The removal of ocular artefacts from the electroencephalogram: a review

Jervis, B.W.; Ifeachor, Emmanuel; Allen, Elizabeth S.

doi:10.1007/bf02441820

Cited by 57 publications

(32 citation statements)

References 39 publications

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“…[1982]), Gratton and collaborators later introduced an expansion of their technique to be used with multiple EOG electrodes, based on a multiple regression method (Gratton & Coles, 1989;Miller, Gratton, & Yee, 1988). Jervis, Ifeachor, and Coelho (1989;see also Ifeachor, Jervis, Morris, Allen, & Hudson, 1986;Jervis et aI., 1985) also proposed a time domain method based on a multiple regression procedure. The major difference between Jervis et als method and the other techniques reviewed earlier is an emphasis on using three EOG channels (one vertical and two horizontal, one for each eye) as predictors, and the use of iterative methods for the computation ofthe scaling factors.…”

Section: Time Domain Techniquesmentioning

confidence: 99%

Dealing with artifacts: The EOG contamination of the event-related brain potential

Gratton

1998

Behavior Research Methods, Instruments, & Computers

122

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Eye movements and blinks represent a major source of artifacts in the electroencephalogram (EEG) and event-related brain potentials (ERPs). The origin of this artifact is the large difference in potential that exists between the cornea and the retina. Eye movements and blinks produce shifts of the electric fields that propagate across the whole head and that can be several times larger than the activity generated by the brain, Ocular activity can be monitored by electrodes located near the eyes (electrooculogram, or EOG). The electric fields associated with eye movements and blinks are somewhat different. The simplest procedure for dealing with ocular artifacts is to eliminate trials on which EOG activity is detected (rejection). However, this technique may result in data loss and biased data samples, especially when one is comparing clinical populations or tasks involving large amounts of eye movements, Another approach involves estimation and correction of the ocular artifact on the EEG and ERP traces. Several techniques have been proposed. Some of them are reviewed in the present paper. Issues related to the accuracy of the various techniques, as well as other advantages and limitations, are also discussed. Finally, general guidelines for how to deal with ocular artifacts are proposed. 44The difference in electric potential between the front and the back ofthe eye generates a large dipole-like electric field (electro-oculogram, or EOG) that may interfere with the surface recording ofthe electrical activity of the brain (electroencephalogram, or EEG), If the EOG does not change over time, its effect on the EEG will be a constant. Since the EEG is measured as a change with respect to a baseline value, the effect of a constant EOG is null. Therefore, EOG contamination of the EEG trace occurs whenever the eyes move, or whenever the propagation ofthe EOG to the scalp electrodes varies over time. This type of contamination is labeled EOG artifact, ocular artifact, or eye-movement artifact.The EOG artifact is of great significance in the registration ofEEG and event-related brain potentials (ERPs). In fact, when measured at the scalp, especially at frontal locations close to the eyes, the EOG signal (several hundred microvolts in amplitude) can be several times larger than the brain-generated scalp potentials (typically, less than 50 JiV). Eye movements and blinks (both of which are associated with large EOG artifacts) are very frequent and may occur during the performance of various tasks or as the result of certain types ofstimulations. The frequency and size ofEOG artifacts may vary, depending I would like to thank Monica Fabiani and two anonymous reviewers for their comments on earlier versions of this manuscript. Send all correspondence to G. Gratton,

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Section: Time Domain Techniquesmentioning

confidence: 99%

Dealing with artifacts: The EOG contamination of the event-related brain potential

Gratton

1998

Behavior Research Methods, Instruments, & Computers

122

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“…The vertical right EOG, VEOG R , the horizontal right EOG, HEOG R , the horizontal left EOG, HEOG L , and the radial right EOG, REOG R , were then given by: 4 , and REOG R ¼ (V 1 +V 2 )/2. EEGs were measured between electrode sites C z , P 4 , F 4 , O 1 and linked mastoids. Quasiperiodic vertical, horizontal, and oblique eye movements, and also random eye movements with blinks, generated corresponding EOGs.…”

Section: Methodsmentioning

confidence: 99%

“…Ocular artefact removal (OAR) has been reviewed in [4]. Methods based on modelling the OAs as a sum of EOG terms and then subtracting the modelled OA from the measured EEG to provide an estimate of the artefact-free EEG have been the most widely used.…”

Section: Introductionmentioning

confidence: 99%

Online removal of ocular artefacts from the electroencephalogram

Jervis

García²,

Thomlinson

et al. 2004

IEE Proceedings - Science, Measurement and Technology

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Abstract:A method by which ocular artefacts (OAs) in the electroencephalogram (EEG) may be removed automatically online by electro-oculogram (EOG) subtraction is demonstrated. This is achieved by a combination of recursively calculating the required cross-correlations, a fast matrix inversion method, and the use of a modern microprocessor with a high clock rate. Although recursive calculations are involved, the method itself is essentially non-recursive, which means that distortion of any evoked potentials is minimised. The method may be applied simultaneously to any number of EEG channels.

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“…ods are based on the assumption that the measured EEG is described as a linear combination of the underlying cortical activity, or background EEG, and the corrupting ocular artifact [5]. They generally use a variation of the following model to relate the measured EEG, background EEG, and ocular artifact (1) y…”

Section: Subtraction Ocular Artifact Removal Methods Subtraction Meth-mentioning

confidence: 99%

“…Automated OAR methods, like the subtraction method discussed in the next section, allow for online removal of ocular artifacts. For a survey of OAR methods see [5]. Figure 1 shows a segment of EEG signals corrupted with ocular artifact.…”

mentioning

confidence: 99%

EEG ocular artifact removal through ARMAX model system identification using extended least squares

Haas

Frei²,

Osorio³

et al. 2003

Communications in Information and Systems

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Abstract. The removal of ocular artifact from scalp electroencephalograms (EEGs) is of considerable importance for both the automated and visual analysis of underlying brainwave activity. Traditionally, subtraction techniques use linear regression to estimate the influence of eye movements on the electrodes of interest. These methods are based on the assumption that the underlying brainwave activity is uncorrelated when, in general, it is not. Furthermore, regression methods assume that the ocular artifact propagation is frequency independent, i.e. all waveforms of the ocular artifact propagate similarly. In this paper, we examine relaxing these assumptions by using a more general autoregressive (AR) moving average (MA) exogenous (X) model and the extended least squares (ELS) algorithm to remove ocular artifact. We demonstrate that in some cases this general ARMAX model can decrease ocular artifact not removable by standard regression techniques. We also show that the incorporation of a forgetting factor to exponentially weight past data can improve ocular artifact removal even for the traditional subtraction method.

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The removal of ocular artefacts from the electroencephalogram: a review

Cited by 57 publications

References 39 publications

Dealing with artifacts: The EOG contamination of the event-related brain potential

Dealing with artifacts: The EOG contamination of the event-related brain potential

Online removal of ocular artefacts from the electroencephalogram

EEG ocular artifact removal through ARMAX model system identification using extended least squares

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