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1、A WAVELET BASED DE-NOISING TECHNIQUE FOROCULAR ARTIFACT CORRECTION OF THE ELECTROENCEPHALOGRAMTatjana Zikov, Stphane Bibian, Guy A. Dumont, Mihai HuzmezanDepartment of Electrical and Computer Engineering, the University of British Columbia, BC, CANADA.Abstract This paper investigates a wavelet based
2、 denoising of the electroencephalogram (EEG) signal to correct for the presence of the ocular artifact (OA).The proposed technique is based on an over-complete wavelet expansion of the EEG as follows: i) a stationary wavelet transform (SWT) is applied to the corrupted EEG; ii) the thresholding of th
3、e coefficients in the lower frequency bandsis performed; iii) the de-noised signal is reconstructed. This paper demonstrates the potential of the proposed technique for successful OA correction. The advantage over conventional methods is that there is no need for the recording of the electrooculogra
4、m (EOG) signal itself. The approach works both for eye blinks and eye movements. Hence, there is no need to discriminate between different artifacts. To allow for a proper comparison, the contaminated EEG signals as well as the corrected signals are presented together with their corresponding power
5、spectra.Keywords-stationary wavelet transform (SWT), electroencephalogram (EEG), electrooculogram (EOG), ocular artifact (OA).I. INTRODUCTIONThe electroencephalogram (EEG) gives researchers a non-invasive insight into the intricacy of the human brain. It is a valuable tool for clinicians in numerous
6、 applications, from the diagnosis of neurological disorders, to the clinical monitoring of depth of anesthesia. For awake healthy subject, normal EEG amplitude is in the order of 20-50V.The EEG is very susceptible to various artifacts, causing problems for analysis and interpretation. In current dat
7、a acquisition, eye movement and blink related artifacts are often dominant over other electrophysiological contaminating signals (e.g. heart and muscle activity, head and body movements), as well as external interferences due to power sources. Eye movements and blinks produce a large electrical sign
8、al around the eyes (in the order of mV), known as electrooculogram (EOG), which spreads across the scalp and contaminates the EEG. These contaminating potentials are commonly referred to as ocular artifact (OA).The rejection of epochs contaminated with OA usually leads to a substantial loss of data.
9、 Asking subjects not to blink or move their eyes or to keep their eyes shut and still, is often unrealistic or inadequate. The fact that the subject is concentrating on fulfilling these requirements might itself influence his EEG. Hence, devising a method for successful removal of ocular artifacts (
10、OA) from EEG recordings has been and still is a major challenge. Widely used time-domain regression methods involve the subtraction of some portion of the recorded EOG from the EEG 1, 2. They assume that the propagation of ocular potentials is volume conducted, frequency independent and without any
11、time delay. However, Gasser et al. in 3 argued that the scalp is not a perfect volume conductor, and thus, attenuates some frequencies more than others. Consequently, frequency domain regression was proposed. In addition, no significant time delay was found, which was in consistency with the EOG bei
12、ng volume conducted.In 4 it was reported that, in reality, the frequency dependence does not seem to be very pronounced, while the assumption of no measurable delay was confirmed. Thus, while some researchers support the frequency domain approach for EOG correction 3, 5, others disputed its advantag
13、es 4, 6, 7. However, neither time nor frequency regression techniques take into account the propagation of the brain signals into the recorded EOG. Thus a portion of relevant EEG signal is always cancelled out along with the artifact. Further, these techniques mainly use different correction coeffic
14、ients for eye blinks versus eye movements. They also heavily depend on the regressing EOG channel.In addition, Croft and Barry 7 demonstrated that the propagation of the EOG across the scalp is constant with respect to ocular artifact types and frequencies. They proposed a more sophisticated regress
15、ion method (the aligned-artifact average solution) that corrects blinks and eye movement artifacts together, and made possible the adequate correction for posterior sites 6. They claim that the influence of the EEG-to-EOG propagation has been minimized in their method.In an attempt to overcome the p
16、roblem of the EEG-to-EOG propagation, a multiple source eye correction method has been proposed by Berg and Scherg 8. In this method, the OA was estimated based on the source eye activity rather than the EOG signal. The method involves obtaining an accurate estimate of the spatial distribution of th
17、e eye activity from calibration data, which is a rather difficult task.Due to its decorrelation efficiency, the principal component analysis (PCA) has been applied for OA removal from the multi-channel EEG and it outperformed the previously mentioned methods. However, it has been shown that PCA cann
18、ot fully separate OA from the EEG when comparable amplitudes are encountered 9.Recently, independent component analysis (ICA) has demonstrated a superior potential for the removal of a wide variety of artifacts from the EEG10, 11, even in a case of comparable amplitudes. ICA simultaneously and linea
19、rly unmixed multi-channel scalp recordings into independent components, that are often physiologically plausible. Also, there is no need for a reference channel corresponding to each artifact source. However, ICA artifact removal is not yet fully automated and requires visual inspection of the indep
20、endent components in order to decide their removal.Other attempts have been based on different adaptive signal processing techniques 12-16. The performance of these methods also relies on a cerebral activity to minimally contaminate the EOG reference.The EEG may contain pathological signals, which r
21、esemble OA. Thus, these signals are most likely to be removed from the EEG recordings, leading to erroneous diagnosis. Therefore, it is important to distinguish between artifacts and pathological EEG signals prior to artifact removal. Artificial intelligence techniques prove to be somehow helpful in
22、 achieving this goal 17.Our aim is to present a real-time OA removal technique, based on stationary wavelet transform (SWT) de-noising of a single frontal channel EEG. The proposed technique is based on an over-complete wavelet expansion of the EEG as follows: i) a stationary wavelet transform is ap
23、plied to the corrupted EEG; ii) the thresholding of the coefficients in the lower frequency bands is performed; iii) the de-noised signal is reconstructed. No reference EOG channel is needed and the same approach is used for both the blinks and eye movement artifacts.The time and frequency character
24、istics of OA are addressed in Section II, while Section III discusses the proposed method. Results are further presented in Section IV.II. OCULAR ARTIFACTSThere are two different originating phenomena for ocular potentials 1, 18, 19. There is a potential difference of about 100 mV between a positive
25、ly charged cornea and negative retina of the human eye, thus forming an electrical dipole (i.e. corneo-retinal dipole). Firstly, the rotation of the eyeball results in changes of the electrical field across the skull. Secondly, eye blinks are usually not associated with ocular rotation; however, the
26、 eyelids pick up the positive potential as they slide over the cornea. T his creates an electrical field that is also propagated through the skull. Hence, ocular potentials spread across the scalp and superimpose on the EEG.The mechanism of origin (eye movements versus eye blinks) and the direction
27、of eye movements determine the resulting EOG wave shape. Vertical, horizontal and round eye movements usually result in square-shaped EOG waveforms, while blinks are spike-like waves.Fig.1 Uncontaminated Baseline EEG and Various Artifacts(a) Uncontaminated baseline EEG (b) EEG contaminated with slow
28、 blink artifact (c) EEG contaminated with fast blink artifact (d) EEG contaminated with vertical eye movement artifact (e) EEG contaminated with horizontal eye movement artifact (f) EEG contaminated with round eye movement artifactOcular artifacts decrease rapidly with the distance from the eyes 18.
29、 Therefore, the most severe interference occurs in the EEG recorded by the electrodes placed on the patients forehead. Yet, this is the most convenient region for their placement. Thereto, the frontal and prefrontal lobes, which are at the origin of higher cognitive functions, are located directly b
30、ehind the forehead. Therefore, the task of EOG correction for frontal channels is challenging.For the purpose of this paper, we have acquired EEG data from an awake healthy male subject. A single frontal channel was recorded, corresponding to the Fpz electrode placement in the nomenclature of the In
31、ternational 10-20 System. The baseline EEG and five various artifacts were recorded in the following fashion. For each artifact, the subject was first instructed to keep his eyes shut and still. Sixty seconds of presumably uncontaminated baseline EEG was thus recorded. Then, for the next 60 seconds,
32、 the subject was instructed to blink or move his eyes in a predetermined fashion. Finally, another resting period of 60 seconds with no EOG activity was recorded. Five ocular artifacts were recorded in this fashion: slow blinks (1 blink per 2 seconds), fast blinks (2 blinks per second), vertical, ho
33、rizontal and round eye movements. The signal was notch filtered at 50-60 Hz and sampled at 128 Hz.Fig.2 Power Spectra of Uncontaminated Baseline EEG and Various Artifacts(a) Uncontaminated baseline EEG and other resting periods (b) EEG contaminated with slow blink artifact (c) EEG contaminated with
34、fast blink artifact (d) EEG contaminated with vertical eye movement artifact (e) EEG contaminated with horizontal eye movement artifact (f) EEG contaminated with round eye movement artifactFig. 2 clearly shows that OA are large, transient slow waves. They occupy the lower frequency range; from 0 Hz
35、up to 6-7 Hz for the eye movement artifacts, and typically up to the alpha band (8-13 Hz), excluding very low frequencies, for the eye blinks. This is a well-known and documented result 3, which our experiments proved consistent with. Clearly, OA amplitudes are of a much higher order than those of t
36、he uncontaminated EEG and have a characteristic pattern of changes. Vertical eye movement artifacts (Fig. 2.d) also seem to produce a rise in the higher frequencies. However, this is most likely due to the increased muscle activity, and it is also present to a lesser degree for the other two eye mov
37、ements (horizontal - Fig. 2.e, and round - Fig. 2.f).III. WAVELET-BASED DE-NOISINGA. Problem StatementAs previously mentioned, the EOG is the non-cortical activity that contaminates the EEG recordings. Thus, since the brain and eye activities have physiologically separate sources, we will assume the
38、 recorded EEG is a superposition of the true EEG and some portion of the EOG signal. Thus, we have: (1) Where is the recorded contaminated EEG, is due to the cortical activity, and is the propagated ocular artifact at the recording site. The dcoffset takes into account the zero mean value of the cor
39、tical EEG, since this might not be true for the recorded EEG due to the process of data acquisition. We are interested in estimating the ocular artifact based on the analysis of the recorded EEG. By subtracting it from the contaminated EEG, we will then obtain a corrected EEG, which minimizes the ef
40、fect of the ocular artifact and gives an appropriate representation of the true cortical EEG.The true EEG is a noise-like signal. We can not observe any clear patterns within it, nor can we simply correlate the particular underlying events with its wave shape 20.Furthermore, in the awake, conscious
41、state, neurons are firing in a more independent fashion. As a result of this resynchronizations, the awake EEG signal is even more random-appearing. The EOG removal can be approached by recovering the regression function ( ) from the recorded data. For this purpose, in the last decade, wavelet thres
42、holding has emerged as a simple, yet effective technique for de-noising 21.B. Wavelet ThresholdingThe main statistical application of wavelet thresholding is a nonparametric estimation of the regression function f , based on observations at time points . The observations are modeled as: (2)Where are
43、 independent and identically distributed random variables at equally spaced time points . Due to the orthogonality of the wavelet transform, we are allowed to perform filtering in the space of wavelet coefficients. The procedure for suppressing the noise involves: i) finding the coefficients of the
44、wavelet transform of si; ii) comparing each wavelet coefficient against an appropriate threshold; iii) keeping only those coefficients larger than a threshold; and iv) applying an inverse wavelet transform to obtain an estimate of f. The assumption is that large coefficients kept after thresholding
45、belong to the function to be estimated, and those discarded belong to the noise. This is a fair assumption due to the good energy compaction of the wavelet transform. Some of the function coefficients might eventually be discarded since they are of the same level as the noise coefficients. Thus, thi
46、s denoising technique works well for functions whose wavelet transform results in only a few nonzero wavelet coefficients, like e.g. functions that are smooth almost everywhere, except for only a few abrupt changes 22.Special care has to be taken when choosing an appropriate threshold, which always
47、involves the estimation of the noise variance based on the data.一种基于小波的眼伪影校正的脑电图去噪技术一、导言脑电图(EEG)使研究人员对复杂的人类大脑进行非侵入性的深入了解。它是一个有价值的工具，临床医师在许多方面应用，从神经系统疾病的诊断，到对临床监测麻醉深度。清醒健康时，正常脑电图振幅是在为20 - 50V，脑电是非常容易受到各种噪声的干扰，导致分析和理解时产生错误。在目前的数据采集中，眼动和眨眼相关动作经常产生其他的电生理污染信号（如心脏和肌肉的活动，头和身体动作），以及由于电力源产生外部干扰。眼球运动和闪烁在眼睛的周围
48、产生一个大的电信号（毫伏秩序），被称为眼电图(EOG)，蔓延整个头皮，污染脑电图。这些污染电位一般称为眼伪(OA)。 为抵制随时间变化的污染信号OA的产生，通常导致数据的重大损失。而要求受试者不闪烁或移动他们的眼睛，或一直保持他们的眼睛关闭，往往是不切实际和不恰当的。事实上，这个主题是致力于满足这些要求，这些要求可能本身影响脑电图。因此，制定一个成功的方法从脑电图记录中去除眼干扰仍然是一个重大挑战。 关于从EEG去除被记录的眼电图的部分，时域回归方法被广泛应用。他们认为，眼的电位是容积进行，频率独立，无时间延迟。然而，加瑟等人，认为头皮是一个不完美的容积导体，因此，对某些频率的抑制多过对其他频
49、率的抑制。因此，提出频域回归方法。此外，没有明显的时间延迟被发现，这是在一致性的眼电图中被卷了。在4中报道，在现实中，频率依赖似乎并不是很明显，而假设没有可衡量的延迟被确认。因此，虽然一些研究者支持频域眼电图校正方法，其他人支持时域眼电图校正方法。然而，在记录眼电图的大脑信号的传播中，都不考虑时间和频率的回归技术。因此，部分相关的脑电信号总是被噪声抵消。此外，这些技术主要使用不同的眨眼与眼球运动的修正系数。他们也在很大程度上取决于眼电图渠道的回归。此外，克罗夫特和巴里7研究表明，整个头皮的眼动传播常数是遵循噪声的类型和频率的，他们提出了一个更复杂的回归方法（回归aligned-artifact平均眨眼和纠正的解决方案），纠正闪烁眼球运动伪影，并提出可能的适当的修正后的网站6。他们声称他们的方法能使眼电干扰对脑电信号的干扰最小。在试图克服眼动传播影响脑电图的问题，Berg和Scherg