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Towards a Robust and Accurate Screening Tool for Dyslexia with Data Augmentation using GANs

Authors:
Towards a robust and accurate screening tool for
dyslexia with data augmentation using GANs
Thomais Asvestopoulou, Victoria Manousaki, Antonis Psistakis, Erjona Nikolli,
Vassilios Andreadakis , Ioannis M. Aslanides§, Yannis Pantazis, Ioannis Smyrnakis†‡ and Maria Papadopouli
Department of Computer Science, University of Crete, Heraklion, Greece
Institute of Computer Science, Foundation for Research and Technology-Hellas, Heraklion, Greece
Optotech Ltd., Heraklion, Greece
§Emmetropia Eye Institute, Heraklion, Greece
Institute of Applied and Computational Mathematics, Foundation for Research and Technology-Hellas, Heraklion, Greece
Abstract—Eye movements during text reading can provide
insights about reading disorders. We developed the DysLexML,
a screening tool for developmental dyslexia, based on various
ML algorithms that analyze gaze points recorded via eye-
tracking during silent reading of children. We comparatively
evaluated its performance using measurements collected from two
systematic field studies with 221 participants in total. This work
presents DysLexML and its performance. It identifies the features
with prominent predictive power and performs dimensionality
reduction. Specifically, it achieves its best performance using
linear SVM, with an accuracy of 97% and 84% respectively,
using a small feature set. We show that DysLexML is also
robust in the presence of noise. These encouraging results set the
basis for developing screening tools in less controlled, larger-scale
environments, with inexpensive eye-trackers, potentially reaching
a larger population for early intervention. Unlike other related
studies, DysLexML achieves the aforementioned performance
by employing only a small number of selected features, that
have been identified with prominent predictive power. Finally, we
developed a new data augmentation/substitution technique based
on GANs for generating synthetic data similar to the original
distributions.
I. INTRODUCTION
Dyslexics manifest significant and persistent reading dif-
ficulties [1], which often involve difficulty in reading due
to word decoding (relating sounds with written phrases, i.e.,
graphemes to phonemes) [2]. Early intervention can be ef-
fective in alleviating the symptoms of the disability. How-
ever, screening large populations of children is rather time-
consuming and expensive [3]. For example, a differential
diagnosis of dyslexia can take up to 14 months [4]. It has
been known that the eye movements during text reading can be
particularly revealing [5]–[9]. For example, dyslexics exhibit
more aberrant eye movements than normal readers at the
This work has been partially funded from the Hellenic Foundation for
Research and Innovation (HFRI) and the General Secretariat for Research
and Technology (GSRT) under grant agreement No. 2285, the Erasmus+
International Mobility between University of Crete and Harvard Medical
School 2017-1-EL01-KA107-035639, the Marie Curie RISE NHQWAVE
project under grant agreement No. 4500, and the Human Resources
Development, Education and Life Lifelong Learning for the implementation
of the European Social Fund and the Youth Employment Initiative.
Contact author: Maria Papadopouli (mgp@ics.forth.gr)
same age level [1], although it is unlikely that the primary
cause of dyslexia is erratic eye movements. Fixations, i.e.,
maintaining the visual gaze on a single location, and saccadic
movements, i.e., quick simultaneous movements of the eyes
between fixations, are important characteristics for screening
dyslexia. Readers with developmental dyslexia generate differ-
ent eye movements than typical readers during text reading:
longer and more frequent fixations, shorter saccade lengths,
more backward refixations than typical readers [6]–[8], [10].
Furthermore, readers with dyslexia have difficulty in reading
long words, lower skipping rate of short words, and high
gaze duration (total fixation duration) on many words [4].
Nonetheless, it is still an open question whether it is possible
to build a screening tool that can reliably identify readers who
may be of high risk for dyslexia by analyzing these distinctive
oculomotor patterns collected during reading and can be robust
under noise.
This work develops DysLexML, a screening tool for
dyslexia, that employs various ML classifiers, such as SVM,
Na¨
ıve Bayes, and comparatively evaluates their performance
using data obtained from two systematic field studies. The
first study (RADAR [4]) involved 69 native Greek speak-
ers, children, 32 of which were diagnosed as dyslexic. The
second field study involved 152 participants, 72 of which
were diagnosed as dyslexic. The diagnosis was performed by
the official governmental agency for diagnosing learning and
reading difficulties in Greece.
To examine the robustness of DysLexML, we assessed its
accuracy under various fixation position noise levels, intro-
duced by the eye-tracking technology or the small screen
size (e.g., the small size of the text when a mobile device
is used). DysLexML can achieve high accuracy and is robust
in the presence of noise. It performs dimensionality reduction,
achieving the aforementioned performance using only a small
number of features, namely the mean and median saccade
length, the number of short forward movements, and the
number of multiply fixated words for the first dataset, and
the number of fixations, the median fixation duration, the
median length of medium forward movements, the number
of short forward movements, the number of multiply fixated
775
2019 IEEE 19th International Conference on Bioinformatics and Bioengineering (BIBE)
2471-7819/19/$31.00 ©2019 IEEE
DOI 10.1109/BIBE.2019.00145
words, and age for the second dataset. Finally, to address
the lack of large-size datasets and privacy requirements, we
developed a methodology based on generative models for
generating realistic synthetic datasets. Such synthetic data can
be used either as additional samples to assist the classifiers to
perform better or to completely replace the original data when
sensitive information cannot be shared with others (privately
or publicly).
The innovative contributions of this work include (i) the
analysis of the robustness in the presence of noise, (ii) the
identification of features with large discriminating power, (iii)
the high accuracy using only a small set of features, (iv) the
comparative evaluation with other screening tools/algorithms,
and (v) a data augmentation/substitution technique based on
generative adversarial networks (GANs), which is, to the best
of our knowledge, one of the first published approaches using
small datasets that achieves training convergence. This is the
first attempt of data augmentation in the context of dyslexia.
The paper overviews briefly the small field study in Section
II. Section III presents the DysLexML and Section IV evalu-
ates its performance for both datasets. The system’s sensitivity
to noise is examined in Section V. Section VI describes the
procedure of creating new synthetic datasets with the use of
generative models. Section VII gives insights in other work
based on dyslexia analysis and prediction, while Section VIII
summarizes our key findings and future work plans.
II. FIRST FIELD STUDY (SMALL POPULATION)
The first field study was performed in Greece and included
69 children, 32 of which were diagnosed as dyslexic by
the official governmental agency for diagnosing learning and
reading difficulties in Greece. Participants age span is between
8.5 and 12.5 years old [4]. The children were instructed to read
two passages, at their own pace. Both texts were written by a
special education teacher in Greek. The first passage (baseline
text) consists of 181 words, many of which multi-syllable. A
second passage, simpler than the first one, targeting to younger
participants, was also given to the subjects. It included 143
words, mostly of one or two syllables. It was also emphasized
that the purpose was to understand the text in order to answer
five comprehension questions at the end. The experimental
procedure consisted of recording the eye movements of the
participants, while they were silently reading the texts in front
of a computer monitor.
A custom-made eye-tracker, developed by Medotics AG was
employed [4]. It consists of two steady cameras that can record
images up to 60Hz with a resolution of 1600×1200 pixels.
Cameras are positioned between the screen and the participant
with a viewing field from down towards the participant’s face.
While the participant performs a reading task, the cameras
record the participant’s face. The images extracted are then
used to detect pupil and corneal reflection coordinates. Based
on the collected raw gazing measurements, the fixations were
identified according to a dispersion algorithm [11]. More
information about the field study, the inclusion and exclusion
criteria, texts, and data collection, can be found in [4]. The
dataset includes for each fixation, its x- and y-axis coordinates,
its starting and termination time, as well as the Region of
Interest (ROI) (i.e., word) the subject is looking at.
Fig. 1. Reading ”path” from a typical reader (top) and from a reader with
dyslexia (bottom). The blue circles are the fixations and the orange lines the
saccadic movements. The larger the circle, the longer the fixation (Figure
appeared in [4]).
III. DYSLEXML SYSTEM
The three main modules of the DysLexML algorithm are
(i) the feature extraction, (ii) the identification of dominant
features (feature selection), and (iii) its classifiers, that employ
these dominant features. DysLexML extracts general (non-
word-specific) features and word-specific ones that take into
account the word the subject is looking at. Examples of non-
word specific features are the number of fixations on the
screen, mean and median duration of fixations, and features
related to saccades, such as the mean and median length
of saccades, i.e., the Euclidean distance between consecutive
fixations, and characterization of the types of eye movements.
DysLexML creates a feature vector of 35 features in total.
People with reading difficulties tend to perform back and
forth movements (saccades) on the text line as they proceed as
a result of difficulty to focus or understand [6]. Thus, the iden-
tification of such movements and definition of features based
on them can provide valuable information about the dyslexic
population. Typical readers tend to perform medium to large
movements (saccades) in terms of length, while readers with
reading difficulties “generate” many choppy movements [10].
A movement is labeled as short, if the Euclidean distance
between its consecutive fixations is less than 100 pixels (about
5 letters in the text). Most of the movements occur within
words. Given that the line of text was about 900 pixels long,
the threshold for medium to long movements was set to be
400 pixels (about half a line). With this threshold a medium
backward movement includes re-reads of small groups of
words but not of entire phrases. That is, short movements are
776
of less than 100 pixels, long ones are of more than 400 pixels,
and medium movements in the range between 100 and 400
pixels. Change-of-line movements have been excluded from
both forward and backward movement sets. We also derive
information about the number of visits of each word, namely
the number of words that were not visited at all (skipped) and
number of words that were visited more than once during the
text reading.
To identify the features with the most predictive power, we
employed the least absolute shrinkage and selection operator
(LASSO) [12], [13], a particular case of penalized least
squares regression with L1-penalty function. LASSO finds the
minimum of the residual sum of squares, subject to the sum of
the absolute value of the coefficients being less than a constant.
The LASSO estimate can be defined by:
βLASSO =arg min
β{1
2N
N
i=1(yiβ0p
j=1
xi,j βj)2+λ
p
j=1βj∣} (1)
In practice, as λgets higher, less features are taken into
account. Specifically, the parameter λin LASSO regression
is estimated using 5-fold cross validation. Two values of λ
were examined, namely the λminMS E that corresponds to the
minimum mean cross-validation error MSE (vertical dotted
line in Fig. 2) and λ1SE which is one standard error of the
mean higher than λminMS E (vertical solid line). The purpose
of the addition of the 1SE is to reduce the number of regression
coefficients, while the mean square error remains close enough
(1SE) to λminMS E . Both variations were considered in our
analysis.
Fig. 2. Cross-Validated MSE of LASSO fit for the baseline text.
DysLexML builds classifiers based on SVM, Na¨
ıve Bayes,
and K-means. The SVM with a linear kernel performs better
than the ones with Gaussian or Polynomial, so only the
performance of the linear kernel is reported here. The K-
Means-based classifier was built as follows: the subjects of the
training set are clustered using k-means, and a label is assigned
to each cluster based on the most frequent label within that
cluster. The distance of the test subject from the centroid of
each cluster was estimated using the Euclidean distance. The
classifier reports the label of the cluster whose centroid has
the shortest distance from the test data.
IV. PERFORMANCE ANALYSIS
Using the first dataset. DysLexML first employs the LASSO
regression five-fold cross-validation to identify the dominant
features. Based on the dominant features, it then applies
various classification algorithms. To evaluate its performance,
we used the Leave One Out Cross Validation (LOOCV), an
appropriate choice given the relatively small size of the dataset.
Given that there were subjects with missing values in the
word specific features, we filled in the missing values with
the median of the corresponding feature values of the training
set.
TABLE I
CLASSIFICATION PERFORMANCE OF THE FIRST DATASET,INCLUDING ALL
SUBJECTS,TREATING THE MISSING VALUES.THE FIRST COLUMN
CORRESPONDS TO THE BASELINE TEXT,WHILE THE SECOND COLUMN TO
THE EASIER TEXT.
Classifier LOOCV accuracy
K-means (k=2) , LASSO (λminMS E ) 86.95 89.39
K-means (k=3) , LASSO (λminMS E ) 91.30 84.84
K-means (k=4) , LASSO (λminMS E ) 81.15 84.84
K-means (k=2) , LASSO (λ1SE) 89.85 78.78
K-means (k=3) , LASSO (λ1SE) 86.95 84.84
K-means (k=4) , LASSO (λ1SE) 89.85 83.33
Linear SVM, LASSO (λminMS E ) 94.20 80.30
Linear SVM, LASSO (λ1SE) 97.10 87.87
Linear SVM, without feature selection 85.50 81.81
Na¨
ıve Bayes, LASSO (λminMS E ) 91.30 86.36
Na¨
ıve Bayes, LASSO (λ1SE) 92.75 84.84
DysLexML can accurately perform classification.
DysLexML, with SVM and LASSO (λ1SE), exhibits an
accuracy of 97.10 % for the baseline text (Table I), while for
the easy text, with K-means (of k equal to 2) and LASSO
(λminMS E ), it reports 89.39% correct classification.
The exclusion of the word-specific features from the feature
vector results to a lower average accuracy for the baseline
(difficult) text. The performance remains the same in the case
of the easier text, indicating that the word-specific features are
not useful when the text is not challenging for the reader.
The dominant features (as selected by the LASSO) for both
texts are the mean saccade length and number of short forward
movements. In the case of the baseline (difficult) text, the
additional dominant features are the median saccade length,
and the number of multiply fixated words. Prior research has
also reported the important role of these dominant features
identified by LASSO (as discussed in Section I). The distri-
butions of the mean and the median saccade length for both
populations are significantly different (as shown in Fig. 3),
which explains their presence as separate dominant features.
Diversity in dyslexic population. Not all cases of dyslexia are
equally severe. Note that in the experiment, the subjects were
instructed to not rush their reading and understand the text
in order to answer some comprehension questions at the end.
This may have prolonged the reading sessions even for typical
777
Fig. 3. Empirical CDF of saccade length features for the first dataset.
readers. The number of short forward movements was ex-
pected to play a prominent role. Dyslexics have been reported
to perform more and shorter saccades during reading, in their
attempt to decode the text [7]. The number of short forward
movements of the dyslexic population has large variance, as
shown in the upper part of Fig. 4. 50% of the dyslexic subjects
have more than twice total short forward movements than the
control population.
Fig. 4. Empirical CDF of number of short forward movements (top) and
number of multiply fixated words, i.e. the words that have been fixated more
than once during the reading session (bottom) for the first dataset.
Dyslexics tend to revisit words more often, especially those
that are long or difficult to read [14]. 90% of the typical readers
have less than 100 words fixated more than once, while this is
the starting value for the dyslexic subjects (Fig. 4 (bottom)).
This illustrates the value of the word specific analysis of the
eye-tracking study.
Second field study with commercial eye-tracker and larger
population. In the second systematic field study, Tobii 4C,
a commercial inexpensive eye-tracker was employed. Unlike
the eye-tracker in the first study that operated at 60Hz, Tobii
4C has sampling frequency of 90Hz. The use of chin rest is
not necessary anymore, hence the participants can freely move
their head making the procedure completely non-invasive. The
field study included 152 participants in total, 72 of which were
diagnosed as dyslexics. All the participants were native Greek
speakers and the age span this time was larger and varied
from 7 to 16 years old. The participants were reading silently
the difficult text that was also used in the previous study. For
the evaluation of DysLexML with the dataset obtained from
the second field study we applied the same methodology as
before.
Validation with the model built from the first study. We first
employed the SVM model built on the first dataset to assess
DysLexML with the second dataset, collected from the larger
scale field study. The model’s accuracy of 75.55% indicates
that the datasets differ. The differences in the two eye trackers,
the higher age variance of the second dataset compared to
the first one, and the absence of chin rest resulting to a
less controlled environment may cause substantial differences.
Thus we repeated the analysis for identifying the dominant
features for the second dataset and assess the impact of the
age on the classification.
Age is a dominant feature. Age was expected to be among
the dominant features, as the reading skills vary with age.
Different eye movement patterns are expected from elementary
school readers compared to high school children. When age
is used as an extra feature in the analysis, the accuracy is
improved in most cases (Table II). Age is selected as a
dominant feature at every iteration of the cross-validation
procedure contributing to an increase of about 12% in clas-
sification accuracy (Table II 72.36% vs 84.21%), achieving
the best score again with the use of linear SVM model. Age
was also reported as an important feature in [15], where the
participants’ age range was wider.
The number of multiply fixated words and the number
of short forward movements remain dominant features.
The number of multiply fixated words and number of short
forward movements remain in the list of important features, as
in the first study (Fig. 5). Moreover, the number of fixations,
the median fixation duration, the median length of medium
forward eye movements, and the age of the participant are
identified as important for the classification.
Increased number of fixations and fixation duration in
dyslexics. The reading skills can also affect the number of
fixations and the fixation duration. The number of fixations
has also been identified as a dominant feature in other studies
[4]; Dyslexics make more choppy eye movements, resulting to
a larger number of fixations than regular readers (Fig. 6 (top)).
Moreover their fixations last more compared to the fixations
of the control population (Fig. 6 (middle)), as also appears in
Fig. 1.
V. ROBUSTNESS TO NOISE
To examine the robustness of DysLexML, we evaluated
its performance in the presence of noise in the form of
small displacements of the fixation points. The noise follows
778
Fig. 5. Empirical CDF of number of short forward movements (top) and
number of multiply fixated words (bottom) for the second dataset.
Fig. 6. Empirical CDF of the number of fixations (top), the median fixation
duration (middle) and median length of medium movements (bottom) for the
second dataset.
a Gaussian distribution with mean value equal to zero and
standard deviation varying from 10 to 100 pixels (with a
step size of 10). For small displacement, only the saccadic
movement features changed. However, large displacements
result to changes in the word-specific features as well. For
each subject, the noise was added and the new feature vectors
were generated. Note that the “shifted” eye-movements result
to different feature vectors. The DysLexML was then evaluated
for this new dataset. Specifically, for each σ, we generated 10
synthetic datasets. We then trained a linear SVM model using
TABLE II
CLASSIFICATION PERFORMANCE OF THE SECOND DATASET.THE FIRST
COLUMN CORRESPONDS TO FEATURE SET WITHOUT THE USE OF AGE,
WHILE THE SECOND COLUMN TO A FEATURE SET THAT CONTAINS AGE IN
IT.
Classifier LOOCV accuracy
without age with age
K-means (k=2) , LASSO (λminMS E ) 71.71 69.07
K-means (k=3) , LASSO (λminMS E ) 63.15 69.07
K-means (k=4) , LASSO (λminMS E ) 65.13 70.39
K-means (k=2) , LASSO (λ1SE) 72.36 68.42
K-means (k=3) , LASSO (λ1SE) 55.26 70.39
K-means (k=4) , LASSO (λ1SE) 71.71 72.36
Linear SVM, LASSO (λminMS E ) 69.07 82.89
Linear SVM, LASSO (λ1SE) 67.76 84.21
Na¨
ıve Bayes, LASSO (λminMS E ) 69.73 71.05
Na¨
ıve Bayes, LASSO (λ1SE) 72.36 71.05
the dominant features that were reported by LASSO using the
original datasets. For testing, we employed the 10 synthetic
datasets with noise for each given σ. Only the children that did
not have missing values from the first dataset were considered
in this analysis. Fig. 7 presents the acquired results for both
original datasets.
The model based on the first small field study exhibits a
robust performance for relatively small noise levels, up to σof
30 pixels, which corresponds to a displacement of something
more than a character on the x-axis and 1/3 of the line on
the y-axis. However, as the noise level increases, the accuracy
drops significantly. Similar trend persists for the model built
using the data of the second field study (larger dataset).
Specifically it exhibits a robust performance for relatively
small noise levels (up to σof 20 pixels), which corresponds to
displacement of something less than a character on the x-axis
and 1/4 of the line on the y-axis. However, as in the case of
the small dataset, as the noise level increases, the accuracy
drops significantly. Through SVM, that behaves well under
generalization, DysLexML addresses the noise in the fixation
coordinates in a robust manner.
VI. SYNTHETIC DATA GENERATION USING GANS
User field studies, like the presented ones, are often time-
consuming and cost-demanding. The small sample size of
datasets makes the use of sophisticated and complex clas-
sification models prohibited due to overfitting issues. More-
over, collected measurements may contain sensitive personal
information. To address these issues, we developed a data
augmentation technique based on generative model for gen-
erating synthetic data that follow the same distributional
characteristics as the original datasets. The aim is to create a
new surrogate dataset, that can be used instead of the original
one, as it follows the same distribution.
In recent years, novel generative modeling approaches
based on neural networks have produced impressive results
in generating realistic samples from complex and unknown
distributions. Two popular families of generative models are
Generative Adversarial Nets (GANs) [16] and Variational
AutoEncoders (VAEs) [17]. Here, we focus on GANs which
779
Fig. 7. Performance of DysLexML under noisy fixation positions. Training
SVM linear model with original data of first field study (top) and larger scale
study (bottom). The testing was performed on noisy data (10 datasets for
each σvalue, σin [10, 100] with stepsize 10). The solid red line indicates
the mean accuracy over the 10 noisy synthetic datasets, while the gray area
represents the range between the lowest and the maximum accuracy achieved
at each noise level.
consist of two powerful neural networks: the Generator and the
Discriminator. The Generator shapes random noise to look like
real data while the Discriminator decides if a given sample
is real or fake. The pair of neural nets compete with each
other forming a two-player zero-sum game. Both nets are
simultaneously trained to achieve the optimum equilibrium
of the game. At equilibrium, the Generator produces samples
from the real distribution, while the Discriminator cannot
distinguish between the original and the fake samples. An im-
portant extension of GAN is the conditional GAN [18], where
the input noise vector is concatenated with a condition vector
in order to produce samples from the conditional distributions.
Thus, a practitioner is able to handle the sampling from a
family of distributions based on the condition vector. We
utilize conditional GANs, where we condition on the disease
state to generate new samples from both classes: dyslexic cases
and controls.
Original
Data
noise
real
fake
generated
sample
class
Generator
Discriminator
class
real
sample
noise
sample
Fig. 8. Conditional GAN architecture.
The main challenge in training (conditional) GANs is the
limited number of real data (152 samples in total). To alle-
viate this, we choose to train, evaluate, and validate GANs’
performance on a reduced dimensional dataset. The reduced
dataset consists of the five most significant features as selected
by LASSO (λ1SE approach). We trained the GAN on this
new dataset and performed a moderate hyper-parameter tuning
for GAN parameters. We ended up with the following setting
(Fig. 8): Both Discriminator and Generator consist of a two-
layer neural network with 32 hidden units each. The input
noise is uniform with dimension 10, while the condition vector
is not only concatenated in the input layer but also in the
hidden layers. ReLU is employed as an activation function
for the hidden layers. The output layer of the Discriminator
has sigmoid activation, while no nonlinearity is applied to the
output layer of the Generator. The training is performed using
stochastic gradient descent with minibatch size of 128. We
utilize Adam as an optimizer with learning rate 0.00001.
GAN-generated data are almost indistinguishable from the
real data. The comparison between the real and synthetic
distributions was performed using several metrics (Fig. 9).
Specifically, Fig. 9 (top) shows the ECDF of the number of
fixations for both control class (dashed lines) and dyslexic
class (solid lines). The ECDFs of GAN-generated data (red
lines) are almost indistinguishable from the ECDFs of the real
data (blue lines). Similar results are obtained for the other four
dominant features. Apart from the marginal distributions, we
compare the overall similarity between the generated and real
distributions using the Maximum Mean Discrepancy (MMD)
distance [19]. MMD takes into account not only the marginals
but also the correlations and the shape of the statistical
distributions. Fig. 9 (bottom) shows the average MMD for
both classes. The gradual decrease of MMD implies that the
GAN indeed learns the real distributions. After 15K iterations
and until the end of training, MMD fluctuates around the very
small value 0.01.
Fig. 9. Real and synthetic empirical CDFs for the number of fixations for both
classes (top). Average MMD after 20 repetitions for both classes (bottom).
780
Slightly increased accuracy when the Na¨
ıve Bayes classifier
is trained on the augmented dataset. We assess the added
value of the synthetic data in classification tasks. Table III
presents the average LOOCV accuracy along with its standard
deviation from the classifiers trained on the augmented dataset
(original and synthetic data). Results are slightly better for
Na¨
ıve Bayes classifier yet not significantly different in sta-
tistical sense, while they slightly deteriorate for linear SVM.
Table IV presents the average accuracy along with its standard
deviation when classifiers are trained on the generated data
and then evaluated on the original data. Results reveal that the
substitution of the original data with synthetic data is feasible
without compromising classification performance.
TABLE III
RESULTS FROM THE CLASSIFIERS TRAINED ON THE AUGMENTED DATASET
(GAN-GENERATED AND ORIGINAL DATA)AND TESTED ON THE ORIGINAL
DATA.
Classifier LOOCV accuracy
Linear SVM, LASSO (λ1SE)61.4±3.5
Na¨
ıve Bayes, LASSO (λ1SE)72.7±0.9
TABLE IV
RESULTS FROM THE CLASSIFIERS TRAINED ON THE GAN-GENERATED
DATA AND TESTED ON THE ORIGINAL DATA.
Classifier Cross accuracy
Linear SVM, LASSO (λ1SE)67.2±5.1
Na¨
ıve Bayes, LASSO (λ1SE)73 ±0.8
VII. RELATED WORK
Although dyslexia has been extensively studied the last three
decades with specialized eye-trackers, there is only a limited
number of eye-tracking-based screening systems, partially due
to the high cost of eye-trackers up to recently and the debate
of the primary cause of dyslexia [8]. The lack of extensive
datasets limits significantly the performance of deep-learning
architectures. On the other hand, SVM is powerful in case
of relatively small datasets. Recent studies have applied ML,
and more specifically SVM, for classification of dyslexia on
data collected from eye-trackers [15], [20]. For example, Rello
and Ballesteros [15] performed a field study that included
97 Spanish language native speakers, aged 11-54 reading 12
different texts. They used a binary polynomial SVM classifier
and achieved classification accuracy of 80.18%. Their feature
set included the age of the participant, the text number,
details about text stylistics, number of visits of an ROI, mean
time spent on an ROI, total reading time, mean of fixation
duration, number of fixations and sum of all fixations duration.
They reported that the reading time, the mean of fixation
duration, and the age of the participant have predictive power.
Benfatto et al. [20] also employed linear SVMs with sequential
optimal optimization for dyslexia screening. Their field study
in Sweden included 185 children, 97 of them with high risk of
dyslexia, speaking Swedish as a first language. All the subjects
were reading from paper a short text adapted to their age
while their eye movements were recorded. The subjects were
equipped with head-mounted goggles with arrays of infrared
transmitters and detectors, arranged around each eye. A chin
and forehead rest was deployed to minimize head movements
and stabilize the viewing distance. Their feature set, produced
using a dynamic dispersion threshold algorithm, consists of
168 features. They also distinguished saccades to progressive
and regressive ones. A recursive feature elimination algorithm
identified the dominant features. They achieved accuracy of
95.6%±4.5% using 48 features of the original feature space.
Al-Edaily et al. [21] developed Dyslexia Explorer in the
Arabic language and performed a study with 14 subjects, 7 of
whom with diagnosed dyslexia. Their system is designed to
help specialists analyze visual patterns of reading and provide
insights into understanding differences between readers with
and without dyslexia. Their measurements included fixation
duration in each/all ROI, mean fixation duration in each/all
ROI, total fixation count for each/ all ROI and backward
saccades.
Unlike the above ML-based approaches, Smyrnakis et al.
[4] developed statistical Bayesian classifiers, using various
thresholds and taking into consideration binary correlations.
They focused on small age span, critical for dyslexia diagnosis.
The size and font of the two texts used was standardized so as
to achieve maximal classification accuracy, unlike in [15]. The
parameters used for classification involved not only direct eye-
tracking parameters, but also relations between eye-tracking
parameters and word properties in the texts read. These pa-
rameters extend the parameter set used in [20]. Including this
set of parameters, it is possible to evaluate word anticipation,
which is often problematic in dyslexics [22]. DysLexML has
been evaluated by employing the same dataset as in [4], in
addition, to the second dataset, collected from a larger field
study using a commercial eye-tracker. DysLexML outperforms
RADAR in terms of classification accuracy: 97.10% vs. 94.2%
for the baseline text. For the easy text, RADAR reports 87.9%
accuracy, while DysLexML, with K-means with k equal to 2,
exhibits an accuracy of 89.39%. As mentioned, the classifier
of DysLexML with the best accuracy on noise-free data is the
linear SVM classifier on features automatically selected by the
LASSO regression at λ1SE. Furthermore, it exhibits a robust
performance under fixation position noise (added artificially).
Its robustness and dimensionality reduction are two innovative
aspects of this work. To the best of our knowledge, we are the
first to examine the impact of noise on the fixation positions
on the accuracy of the classification. Unlike other screening
systems for dyslexia, DysLexML automatically identifies the
features with the largest discriminating power. It achieves high
accuracy using only a small set of features (4 and 6, for the 1st
and 2nd dataset, respectively). Rello and Ballesteros [15] used
only 3 features for their classification by manually selecting
the features.
Finally, modern data augmentation techniques which rely
on deep neural networks and adversarial learning have been
successfully applied in image classification [23], [24] and
speech recognition [25] tasks. Nevertheless, the application
of such techniques in health monitoring systems where the
781
sample size is limited henceforth the convergence of the
training not guaranteed has not been explored. To the best
of our knowledge, we are the first to perform dyslexia data
generation using GANs. It is also among the first published
attempts of training GANs using small datasets.
VIII. CONCLUSIONS
DysLexML’s feature selection, via LASSO with λof one
standard error enabled dimensionality reduction, without com-
promising the accuracy. In the first field study, the mean
and median saccade length, the number of short forward
movements, and the number of multiply fixated words are
the four features with the most prominent predictive power
for the baseline text, while for the easier text only the mean
saccade length and the number of short forward movements
were selected. In the second larger-scale study, the number of
fixations, median fixation duration, number of short forward
movements, median length of medium forward movements,
number of multiply fixated words, and age are the six domi-
nant features.
From the analysis of the small field scale dataset, the text
difficulty does play an important role in the diagnosis: Easy,
less challenging, text reduces the power of the word-specific
features, as they disappear from the dominant feature set. The
text choice has to be relevant to the subjects age and acquired
reading skills. The selected features are easily interpreted and
capture the prior knowledge about eye movements of dyslexic
children. To the best of our knowledge, DysLexML uses the
smallest feature set compared to the other related studies.
Given our vision for screening systems that operate in
less-controlled, larger-scale environments (e.g., potentially in
kindergartens or homes) with commercial eye-trackers, reach-
ing a larger population, the robustness of the system under
noise is critical. As a first step towards this objective, we
added synthetic noise at the fixation positions and assessed
its impact on the accuracy. For noise levels smaller than σ
equal to 20 pixels, the performance of the system remains
robust. Encouraged by the robustness under noise, the team
has been performing follow-up larger-scale field study using
inexpensive non-specialized eye-trackers in more diverse set-
tings (e.g., different countries and under silent and out-loud
reading). One of the future work plans is the identification of
different classes of reading difficulties. Finally, our proposed
synthetic data generation method based on GANs is able to
create synthetic samples of high relevance with the original
dataset. We plan to use such synthetic datasets to train more
sophisticated ML and deep learning methods to improve the
classification accuracy. This work sets the basis for developing
a screening tool that can reach a larger and more diverse
population, in less controlled environments, aiming for early
intervention and potentially larger social impact.
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... Clinical screening and assessment procedures can identify a language or speech delay or disorder. Screening, "the process of identifying healthy people who may have an increased chance of a disease or condition" [5], enables early detection and provides explanations in functioning and/or behavior, allowing the individual to be involved in care/intervention planning when rather little damage has been done and thus (i) improve quality of life and (ii) reduce the chance of developing a serious condition or its complications [5, 9,10]. Assessment, "the ongoing procedures used by qualified personnel to identify the child's unique strengths and needs and …the early intervention services...and includes the assessment of the child...and…the child's family..." [11], must include information such as parents interview/questionnaires, clinician's observations, and standardized age-normed tests or criterion-based assessments, speech and language sample as well as expressive and receptive language [4]. ...
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