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基于深度学习的对早期帕金森病的短时间序列进行分类 Classification of Short Time Series in Early Parkinson’s Disease With Deep Learning of Fuzzy Recurrence Plots
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Classification of Short Time Series in Early
Parkinson’s Disease With Deep Learning
of Fuzzy Recurrence Plots
Tuan D. Pham, Senior Member, IEEE, Karin Wårdell, Member, IEEE, Anders Eklund, and Göran Salerud
Abstract — There are many techniques using sensors and wear-
able devices for detecting and monitoring patients with Parkin-
son's disease (PD). A recent development is the utilization of hu-
man interaction with computer keyboards for analyzing and
identifying motor signs in the early stages of the disease. Current
designs for classification of time series of computer-key hold dur-
ations recorded from healthy control and PD subjects require the
time series of length to be considerably long. With an attempt to
avoid discomfort to participants in performing long physical tasks
for data recording, this paper introduces the use of fuzzy recur-
rence plots of very short time series as input data for the machine
training and classification with long short-term memory (LSTM)
neural networks. Being an original approach that is able to both
significantly increase the feature dimensions and provides the
property of deterministic dynamical systems of very short time
series for information processing carried out by an LSTM layer
architecture, fuzzy recurrence plots provide promising results and
outperform the direct input of the time series for the classifica-
tion of healthy control and early PD subjects.
IndexTerms—Deep learning, early Parkinson’s disease (PD), fuzzy
recurrence plots, long short-term memory (LSTM) neural networks,
pattern classification, short time series.
I. Introduction
P
ARKINSON’S disease (PD) is a neurodegenerative dis-
order that affects dopaminergic neurons [1]. Statistics on
PD have reported it affects approximately 10 million people
worldwide, and about 4% of them before the age of 50 [2].
Symptoms of PD slowly develop over years, and the progres-
sion of PD can be different among individuals because of the
diversity of the disease. People with PD can be observed with
tremor, bradykinesia (slowness of movement), limb rigidity,
and gait and balance problems. The cause of PD remains un-
known [3]. Because a significant amount of the substantia
nigra neurons have already been lost or impaired before the
onset of motor features, people with PD may first start experi-
encing symptoms later in the course of the disease [4], [5].
Treatment options for PD can vary and include medications
and sometimes deep brain stimulation [6], [7]. While Parkin-
son’s itself is not fatal, disease complications can be serious [4].
Many scientific efforts have been spent on exploring
methods for identifying biomarkers for PD [8] with the hope
that these markers can lead to earlier diagnosis and targeted
treatments of the disease. However, present therapies used for
PD cannot slow or stop the disease in a prodromal stage [9].
There are many techniques using sensors for detecting and
monitoring movement patterns on patients with PD. Most
techniques using sensor-induced data focus on studying gait
dynamics and temporal gait parameters[10]–[14], and others
use wrist accelerometers for intraoperative measurements of
tremor during surgery [15]. One issue of using sensor data is
that gait measurements are to be obtained during relatively
long walking periods, causing discomfort to the participants or
impracticability of performance in clinical settings. Therefore,
research into the minimum strides required for a reliable
estimation of temporal gait parameters has recently been
carried out with the purpose of avoiding or minimizing
discomfort to participants in gait experiments [16].
Apart from gait and balance data, the measurement of
computer keystroke time series that contain information of the
hold time occurring between pressing and releasing a key
collected during the sessions of typing activity using a
standard word processor on a personal computer has been
proposed for detecting early stages of PD [17]. Being similar
to the motivation for determining the minimum number of
strides for the analysis of gait dynamics, this study is
interested in answering the question if there are methods that
can process very short time series and achieve good results for
differentiating healthy controls from subjects with early PD. If
being successful, the use of computer keystroke time series
can be equivalent on a practical basis to the use of mobile
sensor data for evaluating upper limbs motor functions by
finger tapping [18] that is typically used in clinical trials. The
finger tapping test requires a participant to press one or two
Manuscript received September 23, 2019; accepted October 24, 2019. Re-
commended by Associate Editor Patrick Schäfer. (Corresponding author: Tu-
an D. Pham.)
Citation: T. D. Pham, K. Wårdell, A. Eklund, and G. Salerud, “Classifica-
tion of short time series in early Parkinson’s disease with deep learning of
fuzzy recurrence plots,” IEEE/CAA J. Autom. Sinica, vol. 6, no. 6, pp.
1306–1317, Nov. 2019.
Tuan D. Pham is with the Department of Biomedical Engineering, the Cen-
ter for Medical Image Science and Visualization, Linköping University,
Linköping, Sweden; the Center for Artificial Intelligence, Prince Mohammad
Bin Fahd University, Al Khobar, Kingdom of Saudi Arabia (e-mail:
tuan.pham@liu.se).
Karin Wårdell is with the Department of Biomedical Engineering,
Linköping University, Linköping, Sweden (e-mail: karin.wardell@liu.se).
Anders Eklund is with the Department of Biomedical Engineering, the De-
partment of Computer and Information Science, the Center for Medical Im-
age Science and Visualization, Linköping University, Linköping, Sweden (e-
mail: anders.eklund@liu.se).
Göran Salerud is with the Department of Biomedical Engineering,
Linköping University, Linköping, Sweden (e-mail: goran.salerud@liu.se).
Color versions of one or more of the figures in this paper are available on-
line at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JAS.2019.1911774
1306
IEEE/CAA JOURNAL OF AUTOMATICA SINICA, VOL. 6, NO. 6, NOVEMBER 2019
buttons on a device such as an iPhone as fast as possible for a
short period of time.
The method of fuzzy recurrence plots [19] developed for
studying nonlinear dynamics in time series data can be useful
for creating feature dimensions of short time series. Therefore,
the deep learning of fuzzy recurrence plots is proposed in this
study for the classification of short time series of computer-
key hold time recorded from two cohorts of healthy control
and early PD.
The rest of this paper is organized as follows. Section II
includes a survey of literature relating to the present study.
Section III outlines the concept and mathematical formulation
of fuzzy recurrence plots. Section IV presents the
implementation of LSTM neural networks with fuzzy
recurrence plots. The public database for testing the proposed
classification approach is described in Section V. Section VI
shows the experimental results together with discussion.
Finally, Section VII consists of the concluding remarks of the
research finding and suggestion of issues for future work.
II. Related Work
Having highlighted earlier, the concept of using fuzzy
recurrence plots of short time series for classification using
LSTM networks is original in that it contributes to the
increase of the feature dimensions of short raw time series,
which, in turn, can improve the classification. No prior work
of similar concepts exists in literature. A survey of recent
reports that applied LSTM and convolutional neural networks
for time-series or sequential data classification is addressed
herein.
The deep-learning method of LSTM neural networks has
been adopted for the classification of time-series or sequential
data [20], including speech recognition [21] and machine
translation [22], [23]. A deep recurrent neural network called
TimeNet for extracting features from time series was
developed [24]. The TimeNet was designed to generalize time
series representation across domains. A trained TimeNet can
be used as a feature extractor for time series and was reported
to be useful for time series classification by performing better
than a domain-specific recurrent neural network and dynamic
time warping [24]. Stacked LSTM autoencoder networks were
applied to extract features of time-series data, which were then
used to train deep feed-forward neural networks for
classification of multivariate time series recorded with sensors
in the steel industry to detect steel surface defects [25]. In this
work, the features extracted with LSTM autoencoders were
found to be useful, and therefore the need for domain expert
knowledge can be alleviated.
Other time-series classification using convolutional neural
networks (CNNs) have recently been introduced. A
convolutional LSTM (ConvLSTM) was introduced for a
spatiotemporal sequence forecasting problem in which both
the input and the prediction target are spatiotemporal
sequences [26]. This ConvLSTM model was constructed by
extending the fully connected LSTM to have convolutional
structures in the input-to-state and state-to-state transitions.
The ConvLSTM network was reported to perform better than
the fully connected LSTM by being able to capture the
spatiotemporal correlations of the sequential data for
precipitation nowcasting. A multi-scale convolutional neural
network [27], which extracts deep-learning features at
different scales and frequencies from three representations of
time series, including the original, down-sampled, and
smoothed data, was reported be capable of extracting effective
features for time series classification. Baseline full
convolutional networks were proposed for time series
classification [28]. The proposed baseline models were
reported to be pure end-to-end without demanding heavy pre-
processing of the raw data or feature crafting, and achieve
competitive performance to other state-of-the-art approaches,
including the multilayer perceptron, fully convolutional
network, and residual network. This network consists of a
branching structure. The first branch is the convolutional part,
whereas the second branch is an LSTM block which receives
a time series in a transposed form as multivariate time series
with a single time step. The outputs of the two branches are
concatenated and then fed to a classifier. These models were
reported to be able to enhance the performance of fully
convolutional networks with a nominal increase in model size
as well as require minimal data pre-processing.
In general, time-series classification has been recognized as
an important and challenging area of research, particularly
with respect to the demand for handling increasing availability
of new data of time series. While numerous algorithms for
time-series classification have been published in literature and
the popularity of deep learning has been pervasive in many
disciplines, only a few deep neural networks have been
applied to solving time-series classification problems. A
recent survey on promising applications of deep neural
networks for time series classification in several areas can be
found in [29],
Having reviewed recent deep neural networks for time-
series classification, the purpose of this study is not to
compare the performance of time-series classification between
LSTM and CNN models. This study introduces the usefulness
of constructing fuzzy recurrence plots of short time series that
can be incorporated into LSTM models to improve the
classification accuracy.
III. Fuzzy Recurrence Plots
The development of constructing a fuzzy recurrence plot
(FRP) of a time series was inspired with the concept of a
recurrence plot (RP). An RP [30] is a visualization method for
studying patterns of chaos in time series. An RP shows the
times at which a phase-space trajectory approximately revisits
the same area in the phase space.
m
τ
X = {x}
x
i
i
m
τ
N × N
(i, k)
i = 1,. . . , N
k = 1, . .. , N
x
i
x
k
ϵ
Based on the Takens’ embedding theorem [31] in the study
of dynamical systems, a phase-space reconstruction of a time
series can be obtained using an embedding dimension and
time delay . Let be a set of phase-space vectors, in
which is the th state of a dynamical system in -
dimensional space and time delay . An RP is constructed as
an matrix in which an element , ,
, is represented with a black dot if and are
considered to be closed to each other. For a symmetrical RP, a
threshold, denoted as , is used to define the similarity of a
PHAM et al.: CLASSIFICATION OF SHORT TIME SERIES IN EARLY PD WITH DEEP LEARNING OF FRPs 1307
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