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# Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG

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In this work, we show the success of unsupervised transfer learning between Electroencephalographic (brainwave) classification and Electromyographic (muscular wave) domains with both MLP and CNN methods. To achieve this, signals are measured from both the brain and forearm muscles and EMG data is gathered from a 4-class gesture classification experiment via the Myo Armband, and a 3-class mental state EEG dataset is acquired via the Muse EEG Headband. A hyperheuristic multi-objective evolutionary search method is used to find the best network hyperparameters. We then use this optimised topology of deep neural network to classify both EMG and EEG signals, attaining results of 84.76% and 62.37% accuracy, respectively. Next, when pre-trained weights from the EMG classification model are used for initial distribution rather than random weight initialisation for EEG classification, 93.82%(+29.95) accuracy is reached. When EEG pre-trained weights are used for initial weight distribution for EMG, 85.12% (+0.36) accuracy is achieved. When the EMG network attempts to classify EEG, it outperforms the EEG network even without any training (+30.25% to 82.39% at epoch 0), and similarly the EEG network attempting to classify EMG data outperforms the EMG network (+2.38% at epoch 0). All transfer networks achieve higher pre-training abilities, curves, and asymptotes, indicating that knowledge transfer is possible between the two signal domains. In a second experiment with CNN transfer learning, the same datasets are projected as 2D images and the same learning process is carried out. In the CNN experiment, EMG to EEG transfer learning is found to be successful but not vice-versa, although EEG to EMG transfer learning did exhibit a higher starting classification accuracy. The significance of this work is due to the successful transfer of ability between models trained on two different biological signal domains, reducing the need for building more computationally complex models in future research.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI
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Date of publication xxxx 00, 0000, date of current version xxxx 00, 0000.
Digital Object Identiﬁer 10.1109/ACCESS.2017.DOI
Cross-domain MLP and CNN Transfer
Learning for Biological Signal
Processing: EEG and EMG
JORDAN J. BIRD1, JHONATAN KOBYLARZ2, DIEGO R. FARIA1, ANIKÓ EKÁRT1, AND
EDUARDO P. RIBEIRO2
1School of Engineering and Applied Science, Aston University, Birmingham, United Kingdom
2Department of Electrical Engineering, Federal University of Parana, Curitiba, Brazil
Emails: {birdj1, d.faria, a.ekart}@aston.ac.uk1, jhonatankobylarz@gmail.com2, edu@eletrica.ufpr.br2
JJB and JK are co-ﬁrst authors
ABSTRACT In this work, we show the success of unsupervised transfer learning between Electroen-
cephalographic (brainwave) classiﬁcation and Electromyographic (muscular wave) domains with both MLP
and CNN methods. To achieve this, signals are measured from both the brain and forearm muscles and EMG
data is gathered from a 4-class gesture classiﬁcation experiment via the Myo Armband, and a 3-class mental
state EEG dataset is acquired via the Muse EEG Headband. A hyperheuristic multi-objective evolutionary
search method is used to ﬁnd the best network hyperparameters. We then use this optimised topology
of deep neural network to classify both EMG and EEG signals, attaining results of 84.76% and 62.37%
accuracy, respectively. Next, when pre-trained weights from the EMG classiﬁcation model are used for
initial distribution rather than random weight initialisation for EEG classiﬁcation, 93.82%(+29.95) accuracy
is reached. When EEG pre-trained weights are used for initial weight distribution for EMG, 85.12% (+0.36)
accuracy is achieved. When the EMG network attempts to classify EEG, it outperforms the EEG network
even without any training (+30.25% to 82.39% at epoch 0), and similarly the EEG network attempting
to classify EMG data outperforms the EMG network (+2.38% at epoch 0). All transfer networks achieve
higher pre-training abilities, curves, and asymptotes, indicating that knowledge transfer is possible between
the two signal domains. In a second experiment with CNN transfer learning, the same datasets are projected
as 2D images and the same learning process is carried out. In the CNN experiment, EMG to EEG transfer
learning is found to be successful but not vice-versa, although EEG to EMG transfer learning did exhibit
a higher starting classiﬁcation accuracy. The signiﬁcance of this work is due to the successful transfer of
ability between models trained on two different biological signal domains, reducing the need for building
more computationally complex models in future research.
INDEX TERMS Applied Machine Learning, Biological Signal Processing, EEG, EMG, Knowledge
Adaptation, Neural Networks, Transfer Learning
I. INTRODUCTION
It is no secret that the hardware requirements of Deep
Learning are far outgrowing the average consumer level
of resource availability, even when a distributed processing
device such as a GPU is considered [1]. In addition to this,
limited data availability often hampers the machine learning
process. It is for these reasons that researchers often ﬁnd
similar domains to transfer the learning between, effectively
saving computational resources through said similarities by
applying cross-domain interpretation. By doing so, once
impossible tasks become possible, despite limited resources.
A well-known example is VGG (Visual Geometry Group), a
set of 16 and 19 hidden-layer Convolutional Neural Networks
(CNNs) which have been trained to the extreme on a large
image dataset [2]. Useful recognisable features from images
such as points, lines, curves, and geometric shapes can be
transferred over to a differing CNN task since these features
always exist within the domain. Thus, cross-domain transfer
learning is enabled in order to interpret new data [3], [4].
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
Electrical biological signals show a similarly non-
juxtapose pattern of behaviour [5], [6], and thus the domain-
transfer may be possible, although it is currently not yet well-
understood. If it is possible, then to what extent and effects
are those possibilities?
Here, we study for the ﬁrst time whether cross-domain
transfer learning can impact the classiﬁcation ability of mod-
els when trained on Electroencephalographic (brainwave)
and Electromyographic (muscular wave) data. This is per-
formed through the transfer of initial weights via the best
models of each, and learning is continued from this ini-
tial starting point. When compared to the classical method
of random weight distribution initialisation, we argue that
knowledge can be transferred from EMG to EEG and vice-
versa, succesfully. We also compare the results to a model
ﬁne-tuned by ImageNet weights in order to discern that use-
ful domain-related knowledge is actually being transferred
rather than simply general image rules, which could be learnt
from any range of sources and domains.
With better classiﬁcation results come higher impact ap-
plications. In the domain of Human-Robot Interaction, the
control of prosthetic devices [7]–[9], enabling telepresence
within settings such as care assistance [10], [11], as well
as within hazardous settings such as bomb disposal [12],
and remote environments [13], as well as risk of potential
injury [14]–[16] are just a few of many possible ﬁelds that
successful knowledge transfer could potentially advance,
through both improved classiﬁcation ability and lower com-
putational expense required to train models.
The most notable scientiﬁc contributions of this work are
the following:
1) The collection of an original EMG dataset of hand
gestures gathered from the left and right forearms.
2) Derivation of a strong set of neural network hyper-
parameters through an evolutionary search algorithm,
via a multi-objective ﬁtness function towards the best
interpretation and classiﬁcation ability of both EEG
and EMG data.
3) Successful transfer of knowledge between the two do-
mains through unsupervised transfer learning, enabling
increased classiﬁcation ability of the neural networks
when weights are transferred between them as opposed
to traditional random initial weight distribution. Bet-
ter starting abilities, learning curves, and asymptotes
of the network learning process are observed when
knowledge is transferred.
4) To the authors’ knowledge, cross-domain transfer
learning is performed between differing biological sig-
nals (EEG and EMG) for the ﬁrst time.
Section II important background on Artiﬁcial Neural Net-
works and the domains of EEG and EMG are described,
and also the concept of Transfer Learning is introduced. In
Section III the experimental setup is described. The MLP and
CNN transfer learning experiments are described in Sections
IV and V, and subsequent ﬁndings are then presented in Sec-
tion VI. Finally, future work and conclusions are presented in
Section VII.
II. BACKGROUND
In this section, the important scientiﬁc concepts as the basis
of this work, as well as notable state-of-the-art research
within Transfer Learning and Biological Signal Processing
are considered.
A. MULTILAYER PERCEPTRON ARTIFICIAL NEURAL
NETWORKS
A Multilayer Perceptron (MLP) is an Artiﬁcial Neural Net-
work (ANN) trained via validation, backpropagation of er-
rors [17] and a subsequent gradient descent optimisation al-
gorithm [18] in order to perform a classiﬁcation or regression
prediction task [19]. The task in the context of this work is to
label a wave, i.e., what class the wave data belong to, based
on statistical descriptions of the wave behaviour. The goal of
the learning process is to reduce the error rate of the output
of the network when compared to the ground truth; the loss
function minimised in the experiments reported here is the
cross-entropy loss [20], [21] of the networks:
M
X
c=1
yo,c log(po,c),(1)
where Mis the number of classes (3 for EEG, 4 for EMG),
yis a binary indicator of whether the prediction that class c
is the class of observed data ois correct, and ﬁnally pis the
probability that aforementioned data obelongs to the class
label c.
Learning by backpropagation is performed through a gra-
dient descent optimisation algorithm that drives the updating
of the weights within the neural network. In this study,
the Adaptive Moment Estimation (ADAM) algorithm is ap-
plied [22]. Inspired by RMSProp [23] and Momentum [24],
the main steps of ADAM are the following:
1) The exponentially weighted average of past gradients,
vdW is calculated.
2) The exponentially weighted averages of the squares of
past gradient, sdW is calculated.
3) The bias towards zero in the previous are corrected,
deriving vcorrected
dW and scorrected
dW respectively.
4) The network parameters are updated through the fol-
lowing process:
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
vdW =β1vdW + (1 β1)J
∂W
sdW =β2sdW + (1 β2)J
∂W 2
vcorrected
dW =vdW
1(β1)t
scorrected
dW =sdW
1(β2)t
W=Wαvcorrected
dW
qscorrected
dW +ε,
(2)
in which β1and β1are tunable hyperparameters, J
∂W is a
cost gradient of the current network layer to be tuned, Wis a
matrix of weights, αis the learning rate, and εis a small value
introduced to prevent the possibility of division by zero.
B. CONVOLUTIONAL NEURAL NETWORKS
A Convolutional Neural Network (CNN) is a Deep Learning
algorithm capable of collecting an input matrix and ascribing
weights and bias in parallel under the constraints of a pre-
dictive problem [25], [26], resulting in speciﬁc features. A
Convolutional layer performs a dot product between two ma-
trices, where one matrix is the set of learnable parameters and
the other one is known as a kernel, producing an Activation
Map, as shown below:
G[m, n]=(fh)[m, n] = X
jX
k
h[j, k]f[mj, n k],
(3)
where the input matrix is fand the kernel is denoted as h.
It has been previously shown that CNNs succeed with
Biological Signal interpretation. Using EEG-based mental
state, Tripathi and Acharya [27] performed two different clas-
siﬁcations using both Deep (DNN) and Convolutional (CNN)
Neural Networks, where DNN achieved 75.78% accuracy
for valence using the 4040 values as input vector, and CNN
achieved 81.41% accuracy in two dimensions, respectively.
Following the Convolutional layers, interpretation layers are
used to learn from convolutional inputs through the same
process as MLP (see Subsection II-A). Additionally, ﬁlters
are employed to extract statistics from the input matrix and a
single interpretation layer concludes the classiﬁcation into K
possible labels.
C. EEG AND EMG
In this subsection, the necessary scientiﬁc background
on Electroencephalography (EEG) and Electromyography
(EMG) is provided.
EEG is the process of measuring and recording electrical
signals produced by the brain through electrodes placed ei-
ther upon or within the cranium, that is, the nervous impulses
produced by the neurological structure of the brain [28,
p. 31]. Since the activities of a person can largely be summed
up by brain activity [29], to classify an EEG pattern would
provide useful information, and is thus used as a point of
input and control for a Brain Computer Interface (BCI).
BCIs have been successfully operated in a variety of
clinical situations such as evaluation of seizures, epilepsy,
confused states and coma [30]. Classiﬁcation of the brain
activity preceding a stroke showed that a stroke could be
predicted before actual occurrence based on abnormalities
in brain activity [31], through a Random Forest algorithm
applied for statistical classiﬁcation of extracted feature sets.
Successful rehabilitation of motor functions within stroke
patients is aided in a process of measurement and classi-
ﬁcation of brain activity along with robotic feedback [32].
Classiﬁcation of brain activity has also been successful,
similarly to stroke, in the preemptive detection of epileptic
seizures before they occur [33], [34].
EMG is a measure of the electrical potential difference
between two points whose origin are individual or groups
of muscle ﬁbres [35]. Similarly to EEG, the activity of the
muscle can largely be summed up by electrical impulses
produced, and can thus form a point of control in a Muscle-
Computer Interface (muCI) [36]. Similarly to the Muse head-
band operated in many EEG studies, due to its consumer-
friendliness and future potential based on its low-cost yet
high-performing nature, the Myo armband is a prominent
device used in muCI systems, frameworks, and applications.
For example, researchers collaborating from multiple ﬁelds
found that accurate gesture classiﬁcation could lead to a
new standard for New Interfaces for Musical Expression
(NIME) [37].
In the Human-Machine Interaction community, Myo has
been succesfully employed in Sign Language Recognition,
with the classiﬁcation of 20 Brazilian Sign Language letters
with 96-99% accuracy [38]. Following Myo’s proprietary
system of classiﬁcation boasting around 83% accuracy, re-
searchers found that through the application of K-Nearest
Neighbour (KNN) and Dynamic Time Warping (DTW)
algorithms, the classiﬁcation of ﬁve Myo gestures could be
improved to around 86% (+3%) [39].
D. TRANSFER LEARNING
Transfer Learning, as the name suggests, consists in trans-
ferring something learnt in one problem or task to another.
Oftentimes, Transfer Learning is the application of a model
trained on source data to unseen data of the same domain
called target data [40]. The model trained on the source data
can be further trained on the target data, before its deploy-
ment on the target data. Cross-domain transfer learning is a
similar application of a pre-trained model from one domain
to another domain of different nature; for example, in this
study, models trained on two different datasets of biological
waves from the brain and forearm muscles are applied to one
another’s data for further training.
Transfer of knowledge is considered successful from one
domain to the other when the starting point, the learning
curves and asymptotes are higher than those of the traditional
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
Accuracy
Time
FIGURE 1: Example of a successful Transfer Learning ex-
periment. Transfer Learning (top line) has a higher starting
point, steeper curve, and higher asymptote in comparison to
learning via random weight distribution (bottom line).
source-train source-classify approach [41]. A visual repre-
sentation of a successful Transfer Learning experiment can
be seen in Figure 1, where the starting point is higher for
transfer learning compared to random distribution, and sub-
sequently the learning curve is also steeper and the asymptote
is higher.
Generally, there are two main reasons for the application
of Transfer Learning [40]. Firstly, pre-trained models and
computational resources have become easily accessible [42],
there are countless available models trained over many
hours on extremely powerful hardware. Examples include
VGG [2], Inception [43], and MobileNet [44]. Secondly, the
lack of a large enough dataset for learning is often negated by
transferring previous knowledge to the domain at hand [40].
Pan and Yang [45] deﬁne three main types of Transfer
Learning as follows:
1) Inductive Transfer Learning is knowledge transfer
when the source and target domains are identical but
a new task is to be learned. For example, if ﬁve EMG
gestures are classiﬁed and further learning enables
the model to learn to recognise additional gestures,
based on the current knowledge, then inductive transfer
learning takes place.
2) Unsupervised Transfer Learning is the transfer of
knowledge between two differing domains and like-
wise differing tasks. In this study, unsupervised trans-
fer learning ﬁnds success in sharing knowledge be-
tween EEG and EMG domains through the mental state
and gesture recognition tasks.
3) Transductive Transfer Learning is the process of shar-
ing knowledge between differing domains but for the
same task. For example, if an EEG headband is to
be calibrated to a subject’s data (a slightly different
domain) to complete the same mental state recognition
task, then transductive transfer learning takes place.
Recently, many Transfer learning techniques have been ap-
plied successfully in real-world problems, for example, can-
cer subtype discovery [46], building-space optimisation [47],
[48], text-mining [49], [50], and reinforcement-learning for
videogame AI [51], [52].
Cross-domain transfer learning has been given relatively
little attention in the ﬁeld of biological signal processing,
with research almost exclusively opting for same-domain
personalisation, or calibration. EEG and EMG signals are ex-
cellent candidates for cross-domain transfer learning, given
their similarities, yet this idea has not been investigated. In
this study we aim to ﬁll this gap and establish cross-domain
transfer learning between EEG and EMG domains.
It has been shown that models do not generalise well
between subjects, thus there is a need for transfer learning
to achieve accurate classiﬁcation results [53], [54].
A highly promising proposal [55] consists of a two-step
ensemble of ﬁlter-bank classiﬁcation of EEG data via two
models, one for the original dataset, and another for a small
dataset collected from a new subject. The baseline classiﬁ-
cation ability for nine individual subjects improves by ap-
proximately 10%. Similarly, the kernel principal component
analysis (kernel PCA) approach in leads to an improvement
from 58.95% to 79.83% (+20.88) classiﬁcation accuracy
when transfer learning from the original dataset is performed
for a new subject [56].
Similarly to EEG, transfer learning in EMG is most of-
ten concerned with cross-subject learning rather than cross-
domain application [57]. Researchers gathered and combined
datasets of EMG data measured from a total of 36 subjects
via the Myo armband (as also used in this study). The dataset
was split into sets of 19 and 17 subjects. Transfer learning
of learnt features of a Convolutional Neural Network led to
a classiﬁcation accuracy improvement of 3.5%. Though this
improvement is small, the achieved accuracy is the state-of-
the-art for the dataset.
Transfer Learning in EMG has been successful in calibrat-
ing to electrode shift, change of posture, and disturbances due
to sweat and fatigue [58] through small calibration recordings
that subsequently require fewer than 60 seconds of training
time. Prahm et al. found that the exercises increased in
accuracy after disturbance from 74.6% to 97.1%.
Motivated by the small successes of cross-subject transfer
learning within EEG and EMG domains independently, as
well as the similar nature and behaviour of these biological
signals, we propose to explore the potential of applying learnt
knowledge from one biological signal domain to the other
and vice versa.
III. EXPERIMENTAL SETUP
In this section,we describe the data acquisition, feature
extraction, topology selection, learning, and transfer learning
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
(a) Muse EEG Headband
(InteraXon)
(b) MYO EMG Armband
(Thalmic Labs)
FIGURE 2: The devices used to acquire data for this experi-
ment.
experiments.
In terms of hardware, all models are trained on Tensor-
Flow via an NVidia GTX980Ti Graphics Processing Unit.
For topology selection, our previously proposed DEvo [59]
algorithm is executed for 15 generations. Hard limits of a
maximum of 5 hidden layers and 512 neurons were set. Evo-
lutionary topology optimisation allowed for 100 epochs of
training and transfer learning was observed with 30 epochs of
training. These values were chosen based on the observation
that in preliminary experiments there was little or no further
improvement after these numbers of generations and epochs,
respectively. Training validation is enabled through 10-fold
cross validation, where the ten folds are shufﬂed. Other
hyperparameters that were chosen were the ReLu activation
function for the hidden layers, and the ADAM optimisation
algorithm [22] for tuning of weights during training.
A. DATA ACQUISITION
Two datasets are used in this experiment, EEG and EMG.
Here we describe the acquisition of the EEG and EMG
datasets. The devices used to acquire datasets can be seen
in Figure 2.
The EEG dataset was collected in a previous study [60].
This dataset was obtained from four subjects, two male and
two female. The subjects performed three tasks, while the
sensors were recording the data. The tasks involved three
different states of brain activity: concentration, relaxation
and neutral. The EEG data were acquired using the Muse
Headband, which is a commercial EEG device with four dry
electrodes (TP9, AF7, AF8 and TP10). The EMG dataset
was gathered via the Myo armband, a commercial elec-
tromyograph monitoring device with 8 dry electrodes for the
measurement of electrical current produced by the skeletal
muscles within the arm. The Myo armband is capable of
recording EMG data at sample rate of 200Hz. In addition,
it also has a nine axis Inertial Measurement Unit (IMU)
operating at a sample rate of 50Hz. For this project, only
the EMG data are used and so the inertia of the arm is not
considered.
Ten subjects contributed to the EMG dataset, six male
and four female all aged 22-40. The subjects performed
four different gestures for 60 seconds each, and the sensors
FIGURE 3: Data collection from a male subject via the Myo
armband on their left arm.
recorded EMG data produced by the muscles in the forearm.
The gestures performed were, clenching and relaxation of the
ﬁst, spreading and relaxation of ﬁngers, swiping right, and
swiping left. The observations were performed twice, once
for the right arm and once for the left. Figure 3 shows the
experimental setup of a subject wearing the Myo armband.
B. FEATURE EXTRACTION
Biological signal processing encounters issues in classiﬁ-
cation due to their non-stationary, nonlinear and random
nature. Since the wave class is observed over time, temporal
observations must be made rather than single points. For
this reason, short segments of the wave are considered and
a statistical feature matrix is generated based on observation
of the temporal segment. Previous studies identiﬁed a set of
statistics to be extracted from EEG brainwaves [59], [60].
In this study, a set of statistics was extracted from both the
EMG and EEG signals of a temporal nature. Initially, the
sampling rate was reduced to a uniform 200Hz based on fast
Fourier transformations along a given axis due to the variable
nature of a Bluetooth Low Energy (BLE) connection. The
signal was assumed to be periodic since a Fourier method
is used. This led to a realistic down-sampling since the
dominant energy was concentrated in the range of 20-500Hz
as observed in [60], even though the frequency range of
the EEG sensor was superior to this. The Muse headband
performed Notch ﬁltering at 50Hz since the experiment was
performed in the United Kingdom.
Both EEG and EMG signals are non-stationary and ran-
dom waves and it is for this reason that feature extraction
must be performed when a non-temporal learning process is
followed. Feature extraction was performed by introducing
sliding windows of length of 1 second at an overlap of 0.5
seconds to segment the data. Algorithm 1 describes the hand-
crafted features extracted in this work. Considering how we
split the 1 second window, the two 0.5 second half-windows
produced due to the differences between two 1 second win-
dows, we compute: (i) the change in both the sample means
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
and in the sample standard deviations between the ﬁrst and
second half-window; (ii) the change in both the maximum
and minimum values between the ﬁrst and second half-
windows. Then, considering the two 0.25 second quarter-
windows produced due to further differences between com-
puted windows, we then compute (i) the sample mean of each
quarter-window; (ii) all paired differences of sample means
between the quarter-windows; (iii) the maximum (minimum)
values of each quarter-window, plus all paired differences of
maximum (minimum) values between the quarter-windows.
A Discrete Fourier Transform is performed on the time
windows and all resultant values are treated as attributes.
Result: Features extracted from raw data for every wt
User deﬁned the size of the sliding window wt= 1s;
Input: sequence of raw data (EEG or EMG);
Initialisation of variables init = 1,wt= 0;
while getting sequence of raw data from sensor (>1min)do
if init then
prev_lag = 0;
post_lag = 1
end
init = 0;
;
for each slide window (wtprev_lag) to (wt+post_lag )
do
Compute the mean of all wtvalues y1, y2, y3...yn;
¯yk=1
NPN
i=1 yki ;
;
Compute the asymmetry and waves peakedness
represented by 3rd and 4th order moments skewness and
kurtosis g1,k =PN
i=1(yki ¯yk)3
Ns3
k
and
g2,k =PN
i=1(yki ¯yk)4
Ns4
k
3;
;
Compute the maximum and minimum value of each
signal wt
max =max(wt)and wt
min =min(wt);
;
Compute the sample variances K×Kmatrix Sof each
signal, plus the sample covariances of all signal pairs
sk =1
N1PN
i=1 (yki ¯yk) (yi ¯y) ;
k,  [1, K];
;
Compute the Eigenvalues of the covariance matrix S,
which are the λsolutions to: det (SλIK)=0, where
IKis the K×Kidentity matrix, and det(·)is the
determinant of a matrix;
;
Compute the upper triangular elements of the matrix
logarithm of the covariance matrix S, where the matrix
exponential for Sis deﬁned via Taylor expansion
eB=IK+P
n=1
Sn
n!, then BCK×Kis a matrix
logarithm of S;
;
Compute the magnitude of the frequency components of
each signal, obtained using a Fast Fourier Transform
(FFT), magFFT(wt);
;
Get the frequency values of the ten most energetic
components of the FFT, for each signal, getFFT(wt, 10);
end
wt=wt+ 1s;
prev_lag = 0.5s;post_lag = 1.5s;
Output Features F wtextracted within the current wt
end
Algorithm 1: Feature extraction algorithm for a se-
quence of data (EEG or EMG signals).
A change in attribute values is also considered via a
lag window, in which each window is passed the previous
extracted value vector from the preceding window. The
ﬁrst window does not receive this vector since no window
preceded it. Due to the redundancies of the maximum, mean,
and minimum values of quarter-windows due to the overlaps,
these values are not considered within lag window attribute
vectors.
The features extend beyond the usual values read by clin-
icians due to the lower quality of the Muse headband as
opposed to what can be expected from clinical EEG, with
dimensionality reduction often then used in order to select a
set of useful features from a large dataset in which weaker
features may exist. In this work, identiﬁcation of weak fea-
tures has to be left to the deep learning algorithms since
identical inputs are needed for the networks in order to enable
the possibility of transfer learning (preventing a mismatch in
topology, or neuron representation etc.). This is a limitation
discussed in Section VII, and future experiments are outlined
in order to produce a synchronised dimensionality reduction
algorithm between the two datasets concurrently in the form
of a combinatorial optimisation problem. Dimensionality
reduction was not performed here since it was not needed for
the exploration of transfer learning between biological signal
domains.
A vector representation of the wave behaviour is formed
through the above process and 988 numerical features are
created. Since the same process is followed for the same
number of electrodes, the datasets describing EMG and EEG
have the same dimensionality and thus transfer learning is
possible with these features as inputs to an ANN.
IV. METHOD I: MLP TRANSFER LEARNING
A. DERIVATION OF BEST MLP TOPOLOGY
Although many studies focus on grid search of topologies
[61]–[63], this study applies a multi-objective evolutionary
algorithm in order to select the best neural network architec-
ture for both classiﬁcation problems. We apply an evolution-
ary algorithm instead of a classical grid search for two main
reasons [64]–[66]:
1) Evolutionary search allows for exploration within
promising areas of the problem space at a ﬁner level.
Previous experiments, such as speech recognition [67],
found complex best solutions for the problem, e.g. a
combination of three deep layers of 599, 1197, and 436
neurons. Including such multiples within a grid search
would increase computational complexity of the search
beyond realistic possibility.
2) With multi-objective optimisation through mean ac-
curacy via equal scalarisation (see equation 4), the
algorithm was able to search for a best solution for
both of the problems rather than having to be executed
twice, followed by statistical analyses to calculate a
best topology.
6VOLUME 4, 2016
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
Deep Evolutionary Optimisation (DEvo) is an evolution-
ary algorithm, inspired by Darwinian evolution in Nature,
used to search the problem space of neural network topology
in order to select a best-performing structure of hidden
layers and neurons i.e. a selection of network hyperpa-
rameters. DEvo has been successfully used in benchmark
problems [68] as well as in phoneme classiﬁcation [67] and
EEG classiﬁcation [59].
Result: Output the best neural network conﬁgurations discovered
per generation
User deﬁned population size p, maximum number of hidden layers
h, and maximum number of neurons n
Initialise prandom solutions with hidden layers in the range [1..h],
each hidden layer is assigned [1..n] neurons ;
Train all neural networks through forward-pass and
backpropagation;
Calculate ﬁtness Fof all solutions;
Sort solutions by F;
while Termination not met do
for For each solution parent1 do
Select a random solution parent2;
Select network depth drandomly from parent1,parent2 or
mutate
while d not met do
Randomly select layer from parent1,parent2 or
mutate
if NOT mutate then
if Selected layer exists within selected parent
then
Amend layer to solution offspring
else
if Selected layer exists within other parent
then
Amend layer to solution offspring
else
Assign random value;
\\since none exist
end
end
end
end
end
Calculate ﬁtness Fof all solutions;
Sort solutions by F;
Keep the top psolutions from all parents and offspring;
Amend best solution to best solutions b;
end
Output all best solutions bof each generation;
Algorithm 2: Generalised process of the evolutionary
search of Neural Network topology
As illustrated by the ﬂow diagram in Figure 4, the general
idea of single-objective DEvo is as follows:
1) Create an initial population of random solutions.
2) Simulate the following until a user-deﬁned termina-
tion:
a) Select parent networks for the current generation
of the evolutionary cycle.
b) Optionally, alter the depth or width of the network
via random mutation, to prevent premature con-
vergence to local minima within the population.
c) Train the neural networks by performing forward-
pass and backpropagation of errors through a
given optimisation function for a user-deﬁned
number of epochs, and calculate the ﬁtness.
A more detailed process of the evolutionary search can be
observed in Algorithm 2. In particular, the re-combinations
of two parent networks is shown in detail.
Since the search must derive a ’best of both worlds’
solution for both the EMG and EEG problems, a new ﬁtness
function is introduced to score a solution:
F(s)=0.5A(EMG)
100 + 0.5A(EEG)
100 ,(4)
where A(EM G)and A(E EG)are the mean accuracy scores
of the networks when trained with EMG and EEG data
respectively through shufﬂed 10-fold cross validation. Equal
weights are allocated to the two components as EEG and
EMG training are equally important. Only hidden layers are
to be optimised, therefore the input and output layers of the
network are simply hard-coded.
B. BENCHMARKING OF TRANSFER LEARNING
For transfer learning, the following process is followed:
1) A neural network with randomly distributed weights is
trained to classify the EMG dataset.
2) A neural network with randomly distributed weights is
trained to classify the EEG dataset.
3) The best weights from the EMG network are applied to
a third neural network, which is then trained to classify
the EEG dataset.
4) Mirroring step 3, the best weights from the EEG net-
work (step 2) are initialised to a fourth neural network,
which is then trained to classify the EMG dataset.
The four networks are then compared. EEG to EMG-EEG
and EMG to EEG-EMG in order to discern whether knowl-
edge has been transferred. If higher starts, curves, and asymp-
totes are observed, then knowledge is considered succesfully
transferred between the two domains.
V. METHOD II: CNN TRANSFER LEARNING
A. REPRESENTING BIOLOGICAL WAVES AS IMAGES
In order to generate a square matrix, after the feature extrac-
tion process, the ﬁnal 28 attributes are removed from each
dataset. This is done because 961 is the closest square number
within the attribute set (31x31) and the ﬁnal attributes are
chosen in order to retain identical inputs to the networks for
both datasets. After normalisation of all attributes between
the values of 0 and 255, they are then projected as 31px
square images. Examples of waves projected into visual
space can be observed in Figures 5 and 6. Though padding
would be applied in the situation where a square reshape is
not possible (if square input is considered), this is not needed
in this experiment since 961 attributes are selected (31x31
reshape).
VOLUME 4, 2016 7
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
START
Generateprandom
neuralnetwork
architectures
Trainallnetworks
throughforward-pass
andbackpropagation
CalculateFitnessFiof
eachneuralnetworki
Sortneuralnetworksin
descendingorderofFi
Selectparentnetworks
forthegeneration
Termination?
Foreachparent1,
chooseparent2and
produceoffspring
Applyrandommutation
Present[b1..bn]best
solutionsobservedat
eachgeneration
END
NO
YES
FIGURE 4: Flow Diagram of the evolutionary search of Neural Network topology. Population size is given as pand ﬁtness
calculation is given as F. Set {b1..bn}denotes the best solution presented at each generation
FIGURE 5: 30 Samples of EEG as 31x31 Images. Top row shows relaxed, middle row shows neutral, and bottom row shows
concentrating.
FIGURE 6: 40 Samples of EMG as 31x31 Images. Top row shows close ﬁst, second row shows open ﬁngers, third row shows
wave in and bottom row shows wave out.
8VOLUME 4, 2016
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
TABLE 1: Network topology and parameters found to be best
in a previous work [69]
Layer Output Parameters
Conv2d (ReLu) (0, 14, 14, 32) 320
Conv2d (ReLu) (0, 12, 12, 64) 18,496
Max Pooling (0, 6, 6, 64) 0
Dropout (0.25) (0, 6, 6, 64) 0
Flatten (0, 2304) 0
Dense (ReLu) (0, 512) 1,180,160
Dropout (0.5) (0, 512) 0
Dense (Softmax) (0, 3) 1,539
B. DERIVATION OF HYPERPARAMETERS
Due to the computational costs of hyperparameter optimisa-
tion used in the MLP experiment when a CNN is consid-
ered, CNN topology is instead inspired by Ashford et al.’s
work [69]. In this work, the topology observed in Table 1
was found to be best after empirical exploration. Dropout is
introduced in order to prevent overﬁtting after two convolu-
tional operations (and subsequent pooling) and again after an
interpretation layer of 512 densely connected neurons.
C. BENCHMARKING OF TRANSFER LEARNING
The benchmark of the CNN transfer learning follows the
same process as detailed in Section IV-B, except the weight
transfer applies to input, convolutional, and hidden interpre-
tation layers.
The hypothesis of this experiment ie. that transfer learning
has occurred cross-domain, not simply through deep learn-
ing, is tested by comparison to a popular pre-trained model.
For this purpose, the ResNet50 architecture and weights [70]
are used when trained on the ImageNet dataset. This archi-
tecture is chosen based on its aptitude for smaller images as
opposed to the previously mentioned VGG16 model, more
ﬁtting to the nature of the images generated by the algorithm.
The experiments are given unlimited time to train in order
to explore this, with an early stop executing after 10 epochs
with no observed improvement of validation accuracy. Other
model hyperparameters are identical to their transfer learning
counterparts.
VI. EXPERIMENTAL RESULTS
In this section, the results from the two experiments are dis-
cussed. Firstly, an MLP network topology is derived through
the previously described DEvo method before transfer learn-
ing capabilities are benchmarked. Initially, the models are
trained starting from random weight distribution (baseline) in
order to provide the baseline. Secondly, the model trained on
EMG dataset is used to transfer knowledge to a model train-
ing to classify the EEG dataset and then vice-versa. These are
then compared to their baseline non-transfer learning coun-
terparts. This is carried out a second time with Convolutional
Neural Networks (without evolutionary search) where signals
have been projected as raster images.
The MLP experiments are presented and discussed in Sub-
section VI-A and the CNN experiments are then presented
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0
0.2
0.4
0.6
0.8
1
Generation
Best Fitness Score
FIGURE 7: Highest (best) ﬁtness observed per generation of
the combined and normalised ﬁtnesses of EEG and EMG data
classiﬁcation. The two ﬁtness components are considered
equally weighted to produce the same topology in order to
allow direct transfer of weights.
TABLE 2: Comparison of the MLP Training Processes of
EMG and EEG with random weight distribution compared
to weight transfer learning between EMG and EEG
Experiment Training Accuracy (%)
Epoch 0 Final Epoch Best Epoch
EMG 62.84 83.57 84.76
EEG 54.7 62.1 62.73
Transfer Learning
(EEG to EMG) 65.22 (+2.38) 85 85.12 (+0.36)
Transfer Learning
(EMG to EEG) 84.95 (+30.25) 93.28 93.82 (+29.95)
and discussed in Subsection VI-B.
A. EXPERIMENT 1: MLP TRANSFER LEARNING
1) Hyperparameter Selection for Initial Random Distribution
Learning
Figure 7 shows the ﬁtness evolution (Equation 4) of neural
network topologies for the two datasets, where each point
is the combined mean ﬁtness for EEG and EMG and the
best topology. The best result was found to be a network of
5 hidden layers, with neuron counts 206,226,298,167,363
respectively at a combined ﬁtness of 0.74. This network
topology is thus taken forward in the experiments towards
transfer learning capability between the networks of EEG to
EMG and vice-versa.
2) MLP Transfer Results
Finally, the transfer learning experiment is executed fol-
lowing the process described in section IV-B. Figure 8 and
Table 2 detail the learning processes of both EMG and
EEG as well as the transfer learning experiments, from one
VOLUME 4, 2016 9
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
FIGURE 8: Test and Training Accuracies of EMG, EEG, and transfer between EMG and EEG. ’EEG Transfer’ denotes EMG
to EEG and likewise for ’EMG Transfer’.
domain to the other and vice-versa. Transfer learning was
most successful when EMG data was used to ﬁne tune the
EEG problem, with an increase of best classiﬁcation accuracy
from 62.37% to 93.82% (+29.95). A very slight increase
was also observed in reverse, when EEG network weights
were used as the initial distribution for the EEG problem,
with best accuracy rising from 84.76% to 85.12% (+0.36). In
terms of starting accuracy, that is, accuracy of classiﬁcation
with no training at all, a success of knowledge transfer also
occured; EEG classiﬁcation increased from 54.7% to 84.95%
(+30.25), and thus even prior to any training the network
outperformed the network initially trained on EEG data.
Likewise, the EMG classiﬁcation prior to any training at
epoch 0 increased from 62.84% to 65.22% (+2.38). It was
observed that learning had ceased prior to epoch 30 being
reached.
The epoch zero results are particularly interesting since
transfer learning has occurred between two completely dif-
ferent domains, from EMG gesture classiﬁcation to EEG
mental state recognition. This shows that knowledge transfer
TABLE 3: Comparison of the CNN Training Processes of
EMG and EEG with random weight distribution compared to
weight transfer learning between EMG and EEG
Experiment Training Accuracy (%)
Epoch 0 Final Epoch Best Epoch
EMG 52.4 88 88.55
EEG 72.5 95.3 96.24
Transfer Learning
(EMG to EEG) 82.39 (+9.89) 96.4 97.18 (+0.94)
Transfer Learning
(EEG to EMG) 58.18 (+5.78) 84.24 85.18 (-3.37)
is possible even without training being required.
B. EXPERIMENT 2: CNN TRANSFER LEARNING
Figure 9 shows the learning processes for the four networks.
It was observed that learning was still occurring at epoch
30 (unlike in the MLPs in Experiment 1), and due to this,
learning time was increased to 100 epochs. Table 3 shows
the outcome of the experiments. Some transfer learning suc-
cesses were achieved, with higher starts in TL experiments,
10 VOLUME 4, 2016
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
FIGURE 9: Test and Training Accuracies of EMG, EEG, and transfer between EMG and EEG with a Convolutional Neural
Network, over 100 epochs. As with the previous ﬁgure, ’EEG Transfer’ denotes EMG to EEG and likewise for ’EMG Transfer’.
TABLE 4: Best CNN accuracy observed for ResNet50, Base-
line (Non-Transfer) Learning, and Transfer Learning
Best CNN Accuracy Observed (%)
ResNet50 Baseline Transfer Learning
EEG EMG EEG EMG EEG EMG
92.34 74.92 96.24 88.55 97.18 85.18
of +9.89% and +5.78% for EEG and EMG, respectively. The
best classiﬁcation accuracy of EEG was improved by 0.94%
whereas this was not the case for EMG, which actually de-
creased by 3.37%. Thus, the CNN transfer learning approach
is only successful in the case of EMG to EEG but not vice-
versa.
It is important to note that previously, the One Rule Ran-
dom Forest approach [60] gained 87.16% accuracy and the
image representation and CNN approach [69] gained 89.38%
accuracy on EEG data. Our network is competitive at 82.39%
accuracy on the same dataset with no training whatsoever,
using simply the weights from the EMG network. Similarly,
it is also important to note that the ﬁnal accuracy of 97.18%
substantially outperforms these previous approaches.
1) Comparison to ResNet50
For comparison of transfer quality, the ResNet 50 CNN ar-
chitecture is used. Table 4 shows that the ResNet50 achieves
weaker results for both problems. The ResNet50 architecture
was observed to stop improving after 35 and 39 epochs for
EEG and EMG respectively, similarly to the behaviour of our
architecture shown in Figure 9.
VII. FUTURE WORK AND CONCLUSION
This study demonstrated that cross-domain transfer learning
is possible between the domains of electroencephalography
and electromyography, between the electrical signals pro-
duced by the frontal lobes of the brain and forearm mus-
cles.To the best of our knowledge, cross-domain transfer
learning with EMG to EEG and vice-versa has not been ex-
plored elsewhere. Firstly, models were trained via a train/test
split and with some hard-coded hyperparameters such as acti-
vation function and gradient descent optimisation algorithm;
future work should explore the effects and implications of
differing hyperparameter sets.
Limited selection of network topologies was performed
through a single multi-objective evolutionary search. With
the possibility of a local minimum being encountered and
stagnation occurring, further executions of the search should
be performed in a subsequent study in order to explore the
problem space and thus reduce the chance of stagnation.
VOLUME 4, 2016 11
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JJ Bird et al.: Cross-domain MLP and CNN Transfer Learning for Biological Signal Processing: EEG and EMG
Scalarisation was considered equal between the two datasets,
though the EMG dataset was more diverse and much larger
than the EEG dataset and thus alternative scalars with prefer-
ence to either dataset should also be benchmarked. Future
work could also involve the possibilities of cross-domain
transfer learning in multiple biological signal domains, such
as including other areas of the muscular system and brain,
and additionally, other domains such as electrocardiography.
The potential for transfer learning between these domains
should be applied in Human-machine interaction in the
future, since the application of a framework as described
here shows not only the advantage of improved accuracy
of classiﬁcation, but additionally, the derivation of a less
computationally expensive process compared to learning
from scratch.
In this study, dimensionality reduction or feature selection
was not performed. Although previous studies have shown
the effectiveness of removing certain features in favour of
others (both for increased classiﬁcation ability and reduction
of complexity), it was important that the neural networks
were identical in structure to one another. In doing this,
transfer of weights between networks was directly compa-
rable to the original network to be transferred. In future, a
strategy of synchronised feature selection could be explored
in order to match network inputs. This could be done in a
relatively simple manner, by ranking a deﬁned number of
attributes (by Information Gain, Symmetrical Uncertainty,
etc.) and choosing those which both algorithms would choose
individually. In a far more complex experiment, the se-
lection of attributes could be presented as a combinatorial
optimisation problem. Analogously to the famous knapsack
problem, transfer learning could provide a ﬁtness function
to the combination of selected attributes similar to that in
Equation 4. A set of attributes useful to both problems could
then be derived to present a synchronised set of inputs for a
ﬁnal transfer learning benchmark. These suggestions could
be benchmarked in an ’optimisation extension’ to this study.
Additionally, dimensional-reshape techniques other than the
traditional 2D image reshape considered in this study could
also be performed.
The ﬁnal comparison experiment was designed speciﬁ-
cally to assess whether the cross-domain transfer of knowl-
edge has occurred as opposed to simply CNN transfer
learning. Success in cross-domain transfer of knowledge
was shown compared to the ResNet50 model. In the future,
further CNN architectures could be explored. For example,
InceptionV3 and VGG16 models outperform the ResNet50
model in the state-of-the-art, but their minimum input di-
mensions are above those of the data in this experiment. With
larger feature selection bounds for the CNN input, in order to
generate larger images (which are also to be benchmarked
in a related experiment prior to discern the best feature
extraction processes), other models such as these could be
implemented and provide further evidence for cross-domain
transfer of knowledge.
To conclude, we argue that, through initial weight distribu-
tion, cross-domain transfer learning between two biological
signal domains is possible and, in some cases, to a great pos-
itive effect. Identical mathematical features were extracted
from the waves to provide a stationary description ﬁt for
classiﬁcation, and transfer between features was also noted.
Initial abilities pre-training were higher than random weight
distribution, the learning curves and ﬁnal classiﬁcation abili-
ties for both domains were also better, indicating that useful
knowledge had been shared between both domains during the
transfer learning process. The exploration of the possibility
of transfer of knowledge from/to other biological signal
domains such as ECG is also an exciting topic for future
study.
ACKNOWLEDGEMENT
This work was partially supported by “UK ACADEMIES -
RESEARCH MOBILITY” PI 02/2018 in Partnership with
CONFAP-Brazil and Fundação Araucária Pr-Brazil (protocol
50715.538.33202.06072018), through the project: "Stepping-
stones to Transhumanism: Merging EMG and EEG signals to
control a low-cost robotic hand" awarded to Diego R. Faria
(Aston University, UK) and Eduardo P. Ribeiro (Federal
University of Parana, Brazil).
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JORDAN J. BIRD achieved a ﬁrst class Bache-
lor’s Degree with Honours in Computer Science
at Aston University in the United Kingdom, be-
fore continuing with PhD studies at the same
institution in 2018 with an awarded scholarship.
Garnered through a deep Scientiﬁc passion from
an early age, his research interests exist largely
within the ﬁeld of Human-robot Interaction; these
include, the Emergence of Artiﬁcial Intelligence,
Intelligent Social Frameworks, Turing’s Imitation
Game, Deep Machine Learning, and Transfer Learning. Jordan is a founding
member of the Aston Robotics, Vision and Intelligent Systems (ARVIS)
laboratory at Aston University.
JHONATAN KOBYLARZ is an Electronic Engi-
neering student at Universidade Federal do Paraná
(UFPR), Brazil. His research interests include
Deep Machine Learning towards Social Robotic
Interaction, Bioengineering and Computer Vision.
DIEGO R. FARIA is a Senior Lecturer in Com-
puter Science. He is with the School of Engineer-
ing and Applied Science, Aston University, Birm-
ingham (UK). He is the coordinator and founder
of the ARVIS Lab (Aston Robotics, Vision and
Intelligent Systems Lab). Currently (2019-2022)
he is the project coordinator of the EU CHIST-
ERA InDex project (Robot In-hand Dexterous ma-
nipulation by extracting data from human manipu-
lation of objects to improve robotic autonomy and
dexterity) funded by EPSRC UK. Dr Faria is also PI and Co-I (2020-2022) of
two projects with industry (KTP-Innovate UK scheme) related to perception
and autonomous systems applied to autonomous vehicles, and NLP and
image processing for multimedia retrieval. He received his Ph.D. degree
in electrical and computer engineering from the University of Coimbra,
Portugal, in 2014. He holds an M.Sc. degree in computer science from the
Federal University of Parana, Brazil, in 2005. In 2001, he earned a bachelor’s
degree in informatics technology (data computing & information) and he has
ﬁnished a computer science specialization in 2002 at the State University of
Londrina, Brazil. From 2014 to 06/2016 Dr. D. Faria was a post-doctoral
fellow at the Institute of Systems and Robotics, University of Coimbra
where he collaborated on different projects funded by EU commission
and the Portuguese government in areas of Robot Grasping and Dexterous
Manipulation, Artiﬁcial Perception, Cognitive Robotics and Assisted Living.
His research interests are: assisted living, perception systems, intelligent and
autonomous systems, and cognitive robotics.
ANIKÓ EKÁRT is a reader in Computer Science
at Aston University, where she is also Associate
Dean for Postgraduate Studies for the School of
Engineering and Applied Science. After obtaining
a PhD from Eötvös Loránd University, Budapest,
Hungary, she held research positions at the Com-
puter and Automation Research Institute, Hun-
garian Academy of Sciences in Budapest, Hun-
gary and a lectureship in Computer Science at
University of Birmingham, UK. Her research is
focused on theory and application of artiﬁcial intelligence techniques, and
in particular genetic programming and evolutionary computation. Areas of
application range from health to engineering, art and economics.
EDUARDO P. RIBEIRO is a Professor at Univer-
sidade Federal do Parana, Brazil, since 1997. He
received Bachelor’s degree in Electrical Engineer-
ing (1990), M.Sc. degree (1992) and Ph.D. degree
(1996) from Pontiﬁcal Catholic University of Rio
de Janeiro. He did research stage at Vanderbilt
University, USA (1995) and at The University of
British Columbia, Canada (2005). His research
interests include machine learning, signal process-
ing, instrumentation and communication.
14 VOLUME 4, 2016
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Domain adaptation is a form of transfer learning, in which the task remains the same, but there is a domain shift or a distribution change between the source and the target. As an example, consider a model that has learned to classify reviews on electronic products for positive and negative sentiments, and is used for classifying the reviews for hotel rooms or movies. The task of sentiment analysis remains the same, but the domain (electronics and hotel rooms) has changed. The application of the model to a separate domain poses many problems because of the change between the training data and the unseen testing data, typically known as domain shift. For example, sentences containing phrases such as “loud and clear” will be mostly considered positive in electronics whereas negative in hotel room reviews. Similarly, usage of keywords such as “lengthy” or “boring” which may be prevalent in domains such as book reviews might be completely absent in domains such as kitchen equipment reviews.