whether they can become an intrinsic part of interactive sound installations, and assessing whether
they are capable of producing “parrot music” (new music that does not have to sound like music to
human ears) by themselves. The project aims to generate artistic output for humans while improving
the quality of life for grey parrots in captivity with musical instruments and interfaces.
Recent research in the field of cognitive biology has focused on the role of animals listening to
human music as a concept of enrichment [
10
–
13
]. Since most of the music is selected by humans,
this can lead to anthropomorphic biases. Therefore, the music should be attuned to the animals’
auditory skills. Studies have shown that non-human species also have musical skills [
14
–
16
] and
display entrainment to auditory stimuli [
17
,
18
]. Animal species such as grey parrots, cockatoos,
elephants, primates, pigeons, and carps have been found to be able to discriminate between different
Multimodal Technologies and Interact. 2018,2, 51; doi:10.3390/mti2030051 www.mdpi.com/journal/mti

Multimodal Technologies and Interact. 2018,2, 51 2 of 14
composers or different genres, prefer music to silence, or move in rhythmic synchronicity to the musical
beat [10,14,17–19].
According to further studies, some parrot species such as grey parrots and cockatoos have musical
skills and a natural feeling for rhythm [
17
,
18
]. Grey parrots, for example, can repeat musical patterns
and have the ability to reproduce sounds [
15
,
20
]. In this respect, it appears appropriate to work with
grey parrots in the context of animal-based music.
In 2011, Clara Mancini in her “Animal–Computer Interaction (ACI): A Manifesto” proposed
the keystones for ACI as a field of research. ACI research aims to understand the interaction
between animals and the computing technology [
21
], and is therefore a significant step toward an
ethical implementation of such technologies. In this manifesto, Mancini discussed ethical principles,
guidelines, and methodologies for the design, analysis, and evaluation of ACI systems. To the best
of our knowledge, there is no comprehensive summary and analysis of projects that involve animals
interacting with music instruments and interfaces. We therefore have examined different approaches
and technologies in diverse fields from a musical interface designer’s perspective. This paper gives an
overview of current technologies and important developments in the context of musical instruments
that include animals in the music and sound-generating process, with a focus on the creative and
artistic outcomes for both animals and humans. We are interested in gaining a better understanding of
the actual role of the animals interacting with such technologies. This paper attempts to determine the
factors that could lead to a suitable design approach and its ethical implementation.
2. Cultural Context
Musical instruments and interfaces that involve animals as musical agents can be found in
different fields such as New Interfaces for Musical Expression (NIME), contemporary art installations,
animal cognition experiments, and ACI. Some of the earliest examples of conjectural musical
instruments that (ab)use animals as sound generators can be found between the 16
th
and 19
th
centuries
under the names of the Cat Organ or the Pig Organ ([
22
], Figure 1). Both are speculative, piano-like
instruments in which cats or pigs are placed inside or attached to the instrument. When the keys are
pressed, thus poking the pig or pinching the tail of the cat, the animal cries out in pain and generates a
pitched sound. Since both instruments are based on inflicting pain upon animals, such bizarre musical
instruments have never been built out of ethical and animal welfare concerns. However, these ideas
provide evidence of an early interest in humans making music involving animals, in this case, as a
structural part of the musical instrument itself. In general, live animals are not usually a constitutive
part of musical instruments, and there are not many examples in which such instruments have actually
been put into practice.
Multimodal Technol. Interact. 2018, 2, x FOR PEER REVIEW 2 of 14
composers or different genres, prefer music to silence, or move in rhythmic synchronicity to the
musical beat [10,14,17–19].
According to further studies, some parrot species such as grey parrots and cockatoos have
musical skills and a natural feeling for rhythm [17,18]. Grey parrots, for example, can repeat musical
patterns and have the ability to reproduce sounds [15,20]. In this respect, it appears appropriate to
work with grey parrots in the context of animal-based music.
In 2011, Clara Mancini in her “Animal–Computer Interaction (ACI): A Manifesto” proposed the
keystones for ACI as a field of research. ACI research aims to understand the interaction between
animals and the computing technology [21], and is therefore a significant step toward an ethical
implementation of such technologies. In this manifesto, Mancini discussed ethical principles,
guidelines, and methodologies for the design, analysis, and evaluation of ACI systems. To the best of
our knowledge, there is no comprehensive summary and analysis of projects that involve animals
interacting with music instruments and interfaces. We therefore have examined different
approaches and technologies in diverse fields from a musical interface designer’s perspective. This
paper gives an overview of current technologies and important developments in the context of
musical instruments that include animals in the music and sound-generating process, with a focus
on the creative and artistic outcomes for both animals and humans. We are interested in gaining a
better understanding of the actual role of the animals interacting with such technologies. This paper
attempts to determine the factors that could lead to a suitable design approach and its ethical
implementation.
2. Cultural Context
Musical instruments and interfaces that involve animals as musical agents can be found in
different fields such as New Interfaces for Musical Expression (NIME), contemporary art
installations, animal cognition experiments, and ACI. Some of the earliest examples of conjectural
musical instruments that (ab)use animals as sound generators can be found between the 16th and 19th
centuries under the names of the Cat Organ or the Pig Organ ([22], Figure 1). Both are speculative,
piano-like instruments in which cats or pigs are placed inside or attached to the instrument. When
the keys are pressed, thus poking the pig or pinching the tail of the cat, the animal cries out in pain
and generates a pitched sound. Since both instruments are based on inflicting pain upon animals,
such bizarre musical instruments have never been built out of ethical and animal welfare concerns.
However, these ideas provide evidence of an early interest in humans making music involving
animals, in this case, as a structural part of the musical instrument itself. In general, live animals are
not usually a constitutive part of musical instruments, and there are not many examples in which
such instruments have actually been put into practice.
Figure 1. Pig Organ, cover illustration for the piece of sheet music “La Piganino” [22].
Figure 1. Pig Organ, cover illustration for the piece of sheet music “La Piganino” [22].

Multimodal Technologies and Interact. 2018,2, 51 3 of 14
The idea of animals becoming anthropomorphic musicians playing traditional acoustic
instruments is much more common in the literature. Well-known fairy tales such as The Town
Musicians of Bremen have popularized this concept. Since the middle of the 20th century, this particular
configuration has often been restaged in a circus environment, where, in general, various species
including primates were capable of performing various musical tasks through conditioning [23].
A related topic within the field of Human–Computer Interaction (HCI) is the design and use
of new technologies for musical expression and artistic performance. Researchers and artists are
developing new musical instruments and interfaces with the aim of understanding and influencing
new forms of artistic expression, usability, and playability. The introduction and development of new
technologies in this field also opens up new possibilities to include animals in the music-generating
process. The projects we could find from this community are mostly combinations of a computer
vision system tracking the animals, and a sound-generating system, which is controlled by the animals’
movements. A common setup in this field is fish-based interfaces [
24
–
30
]. The video tracking system
observes the fish in the aquarium, and the sounds are generated or processed by the generally random
movements of the fish [
24
]. The output of such fish-based musical interfaces can be manifold and
creative, but since the fish are separated from the audio feedback, they lack an animal-centered design
approach. Thus, in some of the projects, all of the plants, stones, and other aquarium furniture have
been removed to provide a better camera view, which leads to the discussion of potential negative
impacts on the living environment of the fish themselves.
Another extensive field in which live animals are used in installations and projects is the
contemporary art world. Various projects such as Harness by Robert Wechsler [
31
] or from here to
ear by Céleste Boursier-Mougenot [
32
] focus on the relationship between animals and humans, and
how animals can become the author or co-author of artworks or the creative process. Some artists
such as Céleste Boursier-Mougenot deal with the musicality of animals by building installations that
involve animals creating sonic environments. Birds, which are known for their musical talents [
14
,
15
],
often play an important role in such art installations.
In the field of animal cognition, test setups, for example with touchscreens, are sometimes used in
experiments to provide an insight into the animals’ musicality or musical preferences [
1
]. However,
these test devices are less attractive for musical interface design, because they generally do not focus
on a playful design. However, their results, such as evidence of the entrainment of animals to music
stimuli [
17
,
18
], have great significance for our study. These kinds of test devices have enabled scientists
to gather evidence that animal species have auditory skills and musical talents, and examine how
these skills work [15,16].
ACI aims at gaining a better understanding of how animals interact with technology, and thus
some ACI employs technology as a mediator between different species, including humans. Even
though ACI is growing fast, the potential opportunities for acoustic enrichment in the context of ACI
technology have received little attention.
3. Musical Animal–Machine Interaction
Based on the more active or passive role that animals can play in the interaction process with
musical instruments and interfaces, we shall now discuss and analyze some projects focusing on
the interaction between animals and music technology that we have identified as relevant to ACI.
The analysis and classification of these projects provides a source of data that could potentially lead
to further contributions in this field, and theoretical improvements in an animal-centered design
process. In the following, we shall define four categories, which are generally based on the degree of
musical agency of the individual species involved: Animal Movement as Control Source; Animals as
Unconscious Performers; Animals as Trained Musicians; and Animals as Voluntary Musicians. Loosely
related to the overall evolutionary stage, these categories are based on the musical understanding and
the related physical and cognitive abilities of the individual species, which range from a complete
lack of any auditory perception and expression to highly developed musical abilities and vocal

Multimodal Technologies and Interact. 2018,2, 51 4 of 14
expression. We intend to implement and extend the findings from the last category (Animals as
Voluntary Musicians) in our ongoing project to develop and design musical instruments and interfaces
for grey parrots.
3.1. Animal Movement as Control Source
The first category of musical interfaces generally involves lower life forms such as bacteria or
other simple animal species such as worms. These animals either completely lack any auditory sensory
capabilities or acoustic expression, or are completely isolated from perceiving any musical stimuli.
Thus, in such a configuration, these animals only play a passive role, since they are unable to interact
with input devices in the sound generation process, and only serve as nearly aleatory control sources.
A typical example of such a musical instrument, which in this case is controlled by live worms, is
the Din Datin Dudero project developed by Peter Blasser. As the worms wriggle around metallic
contact pins, their movements modify the circuits inside the synth and produce different sounds [
33
].
The experimental design of the instrument appears to be suited to the limited physical abilities of
the worms and basically integrates their bodies into the musical circuitry itself. It remains unclear if
the worms are perceiving the low-voltage signals though their bodies and are capable of sensing any
musical result at all.
Many of the musical examples we have found use a video tracking system to observe various
animals and create sounds based on the overall movements of these animals. The most common
species used in these systems are fish in an aquarium. Since the mid-2000s, this idea has been
explored in different fields with various fish species. For example, The Accessible Aquarium is a
long-time project of the Georgia Institute of Technology in Atlanta that is based on the field of informal
learning environments. It attempts to make aquaria, zoos, and science centers more accessible for
visitors through interaction and sonification, and thus enhance the visitors’ learning experience [
25
].
The Accessible Aquarium environment allows visitors to mimic the animals’ movements by using tangible
objects to tell when a new fish enters the viewing range, what kind of fish it is, and in what direction
it’s moving; or add sounds or melodies to a specific aquarium fish. Sonifications corresponding to the
visitors’ movements can be paired with real-time animal-based sonifications produced by the existing
system to generate a musical composition of the visitors and the fish [26].
Other fish-based projects such as FuXi [
24
], Submersed Songs [
27
], Quintetto ([
28
], Figure 2), Musica
sull’Acqua [
29
], and Sonification of Fish Movement [
30
] take a more artistic approach. Here, fish are
employed as musical instruments and interfaces for sound installations or musical performances. Even
though all of the projects mentioned above use a video tracking system, they differ from each other in
the way in which sound is mapped to the fishes’ movement and what kind of sound was generated.
Although it has been shown that some fish species such as carp can discriminate between different
styles of music [34], and gurnard also have audio communication skills [35], these particular abilities
have not been investigated within the fish-based projects found in this field.
Multimodal Technol. Interact. 2018, 2, x FOR PEER REVIEW 4 of 14
abilities and vocal expression. We intend to implement and extend the findings from the last
category (Animals as Voluntary Musicians) in our ongoing project to develop and design musical
instruments and interfaces for grey parrots.
3.1. Animal Movement as Control Source
The first category of musical interfaces generally involves lower life forms such as bacteria or
other simple animal species such as worms. These animals either completely lack any auditory
sensory capabilities or acoustic expression, or are completely isolated from perceiving any musical
stimuli. Thus, in such a configuration, these animals only play a passive role, since they are unable to
interact with input devices in the sound generation process, and only serve as nearly aleatory control
sources. A typical example of such a musical instrument, which in this case is controlled by live
worms, is the Din Datin Dudero project developed by Peter Blasser. As the worms wriggle around
metallic contact pins, their movements modify the circuits inside the synth and produce different
sounds [33]. The experimental design of the instrument appears to be suited to the limited physical
abilities of the worms and basically integrates their bodies into the musical circuitry itself. It remains
unclear if the worms are perceiving the low-voltage signals though their bodies and are capable of
sensing any musical result at all.
Many of the musical examples we have found use a video tracking system to observe various
animals and create sounds based on the overall movements of these animals. The most common
species used in these systems are fish in an aquarium. Since the mid-2000s, this idea has been
explored in different fields with various fish species. For example, The Accessible Aquarium is a
long-time project of the Georgia Institute of Technology in Atlanta that is based on the field of
informal learning environments. It attempts to make aquaria, zoos, and science centers more
accessible for visitors through interaction and sonification, and thus enhance the visitors’ learning
experience [25]. The Accessible Aquarium environment allows visitors to mimic the animals’
movements by using tangible objects to tell when a new fish enters the viewing range, what kind of
fish it is, and in what direction it’s moving; or add sounds or melodies to a specific aquarium fish.
Sonifications corresponding to the visitors’ movements can be paired with real-time animal-based
sonifications produced by the existing system to generate a musical composition of the visitors and
the fish [26].
Other fish-based projects such as FuXi [24], Submersed Songs [27], Quintetto ([28], Figure 2),
Musica sull’Acqua [29], and Sonification of Fish Movement [30] take a more artistic approach. Here,
fish are employed as musical instruments and interfaces for sound installations or musical
performances. Even though all of the projects mentioned above use a video tracking system, they
differ from each other in the way in which sound is mapped to the fishes’ movement and what kind
of sound was generated. Although it has been shown that some fish species such as carp can
discriminate between different styles of music [34], and gurnard also have audio communication
skills [35], these particular abilities have not been investigated within the fish-based projects found
in this field.
Figure 2. Fish-based instrument Quintetto by Quiet ensemble 2009 [19].
Figure 2. Fish-based instrument Quintetto by Quiet ensemble 2009 [19].

Multimodal Technologies and Interact. 2018,2, 51 5 of 14
3.2. Animals as Unconscious Performers
This section deals with projects that involve animals directly touching or triggering the
interfaces, instruments, or sensors, and also involves species that have a developed hearing capability.
Nevertheless, these species lack a general cognitive understanding of the causality of their actions in
relation to the musical results, and are therefore unconscious musical performers. The Intelligent MIDI
Sequencing with Hamster Control project by Levy Lorenzo was a musical instrument that generated
sounds based on the movements of six hamsters. Each hamster was placed inside a narrow single
track where they could only move in a lateral direction. Based on the effective range of a distance
sensor and the typical length of a hamster, Lorenzo decided to build 36-inch tracks. This allowed the
hamsters to walk, sit up, and turn around while still remaining within sight of the distance sensor [
36
].
The sounds generated depended on the distance of the hamster from the sensor. The hamsters had
no free choice to interact with the instrument, and they had only limited space to move within the
instrument tracks. It is unclear how long the hamsters had to stay in the instrument.
Robert Wechsler adopted a different approach in his Harness project [
31
]. He combined a mouse
cage, including a mouse wheel, with a music box situated outside of the cage. When the mouse
exercised on the wheel, it powered the music box, and thus produced the sounds stored within
it. Based on our work with grey parrots [
37
], we assume that it is a good approach for utilizing a
well-established device for pet environments. However, positioning the music box outside of the cage
and therefore shielding it from direct audio feedback in the cage seems disadvantageous, since it also
isolates the mouse from the musical result.
A well-known art project is from here to ear ([
32
], Figure 3) by Céleste Boursier-Mougenot.
Since 1999, he has been developing musical environments and installations for finches in public
exhibitions. The installations combine a set of traditional musical instruments—mainly electric guitars
and drums—that serve as a resting place for the finches and produce sounds upon contact. The public
is invited to enter the installation space and mingle with the finches, whose movements generate a
live musical piece. Each installation can be considered as a unique piece that is living and ephemeral;
it is determined by the given variables of the situation and linked to the circumstances of the given
moment [
32
]. Influenced by the audience, the finches generate a random sound collage that allows for
minimal compositional procedures and choreographies defined by spatial arrangements. This project
is currently one of the most established installations in the field of animal–human sound installation.
Since it has been shown that finches have musical abilities [
14
,
16
], it would be interesting to provide
them with musical instruments that are adapted to the physical skills of the finches.
Multimodal Technol. Interact. 2018, 2, x FOR PEER REVIEW 5 of 14
3.2. Animals as Unconscious Performers
This section deals with projects that involve animals directly touching or triggering the
interfaces, instruments, or sensors, and also involves species that have a developed hearing
capability. Nevertheless, these species lack a general cognitive understanding of the causality of
their actions in relation to the musical results, and are therefore unconscious musical performers.
The Intelligent MIDI Sequencing with Hamster Control project by Levy Lorenzo was a musical
instrument that generated sounds based on the movements of six hamsters. Each hamster was
placed inside a narrow single track where they could only move in a lateral direction. Based on the
effective range of a distance sensor and the typical length of a hamster, Lorenzo decided to build
36-inch tracks. This allowed the hamsters to walk, sit up, and turn around while still remaining
within sight of the distance sensor [36]. The sounds generated depended on the distance of the
hamster from the sensor. The hamsters had no free choice to interact with the instrument, and they
had only limited space to move within the instrument tracks. It is unclear how long the hamsters had
to stay in the instrument.
Robert Wechsler adopted a different approach in his Harness project [31]. He combined a mouse
cage, including a mouse wheel, with a music box situated outside of the cage. When the mouse
exercised on the wheel, it powered the music box, and thus produced the sounds stored within it.
Based on our work with grey parrots [37], we assume that it is a good approach for utilizing a
well-established device for pet environments. However, positioning the music box outside of the
cage and therefore shielding it from direct audio feedback in the cage seems disadvantageous, since
it also isolates the mouse from the musical result.
A well-known art project is from here to ear ([32], Figure 3) by Céleste Boursier-Mougenot. Since
1999, he has been developing musical environments and installations for finches in public
exhibitions. The installations combine a set of traditional musical instruments—mainly electric
guitars and drums—that serve as a resting place for the finches and produce sounds upon contact.
The public is invited to enter the installation space and mingle with the finches, whose movements
generate a live musical piece. Each installation can be considered as a unique piece that is living and
ephemeral; it is determined by the given variables of the situation and linked to the circumstances of
the given moment [32]. Influenced by the audience, the finches generate a random sound collage
that allows for minimal compositional procedures and choreographies defined by spatial
arrangements. This project is currently one of the most established installations in the field of
animal–human sound installation. Since it has been shown that finches have musical abilities [14,16],
it would be interesting to provide them with musical instruments that are adapted to the physical
skills of the finches.
Figure 3. From here to ear by Celeste Boursier-Mougenot [32].
Figure 3. From here to ear by Celeste Boursier-Mougenot [32].

Multimodal Technologies and Interact. 2018,2, 51 6 of 14
3.3. Animals as Trained Musicians
In this section, we shall look at projects in which animals become active music performers.
There are numerous entertaining videos circulating virally around the Internet showing animals
playing musical instruments. On video-sharing websites, there are numerous videos of dogs [
38
] and
cats [
39
] playing the piano, trained seals playing toy wind instruments [
40
], and hens playing toy
instruments [41].
In 2014, the zookeepers of the Smithsonian’s National Zoo in Washington D.C. provided their zoo
animals with some musical instruments [
42
]. For example, they equipped their otters with a keyboard
and the orangutans with xylophones. The zoo’s intention was to enhance the well-being of the captive
animals, keep them active, and enrich their environment. They used traditional instruments designed
for humans such as keyboards, xylophones, and harmonicas, which obviously was a disadvantage
to some animals with different physiologies. Thus, it seems that the zookeepers did not select the
instruments based on the skills of the different animal species.
Elephants are another species that have been given musical instruments to enrich their lives in
captivity. An early attempt in this regard was made by the Thai Elephant Conversation Center with
its Thai Elephant Orchestra (Figure 4). With the intention of creating music for humans, they designed
and built massive musical instruments adapted to the physical abilities of the elephants. In 1997,
the orchestra made its first recording with five elephants, and subsequently released an audio CD in
2001. From the very beginning, it was clear that elephants could, through endless repetition, be taught
to play complex patterns [
43
]. However, the creators decided that making music should also be fun for
the elephants. Thus, the only commands given to the elephants were to start, stop, and occasionally
the number of times to strike an instrument [43].
The instruments that were used in the orchestra were mainly percussion instruments that
the elephants could play with their trunks. The percussion instruments ranged from slit drums,
marimba-like instruments, and gongs to thunder sheets, bells, and xylophones. Other instruments
included single-string instruments, harmonicas, and wind instruments. The designs of the instruments
were adapted to the elephant’s anatomy and skills, and to the Thai cultural environment in which
the animals lived. The elephants often heard Thai music; therefore, the instruments were adapted
to sound similar to Thai instruments [
43
]. It has also been observed that elephants can play a steady
beat [
44
,
45
]. During the course of the project, it was shown that they could make music together with
other elephants and humans [
44
]. The combination of good instrument design and the elephants’
musical skills makes this project a promising approach. That the project was founded to raise money for
the Thai Elephant Conversation Center and thus became a tourist attraction may have had a negative
impact on the animals. For instance, the project focused on training elephants to play human-like
music for a human audience. It would have been interesting to see what would have happened
without any training, by just providing the elephants with musical instruments and seeing what kind
of musical output they created without the need to entertain a crowd of visitors.
Multimodal Technol. Interact. 2018, 2, x FOR PEER REVIEW 6 of 14
3.3. Animals as Trained Musicians.
In this section, we shall look at projects in which animals become active music performers.
There are numerous entertaining videos circulating virally around the Internet showing animals
playing musical instruments. On video-sharing websites, there are numerous videos of dogs [38]
and cats [39] playing the piano, trained seals playing toy wind instruments [40], and hens playing
toy instruments [41].
In 2014, the zookeepers of the Smithsonian’s National Zoo in Washington D.C. provided their
zoo animals with some musical instruments [42]. For example, they equipped their otters with a
keyboard and the orangutans with xylophones. The zoo’s intention was to enhance the well-being of
the captive animals, keep them active, and enrich their environment. They used traditional
instruments designed for humans such as keyboards, xylophones, and harmonicas, which obviously
was a disadvantage to some animals with different physiologies. Thus, it seems that the zookeepers
did not select the instruments based on the skills of the different animal species.
Elephants are another species that have been given musical instruments to enrich their lives in
captivity. An early attempt in this regard was made by the Thai Elephant Conversation Center with
its Thai Elephant Orchestra (Figure 4). With the intention of creating music for humans, they designed
and built massive musical instruments adapted to the physical abilities of the elephants. In 1997, the
orchestra made its first recording with five elephants, and subsequently released an audio CD in
2001. From the very beginning, it was clear that elephants could, through endless repetition, be
taught to play complex patterns [43]. However, the creators decided that making music should also
be fun for the elephants. Thus, the only commands given to the elephants were to start, stop, and
occasionally the number of times to strike an instrument [43].
The instruments that were used in the orchestra were mainly percussion instruments that the
elephants could play with their trunks. The percussion instruments ranged from slit drums,
marimba-like instruments, and gongs to thunder sheets, bells, and xylophones. Other instruments
included single-string instruments, harmonicas, and wind instruments. The designs of the
instruments were adapted to the elephant’s anatomy and skills, and to the Thai cultural
environment in which the animals lived. The elephants often heard Thai music; therefore, the
instruments were adapted to sound similar to Thai instruments [43]. It has also been observed that
elephants can play a steady beat [44,45]. During the course of the project, it was shown that they
could make music together with other elephants and humans [44]. The combination of good
instrument design and the elephants’ musical skills makes this project a promising approach. That
the project was founded to raise money for the Thai Elephant Conversation Center and thus became
a tourist attraction may have had a negative impact on the animals. For instance, the project focused
on training elephants to play human-like music for a human audience. It would have been
interesting to see what would have happened without any training, by just providing the elephants
with musical instruments and seeing what kind of musical output they created without the need to
entertain a crowd of visitors.
Figure 4. The Thai Elephant Orchestra [43].
Figure 4. The Thai Elephant Orchestra [43].

Multimodal Technologies and Interact. 2018,2, 51 7 of 14
3.4. Animals as Voluntary Musicians
In the context of ACI, Fiona French [
6
] is currently investigating the potential of technological
enrichment for captive elephants. The design of the interactive toys for elephants also focuses on
acoustic enrichment experiences. For example, one design concept was based on a harp-like instrument
that the elephants could play with their trunks [
46
]. In collaboration with animal welfare experts
and elephant keepers, French utilized sensors and switches for triggering sounds. The sounds were
adapted to the elephants’ preferences, and 60–70 Hz frequencies were used to arouse the interest of the
participating elephant Valli [
46
]. A further relevant study in ACI was carried out with orangutans in
zoos by Pons [
7
]. A sound-based interactive system was developed for auditory enrichment. Instead of
providing orangutans with human music, the system allowed the animals to explore different types of
sounds by manipulating tangible objects. The sound preferences of the orangutans were incorporated
into the design process, thus making it animal-centered. In our opinion, the works of French and
Pons could be seen as best practices in the context of ACI, and both approaches could be used for the
development of musical instruments and interfaces for other species as well.
metamusic is a project by the artist group alien productions (Martin Breindl, Norbert Math, and
Andrea Sodomka) that was produced in cooperation with a group of African grey parrots. Since 2012,
the artists have been working in close collaboration with zoologists, biologists, and animal keepers.
The initial idea behind the project was to develop interactive sound installations for animals held
in captivity in zoological gardens. Much remains to be done to offer new challenges and diversions to
the zoo inhabitants. metamusic aims to build electronic sound installations to be used by the animals
themselves. Using a wide range of sensors and tools, the animals are able to explore and play with
sounds and sonic moods. Based on the advice of their zoological collaborators, they have centered
their attention on parrots, which have turned out to be the ideal animal species because of their nature.
Although they are regarded as wild animals with complex social flocking behavior, each of the birds
has been bred or at least raised by humans.
The metamusic project has shown that there is great potential for these kinds of open-sound
environments and grey parrots. In collaboration with the grey parrots, the artists have been developing
and modifying mechanical and electronic instruments and tools that can be played by the birds
themselves. Fine-tuned sensors provide interactive modulation of the sounds. Nevertheless, the
musical patterns and sonic structures generated by the parrots are not meant to sound aesthetically
meaningful to human ears. The project also intend for the parrots communicate with each other by
means of their own musical output created by their personalized instruments ([9], Figure 5).
Multimodal Technol. Interact. 2018, 2, x FOR PEER REVIEW 7 of 14
3.4. Animals as Voluntary Musicians
In the context of ACI, Fiona French [6] is currently investigating the potential of technological
enrichment for captive elephants. The design of the interactive toys for elephants also focuses on
acoustic enrichment experiences. For example, one design concept was based on a harp-like
instrument that the elephants could play with their trunks [46]. In collaboration with animal welfare
experts and elephant keepers, French utilized sensors and switches for triggering sounds. The
sounds were adapted to the elephants’ preferences, and 60–70 Hz frequencies were used to arouse
the interest of the participating elephant Valli [46]. A further relevant study in ACI was carried out
with orangutans in zoos by Pons [7]. A sound-based interactive system was developed for auditory
enrichment. Instead of providing orangutans with human music, the system allowed the animals to
explore different types of sounds by manipulating tangible objects. The sound preferences of the
orangutans were incorporated into the design process, thus making it animal-centered. In our
opinion, the works of French and Pons could be seen as best practices in the context of ACI, and both
approaches could be used for the development of musical instruments and interfaces for other
species as well.
metamusic is a project by the artist group alien productions (Martin Breindl, Norbert Math, and
Andrea Sodomka) that was produced in cooperation with a group of African grey parrots. Since
2012, the artists have been working in close collaboration with zoologists, biologists, and animal
keepers.
The initial idea behind the project was to develop interactive sound installations for animals
held in captivity in zoological gardens. Much remains to be done to offer new challenges and
diversions to the zoo inhabitants. metamusic aims to build electronic sound installations to be used
by the animals themselves. Using a wide range of sensors and tools, the animals are able to explore
and play with sounds and sonic moods. Based on the advice of their zoological collaborators, they
have centered their attention on parrots, which have turned out to be the ideal animal species
because of their nature. Although they are regarded as wild animals with complex social flocking
behavior, each of the birds has been bred or at least raised by humans.
The metamusic project has shown that there is great potential for these kinds of open-sound
environments and grey parrots. In collaboration with the grey parrots, the artists have been
developing and modifying mechanical and electronic instruments and tools that can be played by
the birds themselves. Fine-tuned sensors provide interactive modulation of the sounds.
Nevertheless, the musical patterns and sonic structures generated by the parrots are not meant to
sound aesthetically meaningful to human ears. The project also intend for the parrots communicate
with each other by means of their own musical output created by their personalized instruments ([9],
Figure 5).
Figure 5. metamusic, joystick device [37].
Figure 5. metamusic, joystick device [37].

Multimodal Technologies and Interact. 2018,2, 51 8 of 14
4. Animal-Centered Musical Interaction Design
In general, all of the projects presented above have strengths and weaknesses in terms of their
concepts and implementations. There are already promising approaches to improve the quality of
life for animals in captivity with the use of music and sounds. In some of the projects, the selection
of animal species seems arbitrary and not related to the animals’ cognitive and physical skills. We
have shown that some projects lack an animal-centered perspective, and many of the participating
animals do not voluntarily intend to generate music by interacting with the provided interface or
instrument. Additionally, many animals have been specifically trained or conditioned in order to
perform predefined musical patterns.
Some of the projects also raise questions regarding the design of interfaces and musical
instruments, which were not explicitly adapted to the abilities and needs of the participating animals.
The use of traditional musical instruments designed for humans may not be appropriate, and reveals
a lack of an animal-centered design approach, as proposed in Mancini’s ACI manifesto [
9
]. In some
cases, the animals had no choice between different musical instruments or interfaces. It is hard to
imagine that more ambitious musical performances could be performed in these settings, and ethical
principles are another consideration altogether.
Another question that needs to be addressed is the meaning of sound and music to the animals.
Although there are approaches to adapt the frequencies of the sounds [
6
,
37
,
46
] or the music to the
personal preferences of the animals [
1
,
7
], there is no evidence that any species appreciated the acoustic
stimuli. A number of studies in the field of biomusicology, for example [
47
–
50
], have found that animals
prefer sounds and musical arrangements that are biologically relevant for them. This important point
was not considered in the examples provided above, and it is therefore questionable if any of the
approaches provided beneficial enrichment to the animals.
Our work is focused on finding a framework that would fulfill the challenging tasks of
implementing an animal-centered design process, generating an artistic output, and above all, helping
to stimulate discussion about enriching the quality of life of grey parrots in captivity. In the next section,
we shall look at the key points involved in the implementation of musical interfaces for animals.
4.1. Musical Capabilities
The selection of an appropriate animal species for participation in the interaction process with
musical instruments and interfaces is of essential importance. Based on expertise gathered from
animal cognition [
14
], we have identified several animal species that have a particularly high degree of
musicality, such as spontaneous entrainment, in comparison to others. Hoeschele’s list of species with
vocal learning and entrainment abilities also shows those animal species that have been tested and
shown to have musical skills.
As Figure 6shows, vocal learning ability is generally a good indicator that the species has strong
auditory skills. Entrainment in animals is unusual, and involves the ability to align their movements
to a musical beat. This ability has been found in grey parrots, cockatoos, and elephants, and has thus
disproved the claim that entrainment to music is unique to humans [
17
,
18
]. These animals are able
to move synchronously to music. Therefore, it is reasonable to assume that these animals can also
maintain a rhythm with a steady tempo, as the active music performers of the Thai Elephant Orchestra
have demonstrated. The animals on the list have the best prerequisites for playing an active role in
projects that involve musical instruments. The questions of whether these talented animals appreciate
music and whether they like making music or not require further study. Finally, whether the music
created by animals pleases people is secondary to an animal-centered approach, and as such, animal
music need not sound like music to human ears.
A further approach would be to select an animal species that is close to humans such as
chimpanzees. It seems likely that they would share some human cognitive abilities [
14
], and thus
would be interesting for further research. For example, orangutans, another related species,
have shown interest in sound objects for individual auditory enrichment [
7
]. The Toronto Zoo

Multimodal Technologies and Interact. 2018,2, 51 9 of 14
conducted a study to determine the music preference of orangutans by using a participant-controlled,
dichotomous-choice design. The orangutans were trained to indicate their music preference via
touchscreen [
1
]. Furthermore, the orangutans of the Smithsonian’s National Zoo in Washington D.C.
were equipped with xylophones, and it was reported that they showed interest in playing them [42].
Multimodal Technol. Interact. 2018, 2, x FOR PEER REVIEW 9 of 14
Figure 6. Vocal learning and entrainment in animals according to Hoeschele [14].
A further approach would be to select an animal species that is close to humans such as
chimpanzees. It seems likely that they would share some human cognitive abilities [14], and thus
would be interesting for further research. For example, orangutans, another related species, have
shown interest in sound objects for individual auditory enrichment [7]. The Toronto Zoo conducted
a study to determine the music preference of orangutans by using a participant-controlled,
dichotomous-choice design. The orangutans were trained to indicate their music preference via
touchscreen [1]. Furthermore, the orangutans of the Smithsonian’s National Zoo in Washington D.C.
were equipped with xylophones, and it was reported that they showed interest in playing them [42].
4.2. Physical and Cognitive Abilities
Another crucial question concerns the development of an appropriate animal-centered design
approach, which concentrates on the specific physical and cognitive capabilities of each species, as
well as its specific needs. The main problem here is to gather and understand the animal feedback.
This requires different techniques [2], such as a review of the relevant literature on the animal
species, the observation of the animals in their living environment, and the development of testing
prototypes [46]. The design process of such testing prototypes should also be based on the results of
cognitive science and animal behavior, as well as an analysis of the physical capabilities of the
animal participants. The sounds and music that are used need to take the individual differences of
the animals into account. Our work with grey parrots [37] and French’s work with elephants [46]
show that it is also necessary to adjust the pitches of the audio frequencies to make them better
perceptible for the animal species.
4.3. Engagement versus Training
With regard to the various projects presented above, there are several different approaches to
motivating the animal species that are involved in the installations. The scope ranges from
imprinting behaviors, intense training, and rewarding through food to setups in which the animals
were free to interact with the instruments and were only rewarded and motivated though the
sounds generated by their actions. We believe that a setup that relies on the animals’ satisfaction to
motivate them to make sounds with the musical instruments is suited for animal species such as
elephants and grey parrots, which have been proven to have some form of musical skills. An
engaging, playful sonic system may provide a self-training environment to improve the animals’
musical skills. The animals’ intrinsic motivation could be one of the keys to successfully
implementing this kind of technology to improve the well-being of animals in captivity [51,52]. For
example, French uses such an approach in her work with elephants [6,46]. The elephants are already
integrated into the design process by voluntarily testing different sensors for their user interfaces. In
contrast, there are elephants that are trained to play musical instruments [43] or draw pictures under
the guidance of trainers. A study of four captive Asian elephants at the Melbourne Zoo, Australia,
examined painting by elephants in zoos [53], which is believed to be a form of enrichment. However,
the results indicated that the elephants gained little enrichment from the activity of painting.
Figure 6. Vocal learning and entrainment in animals according to Hoeschele [14].
4.2. Physical and Cognitive Abilities
Another crucial question concerns the development of an appropriate animal-centered design
approach, which concentrates on the specific physical and cognitive capabilities of each species, as well
as its specific needs. The main problem here is to gather and understand the animal feedback.
This requires different techniques [
2
], such as a review of the relevant literature on the animal
species, the observation of the animals in their living environment, and the development of testing
prototypes [
46
]. The design process of such testing prototypes should also be based on the results of
cognitive science and animal behavior, as well as an analysis of the physical capabilities of the animal
participants. The sounds and music that are used need to take the individual differences of the animals
into account. Our work with grey parrots [
37
] and French’s work with elephants [
46
] show that it is
also necessary to adjust the pitches of the audio frequencies to make them better perceptible for the
animal species.
4.3. Engagement versus Training
With regard to the various projects presented above, there are several different approaches to
motivating the animal species that are involved in the installations. The scope ranges from imprinting
behaviors, intense training, and rewarding through food to setups in which the animals were free to
interact with the instruments and were only rewarded and motivated though the sounds generated
by their actions. We believe that a setup that relies on the animals’ satisfaction to motivate them to
make sounds with the musical instruments is suited for animal species such as elephants and grey
parrots, which have been proven to have some form of musical skills. An engaging, playful sonic
system may provide a self-training environment to improve the animals’ musical skills. The animals’
intrinsic motivation could be one of the keys to successfully implementing this kind of technology to
improve the well-being of animals in captivity [
51
,
52
]. For example, French uses such an approach
in her work with elephants [
6
,
46
]. The elephants are already integrated into the design process by
voluntarily testing different sensors for their user interfaces. In contrast, there are elephants that are
trained to play musical instruments [
43
] or draw pictures under the guidance of trainers. A study
of four captive Asian elephants at the Melbourne Zoo, Australia, examined painting by elephants
in zoos [
53
], which is believed to be a form of enrichment. However, the results indicated that the
elephants gained little enrichment from the activity of painting.

Multimodal Technologies and Interact. 2018,2, 51 10 of 14
5. Future Work
Our further research aims to show that grey parrots in captivity can generate unique artistic
outcomes with musical instruments and interfaces in playful environments, in which grey parrots can
freely choose which instrument they want to play or interact with, thereby adopting an active role as
a performer. We believe there is a chance to foster relationships between grey parrots and humans
by opening new creative communication processes based on sound and music. The project could
expand the horizon of ACI research by developing musical instruments and interfaces for animals, and
could also provide further contributions to design concepts in other disciplines such as HCI or animal
behavior. Therefore, we conducted a series of experiments with a group of grey parrots at the ARGE
Papageienschutz animal shelter [
37
]. There, we tested the auditory skills and preferences of the grey
parrots as a group, while also identifying any individual characteristics. These tests were carried out in
cooperation with zoologists, and respected the ethical principles of the ACI manifesto [
21
]. However,
a suitable methodology that allows the grey parrots to participate in the design process itself has yet to
be found. Our design intended to avoid any conditioning of the animals, and instead supported their
inherent capabilities and needs. The individuals from our group of grey parrots generally preferred a
loudness and pitch matching their own communication habits, as well as generally faster rhythms [
37
].
Based on the musical skills and preferences of our group of grey parrots, we subsequently moved
on to design a series of analog musical instruments and computer-controlled interfaces that adapt to
these capabilities. We also incorporated our observations of the grey parrots’ behaviors toward and
interactions with our experimental musical interfaces. We observed a general curiosity toward any new
artifact that we added to the environment, leading to an overall exploration and a tendency toward
trying to disassemble the whole artifact. While we initially tried to work around these behaviors,
we eventually decided to incorporate these interactions into the fundamental interaction design of our
musical artifacts. We are currently designing new musical instruments that are based on traditional
design metaphors such as string and percussion instruments, or more contemporary designs such as
digital turntables. Finally, we are also planning to incorporate the animals’ highly developed vocal
capabilities into an acoustic feedback design. All of these designs also consider the particular physical
interaction patterns of grey parrots, which are mostly performed with the parrot beak. So far, we have
implemented two instruments, and according to our observations, the grey parrots playfully interacted
with them. One of them, the disc jockey (DJ) instrument, is based on a foraging wheel for parrot
enrichment, in which the birds have to spin a wheel to collect their treats. When a grey parrot spins
the wheel of the DJ instrument (Figure 7), the parrot will not be rewarded with food but with sound.
Since the parrots are already familiar with this form of interaction, they should be able to learn quickly
how to operate the device to get their reward. In further experiments with the DJ turntable instrument,
we want to find out in more detail whether different sounds are perceived as a reward or a positive
experience by the grey parrots.
Multimodal Technol. Interact. 2018, 2, x FOR PEER REVIEW 10 of 14
5. Future Work
Our further research aims to show that grey parrots in captivity can generate unique artistic
outcomes with musical instruments and interfaces in playful environments, in which grey parrots
can freely choose which instrument they want to play or interact with, thereby adopting an active
role as a performer. We believe there is a chance to foster relationships between grey parrots and
humans by opening new creative communication processes based on sound and music. The project
could expand the horizon of ACI research by developing musical instruments and interfaces for
animals, and could also provide further contributions to design concepts in other disciplines such as
HCI or animal behavior. Therefore, we conducted a series of experiments with a group of grey
parrots at the ARGE Papageienschutz animal shelter [37]. There, we tested the auditory skills and
preferences of the grey parrots as a group, while also identifying any individual characteristics.
These tests were carried out in cooperation with zoologists, and respected the ethical principles of
the ACI manifesto [21]. However, a suitable methodology that allows the grey parrots to participate
in the design process itself has yet to be found. Our design intended to avoid any conditioning of the
animals, and instead supported their inherent capabilities and needs. The individuals from our
group of grey parrots generally preferred a loudness and pitch matching their own communication
habits, as well as generally faster rhythms [37].
Based on the musical skills and preferences of our group of grey parrots, we subsequently
moved on to design a series of analog musical instruments and computer-controlled interfaces that
adapt to these capabilities. We also incorporated our observations of the grey parrots’ behaviors
toward and interactions with our experimental musical interfaces. We observed a general curiosity
toward any new artifact that we added to the environment, leading to an overall exploration and a
tendency toward trying to disassemble the whole artifact. While we initially tried to work around
these behaviors, we eventually decided to incorporate these interactions into the fundamental
interaction design of our musical artifacts. We are currently designing new musical instruments that
are based on traditional design metaphors such as string and percussion instruments, or more
contemporary designs such as digital turntables. Finally, we are also planning to incorporate the
animals’ highly developed vocal capabilities into an acoustic feedback design. All of these designs
also consider the particular physical interaction patterns of grey parrots, which are mostly
performed with the parrot beak. So far, we have implemented two instruments, and according to
our observations, the grey parrots playfully interacted with them. One of them, the disc jockey (DJ)
instrument, is based on a foraging wheel for parrot enrichment, in which the birds have to spin a
wheel to collect their treats. When a grey parrot spins the wheel of the DJ instrument (Figure 7), the
parrot will not be rewarded with food but with sound. Since the parrots are already familiar with
this form of interaction, they should be able to learn quickly how to operate the device to get their
reward. In further experiments with the DJ turntable instrument, we want to find out in more detail
whether different sounds are perceived as a reward or a positive experience by the grey parrots.
Figure 7. metamusic, disc jockey (DJ) device [7].
Figure 7. metamusic, disc jockey (DJ) device [7].

Multimodal Technologies and Interact. 2018,2, 51 11 of 14
The second device is a collaborative instrument (Figure 8) in which a parrot plays “hand-in-hand”
with another parrot or a person. It consists of a tube with a spring reverb installed inside that can
be triggered from both sides by pulling on a rope. The tube instrument is intended to stimulate the
birds’ interest in interacting and making music together, since grey parrots usually live in long-term
couple relationships. We are also focusing on designing further experimental musical instruments and
carrying out different sonic experiments for use in art installations and performances. At the same
time, we are giving careful consideration to whether the musical instruments and interfaces developed
have the potential to improve the quality of life of grey parrots in captivity.
Multimodal Technol. Interact. 2018, 2, x FOR PEER REVIEW 11 of 14
The second device is a collaborative instrument (Figure 8) in which a parrot plays
“hand-in-hand” with another parrot or a person. It consists of a tube with a spring reverb installed
inside that can be triggered from both sides by pulling on a rope. The tube instrument is intended to
stimulate the birds’ interest in interacting and making music together, since grey parrots usually live
in long-term couple relationships. We are also focusing on designing further experimental musical
instruments and carrying out different sonic experiments for use in art installations and
performances. At the same time, we are giving careful consideration to whether the musical
instruments and interfaces developed have the potential to improve the quality of life of grey parrots
in captivity.
Figure 8. metamusic, tube device [8].
6. Conclusions
Our overview of recent musical instruments and interfaces that involve animals in the
interaction and music-generation process is novel, and highlights the costs and benefits of projects of
this kind. It provides insights into current technologies in this field and the musical talents of
animals. Some of the participating animal species such as elephants and grey parrots have shown
musical abilities and skills in musical performances. We have proposed a classification based on the
role that the animals play as a collaborator in the musical environments and performances. Only the
few projects presented above in Section 3.4. Animals as Voluntary Musicians seem to be promising
technological and musical mediators between the animals themselves and between animals and
humans. Thus, they present approaches to improve the quality of life of animals in captivity.
Based on our earlier work, we plan to continue with our practice-based research approach to
develop new musical instruments and interfaces for grey parrots living in captivity. Ultimately, we
hope to identify general cross-species design patterns for musical instruments from our research,
which may even become relevant to human-centered design aspects.
Funding: This project is supported by the Austrian Science Fund FWF through the Programme for Arts-based
Research (PEEK AR 349-G24).
Acknowledgments: We’d also like to thank our partners Martin Breindl, Andrea Sodomka and Norbert Math
from alien productions as well as the animal keepers from ARGE Papageienschutz.
Conflicts of Interest: The authors declare no conflict of interest.
Figure 8. metamusic, tube device [8].
6. Conclusions
Our overview of recent musical instruments and interfaces that involve animals in the interaction
and music-generation process is novel, and highlights the costs and benefits of projects of this kind.
It provides insights into current technologies in this field and the musical talents of animals. Some of
the participating animal species such as elephants and grey parrots have shown musical abilities and
skills in musical performances. We have proposed a classification based on the role that the animals
play as a collaborator in the musical environments and performances. Only the few projects presented
above in Section 3.4.Animals as Voluntary Musicians seem to be promising technological and musical
mediators between the animals themselves and between animals and humans. Thus, they present
approaches to improve the quality of life of animals in captivity.
Based on our earlier work, we plan to continue with our practice-based research approach to
develop new musical instruments and interfaces for grey parrots living in captivity. Ultimately,
we hope to identify general cross-species design patterns for musical instruments from our research,
which may even become relevant to human-centered design aspects.
Funding:
This project is supported by the Austrian Science Fund FWF through the Programme for Arts-based
Research (PEEK AR 349-G24).
Acknowledgments:
We’d also like to thank our partners Martin Breindl, Andrea Sodomka and Norbert Math
from alien productions as well as the animal keepers from ARGE Papageienschutz.
Conflicts of Interest: The authors declare no conflict of interest.

Multimodal Technologies and Interact. 2018,2, 51 12 of 14
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