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  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>Spontaneous emotional speech recordings through a cooperative online video game</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Daniel Palacios-Alonso</string-name>
          <email>daniel@junipera.datsi</email>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Victoria Rodellar-Biarge</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Victor Nieto-Lluis</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Pedro Gomez-Vilda</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Centro de Tecnolog a Biomedica and Escuela Tecnica Superior de Ingenieros Informaticos Universidad Politecnica de Madrid Campus de Montegancedo - Pozuelo de Alarcon - 28223 Madrid -</institution>
          <country country="ES">SPAIN</country>
        </aff>
      </contrib-group>
      <abstract>
        <p>Most of emotional speech databases are recorded by actors and some of spontaneous databases are not free of charge. To progress in emotional recognition, it is necessary to carry out a big data acquisition task. The current work gives a methodology to capture spontaneous emotions through a cooperative video game. Our methodology is based on three new concepts: novelty, reproducibility and ubiquity. Moreover, we have developed an experiment to capture spontaneous speech and video recordings in a controlled environment in order to obtain high quality samples.</p>
      </abstract>
      <kwd-group>
        <kwd>Spontaneous emotions</kwd>
        <kwd>A ective Computing</kwd>
        <kwd>Cooperative Platform</kwd>
        <kwd>Databases</kwd>
        <kwd>MOBA Games</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>-</title>
      <p>
        Capturing emotions is an arduous task, above all when we speak about
capturing and identifying spontaneous emotions in voice. Major progress has been
made in the capturing and identifying gestural or body emotions [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ]. However,
this progress is not similar in the speech emotion eld. Emotion identi cation is
a very complex task because it is dependent on, among others factors, culture,
language, gender and the age of the subject. The consulted literature mentions a
few databases and data collections of emotional speech in di erent languages but
in many cases this information is not open to the community and not available
for research. There is not an emotional voice data set recognized for the research
community as a basic test bench, which makes a real progress in the eld very
complicated, due to the di culty in evaluating the quality of new proposals in
parameters for characterization and in the classi cation algorithms obtained
using the same input data. To achieve this aim, we propose the design of a new
protocol or methodology which should include some features such as novelty,
reproducibility and ubiquity.
      </p>
      <p>Typically, emotional databases have been recorded by actors simulating
emotional speech. These actors read the same sentence with di erent tones. In our
research, we have requested the collaboration of di erent volunteers with di
erent ages and gender. Most of volunteers were students who donated their voices.
First of all, they had to give their consent in order to participate in our
experiment. Therefore, we show a novel way of obtaining new speech recordings.
The next key feature for this task is reproducibility, where each experiment
should provide spontaneity, although the exercise was repeated a lot of times.
This characteristic is the most important drawback we have found in the
literature. Most of the time, when it carries out an experiment, this user is discarded
immediately, because he/she knows perfectly the guideline of the exercise, for
this reason the spontaneity is deleted. The third feature is, ubiquity. When we
speak about this concept, we refer to carrying out the exercise in every part
of the world, but that does not mean that we cannot use the same location or
devices. Nowadays, new technologies such as smart-phones, tablets and the like
are necessary allies in this aspect.</p>
      <p>In view of all the above, multiplayer videogames are the perfect way to achieve
the last three premises. Each game session or scenario can be di erent.
Moreover, we can play at home, in a laboratory or anywhere. Thanks to the Internet,
we can nd di erent players or rivals around the world who speak other
languages and have other cultures, etcetera. Each videogame has its own rules,
thus each player knows the game system and they follow these rules if they want
to participate. For this reason we nd the standardization feature intrinsic in
the videogames. Therefore, we conclude videogames are the perfect tool in order
to elicit spontaneous emotions.</p>
      <p>This research has two main stages; they consist of capturing emotions through
a videogame, more speci cally League of Legends (aka LoL), and identify the
captured emotions through the new cooperative framework developed by our
team. To assess the viability of our protocol, we have developed a controlled
experiment in our laboratory. In subsequent sections, it will be explained in detail.</p>
      <p>The contribution of this work is to establish a community to cooperate in
collecting, developing and analyzing emotional speech data and de ne a standard
corpus in di erent languages where the main source of samples will be emotional
speeches captured through videogame rounds. In this sense, this paper is a rst
step in proposing the design and development of an online cooperative framework
for multilingual data acquisition of emotional speech. This paper is organized as
follows. In the next section, we introduce some emotional databases of speech
and foundations for modeling emotions in games. In section III, we introduce
the proposed experiment. And nally, we conclude with the summary and future
works.</p>
    </sec>
    <sec id="sec-2">
      <title>Previous Works</title>
      <p>Below, we present previous works carried out by di erent researchers who have
focused their attention in emotional areas. Some of them have elaborated
emotional databases, others have developed a ective models to improve the realism
of NPCs (Non Player Character) or have attempted to verbalize certain
situations that happen for game rounds, etc. We are going to attempt to nd a
common ground between using videogames and the design of a protocol to
capture emotions through voice.
2.1</p>
      <sec id="sec-2-1">
        <title>Emotions in Videogames</title>
        <p>
          According to [
          <xref ref-type="bibr" rid="ref2">2</xref>
          ] there exists a lack a common, shared vocabulary that allows
us to verbalize the intricacies of game experience. For any eld of science to
progress, there needs to be a basic agreement on the de nition of terms. This
concept is similar to the lack of agreement for the relevant features in order to
characterize and classify speech emotions. They de ne two concepts, ow and
immersion. Flow can be explained as an optimal state of enjoyment where people
are completely absorbed in the activity. This experience was similar for everyone,
independent of culture, social class, age or gender. This last assertion is a key
point for us, because we are searching for the most suitable method or protocol
for anyone. Immersion is mostly used to refer to the degree of involvement or
engagement one experiences with a game. Regarding arousal and valence,
Lottridge has developed a novel, continuous, quantitative self-report tool, based on
the model of valence and arousal which measures emotional responses to user
interfaces and interactions [
          <xref ref-type="bibr" rid="ref3">3</xref>
          ]. Moreover, Sykes and Brown show the hypothesis
that the player's state of arousal will correspond with the pressure used to press
buttons on a gamepad [
          <xref ref-type="bibr" rid="ref4">4</xref>
          ].
        </p>
        <p>
          Concerning elicit emotion and emotional responses to videogames, in [
          <xref ref-type="bibr" rid="ref5">5</xref>
          ]
presents a comprehensive model of emotional response to the single-player game
based on two roles players occupy during gameplay and four di erent types of
emotion. The emotional types are based on di erent ways players can interact
with a videogame: as a simulation, as a narrative, as a game, and as a crafted
piece of art.
        </p>
        <p>
          On the other hand, some researchers focus their attention on
psychophysiological methods in game research. Kivikangas et al. carry out a complete
review of some works in relation with these kind of methods. They present the
most useful measurements and their e ects in di erent research areas such as
game e ects, game events, game design and elicited emotions.
Electromyography (EMG), Electrodermal activity (EDA), Heart Rate (HR), among others, are
some of these measurements [
          <xref ref-type="bibr" rid="ref6">6</xref>
          ].
        </p>
        <p>
          Another initiative was developed by [
          <xref ref-type="bibr" rid="ref7">7</xref>
          ], designing requirements engineering
techniques to emotions in videogame design, where they introduced emotional
terrain maps, emotional intensity maps, and emotional timelines as in-context
visual mechanisms for capturing and expressing emotional requirements.
        </p>
        <p>
          Regarding emotion modeling in game characters, Hudlicka and Broekens
present theoretical background and some practical guidelines for developing
models of emotional e ects on cognition in NPCs [
          <xref ref-type="bibr" rid="ref8">8</xref>
          ]. In [
          <xref ref-type="bibr" rid="ref9">9</xref>
          ] have developed
a toolkit called the Intelligent Gaming System (IGS) that is based on
Command (Atari, 1980). The aim was to keep engagement as measured by changing
heartbeat rate, within an optimum range. They use a small test group, 8 people,
whose experience was documented and thanks to their conclusions, they could
design a theory of modes of a ective gaming.
2.2
        </p>
      </sec>
      <sec id="sec-2-2">
        <title>Emotional Databases</title>
        <p>
          Most databases have been recorded by actors simulating emotional discourses
and there are a very few of them of spontaneous speech [
          <xref ref-type="bibr" rid="ref10">10</xref>
          ], [
          <xref ref-type="bibr" rid="ref11">11</xref>
          ]. The emotions
are validated and labeled by a panel of experts or by a voting system. Most of the
databases include few speakers and sometimes they are gender [
          <xref ref-type="bibr" rid="ref12">12</xref>
          ] unbalanced,
and most of recorded data do not consider age. Then they restrict to carrying
out research related with subject age range [
          <xref ref-type="bibr" rid="ref13">13</xref>
          ]. It can be noticed in several
publications that the data are produced just for speci c research, and the data
are not available for the community. Some databases related to our research are
brie y mentioned next.
        </p>
        <p>
          Two of the well-known emotional databases for speech are the Danish
Emotional Speech Database (DES) [
          <xref ref-type="bibr" rid="ref14">14</xref>
          ] and the Berlin Emotional Speech Database
(BES) [
          <xref ref-type="bibr" rid="ref15">15</xref>
          ] in German. BES database, also known as Emo-Database, is spoken
by 10 professional native German actors, 5 female and 5 male. It includes the
emotions of neutral, anger, joy, sadness, fear, disgust and boredom. The basic
information is 10 utterances, 5 short and 5 longer sentences, which could be used in
daily communication and are interpretable in all applied emotions. The recorded
speech material was around 800 sentences. All sentences have been evaluated by
20-30 judges. Those utterances for which the emotion was recognized by at least
80% of the listeners will be used for further analysis. DES database is spoken by
four actors, 2 male and 2 female. It contains the emotions of neutral, surprise,
happiness, sadness and anger. Records are divided into simple words, sentences
and passages of uent speech.
        </p>
        <p>
          Concerning stress in speech, the SUSAS English database (Speech Under
Simulated and Actual Stress) is public and widely used [
          <xref ref-type="bibr" rid="ref16">16</xref>
          ]. It contains a set
of 35 aircraft communication words, which are spoken spontaneously by aircraft
pilots during a ight, and also contains other samples of non-spontaneous speech.
        </p>
        <p>
          Finally, the work closest to our approach that we have found in literature,
has been in [
          <xref ref-type="bibr" rid="ref17">17</xref>
          ]. They have developed an annotated database of spontaneous,
multimodal, emotional expressions. Recordings were made of facial and vocal
expressions of emotions while participants were playing a multiplayer rst-person
shooter (fps) computer game. During a replay session, participants scored their
own emotions by assigning values to them on an arousal and a valence scale, and
by selecting emotional category labels.
3
        </p>
      </sec>
    </sec>
    <sec id="sec-3">
      <title>A ective Data Acquisition</title>
      <p>
        As mentioned before, we used a videogame-like source of elicited emotions. The
chosen game was League of Legends (aka LoL). To carry out this task, it was
necessary to organize a little tournament, where the team with the best score at
the end of the tournament, obtained a check for the amount of 20 e per person
as well as a diploma. With the obtained samples, we attempt to nd correlates
between acoustics, glotals or biomechanics parameters and the elicited emotions.
To extract these parameters, we have used [
        <xref ref-type="bibr" rid="ref18">18</xref>
        ].
3.1
      </p>
      <sec id="sec-3-1">
        <title>The Subjects</title>
        <p>The subjects are students of Computer Science at the Universidad Politecnica
of Madrid. Apparently, students had not got any disease in their voices and they
gave their explicit consent in order to participate in the experiment.
3.2</p>
      </sec>
      <sec id="sec-3-2">
        <title>The Game</title>
        <p>
          LoL is a multiplayer online battle arena (MOBA) video game developed and
published by Riot Games. It is a free-to-play game supported by micro-transactions
and inspired by the mod Defense of the Ancients for the video game Warcraft
III: The Frozen Throne. League of Legends was generally well received at
release, and it has grown in popularity in the years since. By July 2012, League of
Legends was the most played PC game in North America and Europe in terms
of the number of hours played [
          <xref ref-type="bibr" rid="ref19">19</xref>
          ]. As of January 2014, over 67 million
people play League of Legends per month, 27 million per day, and over 7.5 million
concurrently during peak hours [
          <xref ref-type="bibr" rid="ref20">20</xref>
          ].
3.3
        </p>
      </sec>
      <sec id="sec-3-3">
        <title>The Environment</title>
        <p>
          Each of the game sessions are carried out in our Laboratory, Neuromorphic
Speech Processing Laboratory, which belongs to R+D group Centro de
Tecnolog a Biomedica. In this laboratory, we possess a quasi-anechoic chamber that
is designed in order to entirely absorb the acoustic waves without echoing o
any surface of the chamber such the oor, roof, walls and the like. The chamber
has a personal computer inside, where a player remains throughout the game
round. Outside of the chamber, there will be another four personal computers
at the disposal of four members of the rest of the team. Concerning the choice
of the anechoic chamber, it is easy to understand that we are looking for ideal
conditions in order to develop of following stages such as characterization and
extraction of parameters, selection of parameters, classi cation and nally,
detection of emotions in speech [
          <xref ref-type="bibr" rid="ref21">21</xref>
          ].
Each player has used a SENNHEISER PC 131 headset with wire connector,
30 - 18000 Hz of headphone frequency, 80 - 15000 Hz of microphone frequency,
2000 Ohms of output impedance and 38 Db of sensitivity. On the other hand,
computers used in the experiment had the following features. Intel Core 2 Quad
- CPU Q6600, 4 cores up to 2.40 Ghz, 4 GB of RAM and Nvidia GForce 8600GT
with 512 MB Graphics Card. Moreover, it has been connected to a webcam in
order to record faces and gestures for each game session. Video recordings could
be crucial in order to recognize in the following stages the saved emotion.
3.5
        </p>
      </sec>
      <sec id="sec-3-4">
        <title>The Experiment</title>
        <p>During two weeks, we will convene ten subjects in two shift sessions. The will
be one shift in the morning and another one in the afternoon. Each session will
have ve players, where each player will log in to LoL's website with his/her
o cial account. Four of ve players remain together in the same room, whereas
the remaining player will be isolated inside of anechoic chamber. Approximately,
each session will be limited to three hours and a half, because the average length
of a game is 40 minutes. Once a game is over, the player who stays inside the
chamber, will come out the chamber and the following mate takes his place.
This continues until ve rounds have been completed. Therefore, each day of
tournament, we will have recorded 10 di erent players for 40 minutes. These
records will be saved in our raw speech recordings database. Each team will play
a maximum of 3 games over the two weeks of tournament. The process of the
experiment is depicted in the Fig. 1. This experiment and others, which although
they are not the objective of this paper to analyze them deeply, have been
developed in order to elicit spontaneous emotions by our team. These experiments
are incorporated inside the framework Emotions Portal.
3.6</p>
      </sec>
      <sec id="sec-3-5">
        <title>The Framework</title>
        <p>The aim of this experiment is the capturing of speech recordings through a
cooperative online videogame. The idea is that our server collects spontaneous
emotional voice in di erent languages, with di erent accents and origins, etc. This
framework can be de ned as cooperative, scalable or modular and not
subjective. According to Fig. 2, we divide our online framework into four stages: User
identi cation, Start of the recording, Play the game and End of the recording
and save the speech and video Recording. At the top of the picture is depicted
the player who is inside of the anechoic chamber. He/she is connected to our
platform which is deployed in our server. The user identi cation step consists
of a sign up process through a web form. In this web form, users provide their
personal data, for instance, their name, native language, country, gender, age
and email. The last requirement, email address, is convenient in order to have
the chance to keep in touch with the user and to give him/her information about
the progress of the project. Once logged in to our platform, the user can choose</p>
        <p>Fig. 1. Scenario of Experiment.
the di erent kind of experiments which are available. The actions mentioned
before are explained as follows.</p>
        <p>First of all, users answer if they su er from any type of disease in their voice
at the moment of performing the test. It is crucial to know if the user su ers
from any organic or functional dysphonia, diseases that can a ect voice and
prevent the use of biometric techniques. Then, the player chooses the language
of procedure. When the user is ready to start playing, he/she presses the Start
button. In this moment, the platform begins recording and throws a call to LoL's
application. LoL's platform opens and the user carries out the process of sign
in with his/her LoL's account. These steps are depicted through numbers 1 and
2 in Figure 2. Approximately 40 minutes later, the game round is over. The
player signs out of the LoL's platform and he/she presses the Stop button in
our platform. Finally, the speech and video recording are saved in our repository
inside of our server. The last steps are depicted through numbers 3 and 4 in
Figure 2.
4</p>
      </sec>
    </sec>
    <sec id="sec-4">
      <title>Summary</title>
      <p>The spontaneous data acquisition is a very complex topic to resolve. However,
it is the key point to improve human computer interfaces, robots, video games
and the like. As mentioned before, some researchers have developed new models
and methodologies to improve NPCs, and others have researched new designs to
evoke certain emotions during the game sessions. We have developed a
methodology in order to capture spontaneous emotions through a cooperative videogame
with high quality audio in a controlled environment. Afterwards, we will be
able to design a well-labeled database and continue with our previous work on
emotion recognition.</p>
      <p>Acknowledgments. This work is being funded by grants
TEC2012-38630-C0401 and TEC2012-38630-C04-04 from Plan Nacional de I+D+i, Ministry of
Economic A airs and Competitiveness of Spain.</p>
    </sec>
  </body>
  <back>
    <ref-list>
      <ref id="ref1">
        <mixed-citation>
          1.
          <string-name>
            <given-names>P.</given-names>
            <surname>Ekman</surname>
          </string-name>
          ,
          <article-title>Handbook of cognition and emotion</article-title>
          . Wiley Online Library,
          <year>1999</year>
          , ch.
          <source>Basic emotions</source>
          , pp.
          <volume>45</volume>
          {
          <fpage>60</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref2">
        <mixed-citation>
          2. W. IJsselsteijn, Y. De Kort,
          <string-name>
            <given-names>K.</given-names>
            <surname>Poels</surname>
          </string-name>
          ,
          <string-name>
            <given-names>A.</given-names>
            <surname>Jurgelionis</surname>
          </string-name>
          , and
          <string-name>
            <given-names>F.</given-names>
            <surname>Bellotti</surname>
          </string-name>
          , \
          <article-title>Characterising and measuring user experiences in digital games," in International conference on advances in computer entertainment technology</article-title>
          ,
          <source>vol. 2</source>
          ,
          <issue>2007</issue>
          , p.
          <fpage>27</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref3">
        <mixed-citation>
          3.
          <string-name>
            <given-names>D.</given-names>
            <surname>Lottridge</surname>
          </string-name>
          , \
          <article-title>Emotional response as a measure of human performance," in CHI'08 Extended Abstracts on Human Factors in Computing Systems</article-title>
          . ACM,
          <year>2008</year>
          , pp.
          <volume>2617</volume>
          {
          <fpage>2620</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref4">
        <mixed-citation>
          4.
          <string-name>
            <given-names>J.</given-names>
            <surname>Sykes</surname>
          </string-name>
          and
          <string-name>
            <surname>S. Brown</surname>
          </string-name>
          , \
          <article-title>A ective gaming: measuring emotion through the gamepad," in CHI'03 extended abstracts on Human factors in computing systems</article-title>
          . ACM,
          <year>2003</year>
          , pp.
          <volume>732</volume>
          {
          <fpage>733</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref5">
        <mixed-citation>
          5.
          <string-name>
            <given-names>J.</given-names>
            <surname>Frome</surname>
          </string-name>
          , \
          <article-title>Eight ways videogames generate emotion,"</article-title>
          Obtenido de http://www. digra. org/dl/db/07311.25139. pdf,
          <year>2007</year>
          .
        </mixed-citation>
      </ref>
      <ref id="ref6">
        <mixed-citation>
          6.
          <string-name>
            <surname>J. M. Kivikangas</surname>
            ,
            <given-names>G.</given-names>
          </string-name>
          <string-name>
            <surname>Chanel</surname>
            ,
            <given-names>B.</given-names>
          </string-name>
          <string-name>
            <surname>Cowley</surname>
            , I. Ekman,
            <given-names>M.</given-names>
          </string-name>
          <string-name>
            <surname>Salminen</surname>
            ,
            <given-names>S.</given-names>
          </string-name>
          <article-title>Jarvela, and</article-title>
          <string-name>
            <given-names>N.</given-names>
            <surname>Ravaja</surname>
          </string-name>
          , \
          <article-title>A review of the use of psychophysiological methods in game research,"</article-title>
          <source>Journal of Gaming &amp; Virtual Worlds</source>
          , vol.
          <volume>3</volume>
          , no.
          <issue>3</issue>
          ,
          <issue>2011</issue>
          , pp.
          <volume>181</volume>
          {
          <fpage>199</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref7">
        <mixed-citation>
          7.
          <string-name>
            <given-names>D.</given-names>
            <surname>Callele</surname>
          </string-name>
          , E. Neufeld, and
          <string-name>
            <given-names>K.</given-names>
            <surname>Schneider</surname>
          </string-name>
          , \
          <article-title>Emotional requirements in video games," in Requirements Engineering, 14th IEEE International Conference</article-title>
          . IEEE,
          <year>2006</year>
          , pp.
          <volume>299</volume>
          {
          <fpage>302</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref8">
        <mixed-citation>
          8.
          <string-name>
            <given-names>E.</given-names>
            <surname>Hudlicka</surname>
          </string-name>
          and
          <string-name>
            <given-names>J.</given-names>
            <surname>Broekens</surname>
          </string-name>
          , \
          <article-title>Foundations for modelling emotions in game characters: Modelling emotion e ects on cognition," in A ective Computing and Intelligent Interaction</article-title>
          and Workshops,
          <year>2009</year>
          .
          <source>ACII</source>
          <year>2009</year>
          . 3rd International Conference on. IEEE,
          <year>2009</year>
          , pp.
          <volume>1</volume>
          {
          <fpage>6</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref9">
        <mixed-citation>
          9.
          <string-name>
            <given-names>K.</given-names>
            <surname>Gilleade</surname>
          </string-name>
          ,
          <string-name>
            <given-names>A.</given-names>
            <surname>Dix</surname>
          </string-name>
          , and
          <string-name>
            <given-names>J.</given-names>
            <surname>Allanson</surname>
          </string-name>
          , \
          <article-title>A ective videogames and modes of a ective gaming: assist me, challenge me, emote me,"</article-title>
          <source>DiGRA - Digital Games Research Association</source>
          ,
          <year>2005</year>
          .
        </mixed-citation>
      </ref>
      <ref id="ref10">
        <mixed-citation>
          10. S. Ramakrishnan, \
          <article-title>Recognition of emotion from speech: a review," Speech Enhancement, Modeling and recognition{algorithms and</article-title>
          <string-name>
            <surname>Applications</surname>
          </string-name>
          ,
          <year>2012</year>
          , p.
          <fpage>121</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref11">
        <mixed-citation>
          11.
          <string-name>
            <given-names>D.</given-names>
            <surname>Ververidis</surname>
          </string-name>
          and
          <string-name>
            <given-names>C.</given-names>
            <surname>Kotropoulos</surname>
          </string-name>
          , \
          <article-title>A review of emotional speech databases,"</article-title>
          <source>in Proc. Panhellenic Conference on Informatics (PCI)</source>
          ,
          <year>2003</year>
          , pp.
          <volume>560</volume>
          {
          <fpage>574</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref12">
        <mixed-citation>
          12.
          <string-name>
            <given-names>V.</given-names>
            <surname>Rodellar</surname>
          </string-name>
          ,
          <string-name>
            <given-names>D.</given-names>
            <surname>Palacios</surname>
          </string-name>
          ,
          <string-name>
            <given-names>P.</given-names>
            <surname>Gomez</surname>
          </string-name>
          , and E. Bartolome, \
          <article-title>A methodology for monitoring emotional stress in phonation," in Cognitive Infocommunications (CogInfoCom</article-title>
          ),
          <source>2014 5th IEEE Conference on. IEEE</source>
          ,
          <year>2014</year>
          , pp.
          <volume>231</volume>
          {
          <fpage>236</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref13">
        <mixed-citation>
          13.
          <string-name>
            <surname>C.</surname>
          </string-name>
          <article-title>Mun~oz-</article-title>
          <string-name>
            <surname>Mulas</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          <string-name>
            <surname>Mart</surname>
            nez-Olalla,
            <given-names>P.</given-names>
          </string-name>
          <string-name>
            <surname>Gomez-Vilda</surname>
            ,
            <given-names>E. W.</given-names>
          </string-name>
          <string-name>
            <surname>Lang</surname>
            ,
            <given-names>A.</given-names>
          </string-name>
          <string-name>
            <surname>AlvarezMarquina</surname>
            ,
            <given-names>L. M.</given-names>
          </string-name>
          <string-name>
            <surname>Mazaira-Fernandez</surname>
            , and
            <given-names>V.</given-names>
          </string-name>
          <string-name>
            <surname>Nieto-Lluis</surname>
          </string-name>
          , \
          <article-title>Kpca vs. pca study for an age classi cation of speakers,"</article-title>
          <source>in Advances in Nonlinear Speech Processing</source>
          . Springer,
          <year>2011</year>
          , pp.
          <volume>190</volume>
          {
          <fpage>198</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref14">
        <mixed-citation>
          14.
          <string-name>
            <given-names>I. S.</given-names>
            <surname>Engberg</surname>
          </string-name>
          and
          <string-name>
            <given-names>A. V.</given-names>
            <surname>Hansen</surname>
          </string-name>
          , \
          <article-title>Documentation of the danish emotional speech database DES," Internal AAU report, Center for Person Kommunikation</article-title>
          , Denmark,
          <year>1996</year>
          .
        </mixed-citation>
      </ref>
      <ref id="ref15">
        <mixed-citation>
          15.
          <string-name>
            <given-names>F.</given-names>
            <surname>Burkhardt</surname>
          </string-name>
          ,
          <string-name>
            <given-names>A.</given-names>
            <surname>Paeschke</surname>
          </string-name>
          ,
          <string-name>
            <given-names>M.</given-names>
            <surname>Rolfes</surname>
          </string-name>
          ,
          <string-name>
            <given-names>W. F.</given-names>
            <surname>Sendlmeier</surname>
          </string-name>
          , and
          <string-name>
            <given-names>B.</given-names>
            <surname>Weiss</surname>
          </string-name>
          , \
          <article-title>A database of german emotional speech." in Interspeech</article-title>
          , vol.
          <volume>5</volume>
          ,
          <issue>2005</issue>
          , pp.
          <volume>1517</volume>
          {
          <fpage>1520</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref16">
        <mixed-citation>
          16.
          <string-name>
            <surname>J. H. Hansen</surname>
            ,
            <given-names>S. E.</given-names>
          </string-name>
          <string-name>
            <surname>Bou-Ghazale</surname>
            ,
            <given-names>R.</given-names>
          </string-name>
          <string-name>
            <surname>Sarikaya</surname>
            , and
            <given-names>B.</given-names>
          </string-name>
          <string-name>
            <surname>Pellom</surname>
          </string-name>
          , \
          <article-title>Getting started with SUSAS: a speech under simulated and actual stress database." in Eurospeech</article-title>
          , vol.
          <volume>97</volume>
          , no.
          <issue>4</issue>
          ,
          <issue>1997</issue>
          , pp.
          <volume>1743</volume>
          {
          <fpage>46</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref17">
        <mixed-citation>
          17.
          <string-name>
            <given-names>P.</given-names>
            <surname>Merkx</surname>
          </string-name>
          ,
          <string-name>
            <given-names>K. P.</given-names>
            <surname>Truong</surname>
          </string-name>
          , and
          <string-name>
            <given-names>M. A.</given-names>
            <surname>Neerincx</surname>
          </string-name>
          , \
          <article-title>Inducing and measuring emotion through a multiplayer rst-person shooter computer game,"</article-title>
          <source>in Proceedings of the Computer Games Workshop</source>
          ,
          <year>2007</year>
          , pp.
          <volume>06</volume>
          {
          <fpage>07</fpage>
          .
        </mixed-citation>
      </ref>
      <ref id="ref18">
        <mixed-citation>
          18.
          <string-name>
            <surname>\BioMetroPhon - O cial Webpage</surname>
          </string-name>
          ,
          <article-title>"</article-title>
          <year>2008</year>
          , URL: http://www.glottex.com/ [accessed:
          <fpage>2015</fpage>
          -05-04].
        </mixed-citation>
      </ref>
      <ref id="ref19">
        <mixed-citation>
          19.
          <string-name>
            <given-names>J.</given-names>
            <surname>Gaudiosi</surname>
          </string-name>
          .
          <article-title>Riot games' league of legends o cially becomes most played pc game in the world</article-title>
          . [Online]. Available: "http://www.forbes.com/sites/johngaudiosi/2012/07/11/ riot-games
          <article-title>-league-of-legends-o cially-becomes-most-played-pc-game-in-the-world/ " (</article-title>
          <year>2012</year>
          )
        </mixed-citation>
      </ref>
      <ref id="ref20">
        <mixed-citation>
          20. I. Sheer.
          <article-title>Player tally for league of legends surges</article-title>
          . [Online]. Available: "http://blogs. wsj.com/digits/2014/01/27/player-tally
          <article-title>-for-league-of-legends-surges/" (</article-title>
          <year>2014</year>
          )
        </mixed-citation>
      </ref>
      <ref id="ref21">
        <mixed-citation>
          21. V.
          <string-name>
            <surname>Rodellar-Biarge</surname>
            ,
            <given-names>D.</given-names>
          </string-name>
          <string-name>
            <surname>Palacios-Alonso</surname>
            ,
            <given-names>V.</given-names>
          </string-name>
          <string-name>
            <surname>Nieto-Lluis</surname>
            , and
            <given-names>P.</given-names>
          </string-name>
          <string-name>
            <surname>Gomez-Vilda</surname>
          </string-name>
          ,
          <article-title>\Towards the search of detection in speech-relevant features for stress</article-title>
          .
          <source>expert systems," Expert Systems</source>
          ,
          <year>2015</year>
          .
        </mixed-citation>
      </ref>
    </ref-list>
  </back>
</article>