CN113705324A - Multi-class motor imagery electroencephalogram signal feature extraction and classification method based on decision tree and CSP-SVM - Google Patents

Multi-class motor imagery electroencephalogram signal feature extraction and classification method based on decision tree and CSP-SVM Download PDF

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CN113705324A
CN113705324A CN202110726186.9A CN202110726186A CN113705324A CN 113705324 A CN113705324 A CN 113705324A CN 202110726186 A CN202110726186 A CN 202110726186A CN 113705324 A CN113705324 A CN 113705324A
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motor imagery
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罗元
何小义
任科
盖新晨
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Chongqing University of Post and Telecommunications
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Abstract

The invention requests to protect a multi-class motor imagery electroencephalogram signal feature extraction and classification method based on a decision tree and a CSP-SVM, and the method comprises the following steps: s1, preprocessing the N motor imagery electroencephalogram signals; s2, processing the preprocessed motor imagery electroencephalogram data by utilizing the one-to-many CSP, thereby constructing N spatial filters and acquiring electroencephalogram signal characteristics; s3, classifying the features by using an SVM; s4, selecting the combination of the optimal spatial filter and the SVM to construct decision tree branches according to the classification result, so that one class of motor imagery electroencephalogram signals can be distinguished, and the N classification problem is changed into an N-1 classification problem; and S5, repeatedly constructing branches of the decision tree by using the one-to-many CSP and the SVM until a decision tree capable of distinguishing all classes is constructed. The result shows that the method for extracting and classifying the characteristics of the multi-class motor imagery electroencephalogram signals has a better classification result by verifying the BCI compatibility III Data Sets IIIa in the public Data set and comparing the verification with related documents.

Description

Multi-class motor imagery electroencephalogram signal feature extraction and classification method based on decision tree and CSP-SVM
Technical Field
The invention belongs to the field of signal processing and pattern recognition, and particularly relates to a multi-class motor imagery electroencephalogram signal feature extraction and classification method based on a decision tree and a CSP-SVM.
Background
With the rapid development of fields such as brain science, artificial intelligence, computer technology and the like, brain-computer interface technology has brought about extensive research and attention. The brain-computer interface technology avoids the traditional neuromuscular channel, and directly realizes the communication control between the human brain and a computer or other external auxiliary equipment. In the medical field, the brain-computer interface can help a patient losing physical functions (a patient suffering from paralysis caused by diseases such as spinal cord lateral sclerosis, spinal cord injury, cerebral apoplexy, muscular atrophy and the like) to solve the problem of dyskinesia, so that the patient has certain ability of self-care life again. In addition, the brain-computer interface is widely applied to the fields of traffic driving, man-machine interaction, engineering and the like.
The brain-computer interface technology comprises 3 parts of signal acquisition, signal processing and external control system. Firstly, recording the brain activity of a user through the acquisition of electroencephalogram signals; then, the acquired electroencephalogram signals are converted into corresponding control instructions through signal processing and output to an external control system; and finally, the external control system completes related activities according to the received instruction. The signal processing is the most important component in the brain-computer interface technology, and comprises preprocessing, feature extraction and classification. The accuracy of signal processing directly affects the reliability and safety of the whole brain-computer interface. At present, methods for extracting electroencephalogram signal features include Common Spatial Pattern (CSP), deep learning, power spectrum estimation, and the like. The CSP algorithm obtains good effect in the feature extraction of two types of motor imagery electroencephalogram signals. In 2018, Zhang Wenqing et al combined with S transformation and a common space mode to perform feature extraction and classify by using SVM, and the average accuracy rate obtained on a Data set BCI composition IV Data sets 2b is 92.8%. In 2020, Meng et al optimized the co-spatial mode by selecting channels and bands, and the experimental results showed that the method gave an average accuracy of 90.25% on the public Data set BCI composition III Data set IVa. However, when the CSP is applied to feature extraction of multi-class motor imagery electroencephalogram signals, as the CSP needs to construct a plurality of spatial filters, the extracted features are complex in spatial distribution and low in discrimination, and further the identification accuracy is low.
Disclosure of Invention
In order to solve the problems, the invention provides a multi-class motor imagery electroencephalogram signal feature extraction and classification method based on a decision tree and a CSP-SVM. In the method, the multi-classification problem is converted into a plurality of two-classification problems by utilizing the decision tree idea, so that the advantages of the CSP and the SVM on the two-classification problems are fully exerted, and the motor imagery electroencephalogram signal can be more accurately identified.
In order to achieve the purpose, the invention adopts the technical scheme that: the multi-class motor imagery electroencephalogram signal feature extraction and classification method based on the decision tree and the CSP-SVM comprises the following steps:
s1, preprocessing the N motor imagery electroencephalogram signals;
s2, processing the preprocessed motor imagery electroencephalogram signal by using the one-to-many CSP, thereby constructing N spatial filters and acquiring the final characteristics of the electroencephalogram signal;
s3, classifying the characteristics in the step S2 by using SVM;
s4, selecting the combination of the optimal space filter and the SVM to construct the branch of the decision tree according to the classification result;
and S5, repeatedly constructing branches of the decision tree by using the one-to-many CSP and the SVM until a decision tree capable of distinguishing all classes is constructed.
The invention has the following advantages and beneficial effects:
aiming at the problem that when CSP extracts multi-class motor imagery features, the extracted features are complex in spatial distribution and low in discrimination so that the recognition accuracy is low, a multi-class motor imagery electroencephalogram feature extraction and classification method based on a decision tree and a CSP-SVM is provided, an optimal spatial filter constructed by one-to-many CSPs is selected by using classification results of the SVM, and the multi-classification problem is converted into a plurality of two-classification problems by using a decision tree idea so as to improve the accuracy of multi-class motor imagery electroencephalogram classification.
Tests on the public Data set BCI coordinates Data sets IIIa show that an optimal spatial filter is selected, better feature vectors can be obtained, and robustness of classification of a hyperplane constructed by the SVM can be improved by converting multi-classification into a plurality of two-classification. In order to more clearly compare the classification effects of different methods on multi-class motor imagery electroencephalogram signals, the algorithm model provided by the invention is compared with other CSP and SVM-based methods, and the algorithm provided by the invention has stronger robustness and higher accuracy.
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FIG. 1 is a branch creation process of multi-class motor imagery electroencephalogram signal decision trees based on decision trees and CSP-SVM;
fig. 2 is a classification decision tree for N classes of motor imagery electroencephalogram signals.
Detailed Description
The technical solutions in the embodiments of the present invention will be described in detail and clearly with reference to the accompanying drawings. The described embodiments are only some of the embodiments of the present invention.
As shown in fig. 1, the scheme of the present invention comprises the following steps:
and S1, preprocessing the N motor imagery electroencephalogram signals. EEGLAB band-pass filtering processing is adopted for the original motor imagery electroencephalogram signals, and a band-pass frequency band of 8-30 Hz is set, so that mu rhythms (8-13 Hz) and beta rhythms (13-30 Hz) related to motor imagery are reserved.
S2, the N spatial filters are constructed using "one-to-many" CSP for processing. One of the motion imagination types is regarded as one type, and the other N-1 motion imagination types are regarded as the other type, and CSP processing is carried out so as to construct a spatial filter. For N classes of motor imagery electroencephalogram signals, each class of motor imagery and other N-1 classes of motor imagery need to be regarded as another class, so N spatial filters are constructed. Mapping the original motor imagery electroencephalogram signal through a spatial filter to obtain a new signal ZpIn order to increase the variance of the characteristic values, a new signal Z is appliedp(p ═ 1,2, … 2m) variance logarithm and normalization processing as final feature fp: Zp(p=1,2,…2m)
Figure RE-GDA0003297541740000021
Where var () represents variance, and m represents m rows before and after the projection matrix when constructing the spatial filter.
And S3, classifying the features by using SVM. The features obtained in S2 are classified by SVM and parameters (radial basis kernel parameters and error penalty factors) influencing the support vector machine are selected by a grid search method and a cross validation method. The final decision function expression of the SVM becomes:
Figure RE-GDA0003297541740000031
where f (x) is a decision function, sign () denotes a sign function, λiIs the Lagrangian vector, yiFor training set sample labels, x is the independent variable of the decision function, i.e. the feature vector of the test set sample, xiIs the feature vector of the training set sample, b is the offset vector, K (x, x)i) As a function of the radial basis kernel, K (x, x)i) The expression of (a) is:
K(x,xi)=exp(-γ||x-xi||2) (3)
wherein γ is the function bandwidth of the radial basis kernel function and γ > 0.
S4, selecting the combination of the optimal space filter and SVM to construct decision tree branch, the 'one-to-many' CSP processes N classification problem to construct N space filters, each of which takes one class as one class and the rest N-1 classes as another class, so each space filter can only distinguish one class of signals. Each class of spatial filter performs feature extraction on the electroencephalogram signals, then, one SVM is used as a classifier to classify the features extracted by each spatial filter, N classification results are obtained, and then the combination of the filter (the optimal filter) corresponding to the best classification result and the SVM is selected to construct decision tree branches.
S5, repeatedly constructing branches of the decision tree by using the one-to-many CSP and the SVM. The construction of each decision tree branch enables the N classification problem to be changed into the N-1 classification problem, the distinguishable categories are removed from the data set after each decision tree branch is constructed, then the decision tree branches are repeatedly constructed by utilizing the 'one-to-many' CSP and the SVM, namely the steps S2, S3 and S4 are repeated, but the 'one-to-many' CSP becomes the original CSP method when only two categories are left, because the 'one-to-many' CSP takes one category as one category and the other categories as the other categories, because only two categories are left at the moment, the other categories are one category in total, and at the moment, the final complete binary tree is obtained by the CSP and SVM decision tree branch to realize the classification of all the categories. FIG. 2 is a complete binary tree established by classifying N types of motor imagery tasks by the multi-type motor imagery electroencephalogram signal feature extraction and classification method based on the decision tree and the CSP-SVM.
The above examples are to be construed as merely illustrative and not limitative of the remainder of the disclosure. After reading the description of the invention, the skilled person can make various changes or modifications to the invention, and these equivalent changes and modifications also fall into the scope of the invention defined by the claims.

Claims (7)

1. The multi-class motor imagery electroencephalogram signal feature extraction and classification method based on the decision tree and the CSP-SVM is characterized by comprising the following steps:
s1, preprocessing the N motor imagery electroencephalogram signals;
s2, processing the preprocessed motor imagery electroencephalogram signal by using the one-to-many CSP, thereby constructing N spatial filters and acquiring the final characteristics of the electroencephalogram signal;
s3, classifying the characteristics in the step S2 by using SVM;
s4, selecting the combination of the optimal space filter and the SVM to construct the branch of the decision tree according to the classification result;
and S5, repeatedly constructing branches of the decision tree by using the one-to-many CSP and the SVM until a decision tree capable of distinguishing all classes is constructed.
2. The method for extracting and classifying the characteristics of the multi-class motor imagery electroencephalogram signal based on the decision tree and the CSP-SVM as claimed in claim 1, wherein: the preprocessing of the N classes of motor imagery electroencephalogram signals specifically comprises the following steps: and carrying out band-pass filtering processing on the original motor imagery electroencephalogram signals by EEGLAB, and setting a band-pass frequency band of 8-30 Hz.
3. The method for extracting and classifying the characteristics of the multi-class motor imagery electroencephalogram signal based on the decision tree and the CSP-SVM as claimed in claim 1, wherein: the step of constructing the N spatial filters is as follows: one type of motor imagery is regarded as one type, the other N-1 types of motor imagery are regarded as the other type, CSP processing is carried out to construct a spatial filter, and therefore, for N types of motor imagery electroencephalogram signals, each type of motor imagery and the other N-1 types of motor imagery need to be subjected to CSP processing to construct N spatial filters.
4. The method for extracting and classifying the characteristics of the multi-class motor imagery electroencephalogram signal based on the decision tree and the CSP-SVM as claimed in claim 1 or 3, wherein: mapping the original motor imagery electroencephalogram signal through a spatial filter to obtain a new signal ZpNew signal Zp(p ═ 1,2, … 2m) variance logarithm and normalization processing as final feature fp
Figure FDA0003138744950000011
Where var () represents variance, and m represents m rows before and after the projection matrix when constructing the spatial filter.
5. The method for extracting and classifying the characteristics of the multi-class motor imagery electroencephalogram signal based on the decision tree and the CSP-SVM as claimed in claim 1, wherein: the classifying the features in step S2 by using SVM is specifically to classify the final features by using SVM and select parameters (radial basis kernel parameters and error penalty factors) affecting the support vector machine by using a grid search method and a cross validation method.
6. The method for extracting and classifying the characteristics of the multi-class motor imagery electroencephalogram signal based on the decision tree and the CSP-SVM as claimed in claim 5, wherein: the final decision function expression of the SVM is as follows:
Figure FDA0003138744950000012
where sign () represents a sign function, λiIs the Lagrangian vector, yiFor training set sample labels, x is the independent variable of the decision function, i.e. the test set sample feature vector, xiFor training set sample feature vectors, b is a bias vector, K (x, x)i) As a function of the radial basis kernel, K (x, x)i) The expression of (a) is:
K(x,xi)=exp(-γ||x-xi||2)
where γ is the function bandwidth of the radial basis kernel function.
7. The method for extracting and classifying the characteristics of the multi-class motor imagery electroencephalogram signal based on the decision tree and the CSP-SVM as claimed in claim 1, wherein: the process of repeatedly building decision trees removes the categories that can be distinguished from all categories, and then repeatedly builds decision tree branches using "one-to-many" CSPs and SVMs until all categories can be distinguished.
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