Vladimir Bostanov

BCI competition 2003-data sets Ib and IIb: feature extraction from event-related brain potentials with the continuous wavelet transform and the t-value scalogram

The t-CWT, a novel method for feature extraction from biological signals, is introduced. It is based on the continuous wavelet transform (CWT) and Students t-statistic. Applied to event-related brain potential (ERP) data in brain- computer interface (BCI) paradigms, the method provides fully automated detection and quantification of the ERP components that best discriminate between two samples of EEG signals and are, therefore, particularly suitable for classification of single-trial ERPs. A simple and fast CWT computation algorithm is proposed for the transformation of large data sets and single trials. The method was validated in the BCI Competition 2003 , where it was a winner (provided best classification) on two data sets acquired in two different BCI paradigms, P300 speller and slow cortical potential (SCP) self-regulation. These results are presented here.

Mindfulness-based cognitive therapy (MBCT), cognitive style, and the temporal dynamics of frontal EEG alpha asymmetry in recurrently depressed patients.

Mindfulness-based cognitive therapy (MBCT), a meditation-based maintenance therapy, reduces the relapse risk in individuals suffering from major depressive disorder (MDD). However, only a few studies investigated the psychophysiological mechanisms underlying this protective effect. We examined effects of MBCT on trait rumination and mindfulness, as indicators of global cognitive style, as well as on residual depressive symptoms in a group of recurrently depressed patients (n=78) in remission. Additionally, alpha asymmetry in resting-state electroencephalogram (EEG) was assessed. Alpha asymmetry has been found to be predictive of affective style and a pattern indicative of stronger relative right-hemispheric anterior cortical activity may represent a trait marker for the vulnerability to develop MDD. In line with previous findings, residual depressive symptoms and trait rumination decreased, whereas trait mindfulness increased following MBCT, while no such changes took place in a wait-list control group. Mean values of alpha asymmetry, on the other hand, remained unaffected by training, and shifted systematically toward a pattern indicative of stronger relative right-hemispheric anterior cortical activity in the whole sample. These findings provide further support for the protective effect of MBCT. In the examined patients who were at an extremely high risk for relapse, however, this effect did not manifest itself on a neurophysiological level in terms of alpha asymmetry, where a shift, putatively indicative of increased vulnerability, was observed.

Cortical processing in Guillain-Barré syndrome after years of total immobility.

Sirs: Most people with severe motor paralysis can still communicate with their environment using minor finger or eye movements. However, patients totally paralyzed due to pontine stroke [9, 12] or amyotrophic lateral sclerosis [5, 6] have also been reported where the complete breakdown of communication precludes any conclusion as to their mental state. Being completely immobile, artificially ventilated and fed, such patients can hardly be investigated using modern functional brain imaging techniques. It remains unclear as to what degree their cognitive functions are still preserved. GuillainBarré syndrome (GBS), accompanied by oculomotor palsy, can also result in a total paralysis with complete separation of the patient from the outer world (e. g., [8]). Despite severe motor dysfunction, the sensitivity of patients is often only slightly impaired. The state of higher cortical functions remains unknown as long as no neuropsychological testing is possible. Recently two completely paralyzed patients with acute GBS were examined by means of event-related brain potentials (ERPs) [10]. In both patients, ERPs recorded after a few days of the disease onset indicated severe cognitive deficits; later on they normalized in one patient in parallel with his clinical improvement. However, this study employed a very limited range of stimuli which only deliver information of relatively simple cortical functions. We report a patient (female, aged 36 years) suffering from GBS for the exceptionally long period of five years. Soon after an acute genital infection, she felt progressive weakness in extremities that developed within several weeks to severe paralysis, whereupon tracheostomy was performed for artificial ventilation. Despite intensive immunodepressive therapy, her condition continued to deteriorate. During the initial two years after disease onset, she was able to maintain distinct yes/no communication by means of vertical eye movements. After this, these movements weakened until they could no longer be understood clearly by her caregivers. Because of the unusual course, the diagnosis of GBS was checked twice in two different neurological clinics. During our observation, the patient was completely non-responsive with very low muscle tonus and lacking oculocephalic, corneal, and tendon reflexes. The background EEG was very flat (5–10 μV) without any apparent response to stimulation. However, the spectral analysis of the EEG revealed a normal pattern with predominant slow alpha (7.5–8/s) oscillations over the posterior cortex. Moreover, visual, somatosensory and auditory evoked potentials were intact, indicating well functioning sensory pathways. Our examinations were affirmed by the Ethical Committee of the University of Tübingen and carried out with the informed consent of the patient’s brother. In order to explore the patient’s cognitive processing, we started with a recording of the electrooculogram (EOG) using two electrodes above and below the left eye. Twenty pairs of statements were presented, whose correctness the patient presumably had to know, with one statement in each pair being correct and the other incorrect [e. g.,“Your name is (correct name)”,“Your name is (wrong name)”]. The patient was asked to move her eyes up and down to signal “yes” and “no”, respectively. A comparison of EOG responses to correct and wrong statements revealed no difference. This might imply, either that the patient did not understand the statements or the instruction, or that her eyes were completely paralyzed. To resolve this problem, again the ERP technique was used. The EEG was recorded (digitalization rate 2 ms, filters 0.3–40 Hz) at F3, F4, Fz, C3, C4, Cz, P3, P4, and Pz sites (international 10–20 system) using mastoid electrodes linked via a 15-kΩ resistance as reference. Although a standard regression paradigm [4] was applied for eye artifact correction, there was nothing to correct owing to the lack of any detectable eye movements. A battery of stimulation paradigms tested cortical processing at different levels of complexity [7]. In the oddball I paradigm, two tones of 700 and 1300 Hz were presented with frequencies of 85 % and 15 %, respectively; in oddball II, the tones were replaced by vowels /e/ and /i/. In the prosody paradigm, rare (20 %) sad exclamations were interspersed with four different exclamations of joy (20 % each). In the word pair paradigm, 50 strongly related (e. g., day-night) and 50 semantically unrelated word pairs were randomly presented. Finally, in the sentence paradigm, 100 different 7-word sentences were used, in half of which the final word was semantically inappropriate, while in the other half it was entirely as expected from the context of the sentence. All stimuli were presented LETTER TO THE EDITORS

Multivariate assessment of event-related potentials with the t-CWT method

BackgroundEvent-related brain potentials (ERPs) are usually assessed with univariate statistical tests although they are essentially multivariate objects. Brain–computer interface applications are a notable exception to this practice, because they are based on multivariate classification of single-trial ERPs. Multivariate ERP assessment can be facilitated by feature extraction methods. One such method is t-CWT, a mathematical-statistical algorithm based on the continuous wavelet transform (CWT) and Student’s t-test.ResultsThis article begins with a geometric primer on some basic concepts of multivariate statistics as applied to ERP assessment in general and to the t-CWT method in particular. Further, it presents for the first time a detailed, step-by-step, formal mathematical description of the t-CWT algorithm. A new multivariate outlier rejection procedure based on principal component analysis in the frequency domain is presented as an important pre-processing step. The MATLAB and GNU Octave implementation of t-CWT is also made publicly available for the first time as free and open source code. The method is demonstrated on some example ERP data obtained in a passive oddball paradigm. Finally, some conceptually novel applications of the multivariate approach in general and of the t-CWT method in particular are suggested and discussed.ConclusionsHopefully, the publication of both the t-CWT source code and its underlying mathematical algorithm along with a didactic geometric introduction to some basic concepts of multivariate statistics would make t-CWT more accessible to both users and developers in the field of neuroscience research.

Measuring Mindfulness: A Psychophysiological Approach

Mindfulness-based interventions have proved effective in reducing various clinical symptoms and in improving general mental health and well-being. The investigation of the mechanisms of therapeutic change needs methods for assessment of mindfulness. Existing self-report measures have, however, been strongly criticized on various grounds, including distortion of the original concept, response bias, and other. We propose a psychophysiological method for the assessment of the mindfulness learned through time-limited mindfulness-based therapy by people who undergo meditation training for the first time. We use the individual pre-post-therapy changes (dERPi) in the event-related brain potentials (ERPs) recorded in a passive meditation task as a measure of increased mindfulness. dERPi is computed through multivariate assessment of individual participants ERPs. We tested the proposed method in a group of about 70 recurrently depressed participants, randomly assigned in 1.7:1 ratio to mindfulness-based cognitive therapy (MBCT) or cognitive therapy (CT). The therapy outcome was measured by the long-term change (dDS) relative to baseline in the depression symptoms (DS) assessed weekly, for 60 weeks, by an online self-report questionnaire. We found a strong, highly significant, negative correlation (r = −0.55) between dERPi (mean = 0.4) and dDS (mean = −0.7) in the MBCT group. Compared to this result, the relationship between dDS and the other (self-report) measures of mindfulness we used was substantially weaker and not significant. So was also the relationship between dERPi and dDS in the CT group. The interpretation of dERPi as a measure of increased mindfulness was further supported by positive correlations between dERPi and the other measures of mindfulness. In this study, we also replicated a previous result, namely, the increase (dLCNV) of the late contingent negative variation (LCNV) of the ERP in the MBCT group, but not in the control group (in this case, CT). We interpreted dLCNV as a measure of increased meditative concentration. The relationship between dLCNV and dDS was, however, very week, which suggests that concentration might be relatively unimportant for the therapeutic effect of mindfulness. The proposed psychophysiological method could become an important component of a “mindfulness test battery” together with self-report questionnaires and other newly developed instruments.