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基于运动想象的脑—机接口系统模式识别算法研究
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摘要
脑-机接口(brain-computer interface, BCI)是一种不依赖于正常的由外围神经和肌肉组成的输出通道的通讯系统。它在医学领域、人工智能的实现以及新型的娱乐研究等方面有重要应用,其实现涉及多个学科,触及许多前沿技术。因此BCI技术的研究具有科学与应用的双重价值,已经在国际上引起了广泛的兴趣并获得了快速的发展。其中,信号处理环节是实现BCI系统、提高系统可靠性与性能的关键,其实质是BCI信号的模式识别,主要包括三部分:信号预处理、特征提取和分类,本文着重研究后两部分,以期提高BCI系统的识别率。针对BCI发展过程中遇到的主要问题,如提高系统模式识别的精度和鲁棒性、缩短训练时间、发展异步工作模式下BCI系统的模式识别等,本文开展了如下工作:
     1、针对脑电信号噪声强、数据分布复杂、维数高、训练样本相对不足等特点,本文提出了两种不同的运动相关电位(movement related potentials, MRPs)特征提取算法——邻域空间模式和自适应空间模式算法。前者基于流形学习思想,利用样本向量之间的邻域关系和标签信息构造目标函数确定映射矩阵,不依赖于样本的分布;后者采用特征向量的欧式距离估计样本之间的相似关系,并基于这种关系建立目标函数,通过自适应迭代确定样本之间的邻域关系和最佳的特征投影矩阵,该算法的实施同样不需要对样本数据的分布做任何假设,还给出了高维EEG信号数据相似性的一种有效度量方法。实验证明,这两种算法均能增强MRPs特征的鲁棒性,提高分类精度;
     2、事件相关失同步/同步(event-related desynchronization/synchronization, ERD/ERS)和MRPs是BCI特征提取中的两大生理背景基础,它们都是在运动/运动想象前后或者过程当中产生,在某一特定的时间、某些电极位置、某一特定频段,特性表现比较明显,即对时空频信息敏感。针对这些性质,本文提出广义时空特征提取算法(general temporal- spatial extraction, GTSE),同时优化时空判别信息。ERD/ERS和MRPs的特性不同,针对它们的GTSE算法也不同。实验结果证明,GTSE算法能有效捕捉ERD/ERS和MRPs的时频特性,有效提高分类精度。
     3、针对BCI发展过程中缩短训练时间的要求,本文提出了两种针对BCI系统的半监督学习算法,一是基于联合特征的半监督学习算法,选择置信度高的无标样本(连同预测标签)迭代扩充训练集,并基于现有的训练集更新特征提取器和分类器,从而达到利用大量未标样本和少量的有标训练集学习模式识别系统参数的目的;二是针对MRPs特征的半监督特征提取算法,直接利用标签和未标签样本信息优化特征提取模型的参数。实验结果证明,基于少量的有标签样本,这两种半监督学习算法都能取得较为满意的分类效果。
     4、异步BCI系统的模式识别是近期的研究热点,其难点之一是识别实验过程中用户思维想象状态和空闲状态。由于空闲状态具有多样性,往往没有有效的空闲状态样本。本文提出了一种自适应的空闲状态检测方法。它以训练集(只有各类运动想象样本)的正确检测率和正确分类样本的类内散度为指标,根据接受者操作特性曲线的最佳临界点选择原则,构造目标函数,确定三分类器的上下阈值。为了进一步优化分类结果,该算法还采用了模糊化技术对预测标签进行处理。实验结果证明了算法的有效性。
     总的说来,本论文着眼于EEG作为生物医学信号固有的处理难点和BCI技术的进一步发展和实际应用推广过程中出现的新研究课题,研究更加有效、实用的BCI系统特征提取和分类方法,并用实验证明相关算法的有效性。
A brain-computer interface (BCI) is a communication system. The messages or commands in the system sends from an individual to the external world not passing through the brain’s normal output pathways of nerves system and muscles. The BCI has very important application in medical, artificial intelligence, novel entertainment and so on. Its implement refers to many subjects and frontier technologies. So the study about BCI has both science and application value. It has attracted much interest and been well developed. Signal processing is the key issue for implementing a BCI system as well as improving its reliability and performance, whose essence is the signal pattern recognition. It mainly includes three parts: signal preprocess, feature extraction and classification. The study emphasis of this paper is concentrated on the last two parts and is proved of improving the recognition accuracy of BCI system. Focusing on the main problem of BCI signal processing methods during the development, such as how to enhance classification accuracies and robustness, reduce training time and develop new effective methods for the pattern recognition in asynchronous modes, and so on, our research results and main contribution include:
     1. Focusing on the EEG characteristic, such as fully noisy, complicated data distribution, high dimensions, relative-un-abound training samples, and so on, two algorithms for the extraction of movement related potentials (MRPs) have been proposed: neighborhood spatial pattern algorithm and adapting spatial pattern algorithm. Based on the idea of manifold learning, the former one constructs the target function to estimate the optimal map matrix, only utilizing the neighborhood and label information, while not depending on the underlying data distribution. The latter estimates the similarity between samples by the corresponding features’distance, and then constructs the target function based on this similar relationship. It adaptively determines the neighbor relationship and the optimal direction via iterations. Its implement also doesn’t need any assumption about the underlying data distribution, but proposes an alternative for the similar measurement of EEG data with high dimension. Experimental results show that: the two algorithms all strengthen MRPs feature’s robustness, and improve classification accuracy.
     2. Event-related desynchronization/synchronization (ERD/ERS) and MRPs are two of the most important neurophysiological backgrounds of motor imagine utilized in the feature extraction of BCI pattern recognition. They appear before/after or during the process of motor imagery. Their characteristics are distinct at special temporal area, electrodes and band of frequency, i.e., they are sensitive to time, space and frequency. Based on these, we proposed general temporal-spatial extraction (GTSE) algorithm which can optimize temporal and spatial discriminative information together. The characteristics of ERD/ERS and MRPs are different, so the corresponding GTSE algorithms are different, too. Experimental results show that: the algorithms can catch more essential discriminative information of ERD/ERS and MRPs, and efficiently improve classification accuracy.
     3. Focusing on reducing training time to meet the requirement in the development of BCI, two semi-supervised learning algorithms for BCI are proposed. One is a learning algorithm based on combining-features. It chooses the most confident unlabeled samples (with their predicted labels) to enlarge the training set via iterations, and then based on the enlarged training set, retrains the parameters of the feature extractor and classifier. And consequently it gains its ends of using the information of large number of unlabeled samples with a few labeled samples to train feature extractor and classifier. The other is a semi-supervised feature extraction algorithm for MRPs. It straightly utilizes the information of labeled and unlabeled samples together to optimize the parameters of feature extraction model. Experimental results show that: based on a few labeled samples, the two semi-supervised learning algorithms can also obtain favorable classification accuracy.
     4. The pattern recognition of asynchronization BCI system recently is a hot topic. One of its biggest challenges is to discriminate which state the user is during experiment processing: imagining or idle? Because of the diversity of idle state, there are no effective training samples for idle sate. A novel algorithm is proposed for the detection of the idle state. Using the classification accuracy and the within-class scatter of the samples classified correctly in the training set (only with motor imagine samples) as two indexes, the algorithm constructs the target function according to the chosen criterion for the optimal operating point of the receiver operating characteristic curve, and determines the minimal/maximal decision thresholds for classification. To further optimize the classification effect, the algorithm treats the prediction labels by a fuzzy way. Its efficiency was demonstrated by experiment.
     To sum up, in this paper, we studied feature extraction and classification algorithms for BCI, focusing on the processing challenges including intrinsic nodus for EEG as biomedicine signal and the topics come forth from further development and practical application of BCI technique. The efficiency of the algorithms was all demonstrated by related experiments.
引文
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