Analysis of the electroencephalographic activity during the Necker cube reversals by means of the wavelet transform (original) (raw)
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In previous studies, a perceptual switching related potential was obtained during the observation of a multistable dynamic reversal pattern, where the averaging of the single responses was triggered by subjects pressing a button. The present methodological study aims to increase the signal quality of perceptual switching related potentials considering the dependence of the measurement method on the reaction time of the subject, which may vary signi®cantly during a session, leading to low-amplitude waveform in the averaged event-related-potential (ERP). To overcome this problem in measuring the electrophysiological correlate of an internal event, a pattern selection method based on the wavelet transform (WT) is proposed to choose a subset of single ERPs with more homogenous latencies. Nine subjects observed a Necker cube and were instructed to press the button immediately after perceptual switching. A slow, low-amplitude positive wave with frontocentral amplitude maxima was observed around 250 ms prior to the button press. After the application of a 5 octave WT on single sweeps, the time-frequency coecients obtained in each octave were averaged across trials. The most dominant feature representing the averaged ERP was the delta (0.5±4 Hz) coecient occurring between 250 and 125 ms before the button press. By averaging the subset of the single sweeps containing this property, a sharpening and signi®cant amplitude increase of the response peak was observed.
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IAEME PUBLICATION, 2013
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Adaptive wavelet filtering for analysis of event-related potentials
A challenging task in psychophysiology is the extraction of event-related potentials (ERPs) from the background electro-encephalogram. The task is made more difficult by the properties of ERPs, which typically consist of multiple features of variable latency, Iocalised in time and frequency. A novel technique is described for analysis of ERPs, adaptive wavelet filtering (AWF), which is proposed as an alternative to trial averaging. Band-limited detail representations of each trial are obtained using wavelet analysis. The Woody adaptive filter is then used to align trials with respect to the evoked response. In a simulation study, the AWF extracts 39% of higher-frequency signal variance from background noise, compared with less than 1% for standard averaging and the Woody filter. The AWF is applied to a dataset of 448 ERPs, comprising right-finger button presses from eight subjects. Average split-half reliability of the AWF on scales up to 12Hz was 0.51.
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2001
In this work we show the application of a measure of entropy de®ned from the wavelet transform, namely the wavelet entropy (WS), to the study of eventrelated potentials (ERPs). WS was computed for ERPs recorded from nine healthy subjects with three dierent types of stimuli, among them target stimuli in a cognitive task. A signi®cant decrease of entropy was correlated with the responses to target stimuli (P300), thus showing that these responses correspond to a more``ordered'' state than the spontaneous EEG. Furthermore, we propose the WS as a quantitative measure for such transitions between EEG (``disordered state'') and ERP (``ordered state'').
Event-related oscillations are 'real brain responses' - wavelet analysis and new strategies
The EEG consists of the activity of an ensemble of generators producing rhythmic activity in several frequency ranges. These oscillators are active usually in a random way. However, by application of sensory stimulation these generators are coupled and act together in a coherent way. This synchronization and enhancement of EEG activity gives rise to 'evoked' or 'event-related oscillations'. The compound e¨oked potential manifests as superimposition of Ž . evoked rhythms in the EEG frequencies ranging from delta to gamma 'natural frequencies of the brain' . The superimposition principle is described with efficient strategies and by utilization of an efficient algorithm. The wavelet analysis confirms the results of the combined analysis procedure obtained by using the amplitude frequency Ž . characteristics AFCs and digital filtering. The AFC and adapted digital filtering methods are based on the first approach to analyze average evoked potentials. In contrast, the wavelet analysis is based on signal retrie¨al and selection among a large number of sweeps recorded in a given physiological or psychological experiment. By combining all these results and concepts, it can be stated that the wavelet analysis underlines and extends the expression that alpha-, theta-, delta-, and gamma-responses described in this report are the most important brain responses related to psychophysiological functions. The wavelet analysis confirms once more the expression 'real signals' which we attribute to EEG frequency responses of the brain. It will be demonstrated that the delta, theta, Ž . and alpha responses i.e. the rhythms 'predicted' by digital filtering are real brain oscillations. The frequency Ž . components of the event-related potential vary independently of each other with respect to: a their relation to the Ž . Ž . event; b their topographic distribution; and c with the mode of the physiological measurements. ᮊ
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PubMed, 1999
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Biological Cybernetics, 2000
Since the ®rst observation of perceptual reversal by Necker, many theoretical approaches have been proposed. In a previous study, we showed that a positive wave appeared approximately 250 ms prior to the button press of the subjects, indicating perceptual reversal during the observation of the Necker cube ®gure. A basic diculty in this type of study is the possible jitter in the latency of the button press due to the variability of the subjects' reaction time during a recording session. To overcome this diculty, a pattern selection method based on the wavelet transform was proposed in the previous study. A dominant positive wavelet coecient in the delta band was found to represent the perceptual-reversal-related positivity. In the present study, we aim to analyze the changes in the alpha frequency band during perceptual reversal by using the Necker cube. The RMS values of the alpha frequency band were measured for two time periods: 3 SD around the mean peak latency of the perceptualreversal-related positivity and a time window of the same length before the positive wave. We found significantly increased delta power and decreased alpha power during the perceptual-reversal-related positivity.