Vasil Kolev

Wavelet entropy: a new tool for analysis of short duration brain electrical signals.

Since traditional electrical brain signal analysis is mostly qualitative, the development of new quantitative methods is crucial for restricting the subjectivity in the study of brain signals. These methods are particularly fruitful when they are strongly correlated with intuitive physical concepts that allow a better understanding of brain dynamics. Here, new method based on orthogonal discrete wavelet transform (ODWT) is applied. It takes as a basic element the ODWT of the EEG signal, and defines the relative wavelet energy, the wavelet entropy (WE) and the relative wavelet entropy (RWE). The relative wavelet energy provides information about the relative energy associated with different frequency bands present in the EEG and their corresponding degree of importance. The WE carries information about the degree of order/disorder associated with a multi-frequency signal response, and the RWE measures the degree of similarity between different segments of the signal. In addition, the time evolution of the WE is calculated to give information about the dynamics in the EEG records. Within this framework, the major objective of the present work was to characterize in a quantitative way functional dynamics of order/disorder microstates in short duration EEG signals. For that aim, spontaneous EEG signals under different physiological conditions were analyzed. Further, specific quantifiers were derived to characterize how stimulus affects electrical events in terms of frequency synchronization (tuning) in the event related potentials.

Parallel systems of error processing in the brain.

Major neurophysiological principles of performance monitoring are not precisely known. It is a current debate in cognitive neuroscience if an error-detection neural system is involved in behavioral control and adaptation. Such a system should generate error-specific signals, but their existence is questioned by observations that correct and incorrect reactions may elicit similar neuroelectric potentials. A new approach based on a time-frequency decomposition of event-related brain potentials was applied to extract covert sub-components from the classical error-related negativity (Ne) and correct-response-related negativity (Nc) in humans. A unique error-specific sub-component from the delta (1.5-3.5 Hz) frequency band was revealed only for Ne, which was associated with error detection at the level of overall performance monitoring. A sub-component from the theta frequency band (4-8 Hz) was associated with motor response execution, but this sub-component also differentiated error from correct reactions indicating error detection at the level of movement monitoring. It is demonstrated that error-specific signals do exist in the brain. More importantly, error detection may occur in multiple functional systems operating in parallel at different levels of behavioral control.

Abnormal early stages of task stimulus processing in children with attention-deficit hyperactivity disorder – evidence from event-related gamma oscillations

OBJECTIVES Attention-related differences in early stages of stimulus processing were assessed in healthy controls and children with attention-deficit hyperactivity disorder (ADHD) by analyzing phase-locked gamma band (31-63 Hz) responses to auditory stimuli in a selective-attention task. METHODS A total of 28 children aged 9-12 years (ADHD and matched healthy controls) pressed a button in response to each target stimulus presented at the attended side (right or left). Auditory gamma band responses (GBRs) within 0-120 ms were analyzed at 8 electrodes with wavelet transform. Effects of attended channel, stimulus type, and group were evaluated for GBR power and phase-locking. RESULTS For both groups, GBRs had a frontal-central distribution, were significantly larger and more strongly phase-locked to target than to non-target stimuli, and did not differentiate the attended from the unattended channel. ADHD children produced larger and more strongly phase-locked GBRs than controls only to right-side stimuli, irrespective of whether these were the attended or the ignored stimuli. CONCLUSIONS The association between auditory GBR and motor task stimulus in children suggests that phase-locked gamma oscillations may reflect processes of sensory-motor integration. ADHD-related deviations of GBRs indicate that early mechanisms of auditory stimulus processing are altered in ADHD, presumably as a result of impaired motor inhibition.

Single-sweep analysis of the theta frequency band during an auditory oddball task

The P300 component and the oscillatory 4-7 Hz electroencephalographic activity of auditory event-related brain potentials (ERPs) were assessed to study differences between passive and oddball task conditions. Theta responses from 15 adults were analyzed for single-sweep amplitude, phase locking, and enhancement against prestimulus activity. ERPs were characterized by enhanced and strongly phase-locked theta oscillations in the early (0-300 ms) poststimulus epoch, with only the late (300-600 ms) theta responses at Fz and Pz affected by the oddball condition. P300 was strongly associated not only with the concurrent theta oscillations but also with the evoked theta activity preceding P300 (0-300 ms). It was concluded that single theta response parameters can reveal specific functional differences between passive and oddball conditions and that a strong relationship exists between the theta frequency component and the time domain P300 ERP component.

Time-Frequency Analysis Reveals Multiple Functional Components During Oddball P300

A time–frequency decomposition was applied to rare target and frequent non-target event-related potentials (ERPs) elicited in an oddball condition to assess whether multiple functional components occur in the P300 latency range. The wavelet transform (WT) was used because it allows capture of simultaneous or partly overlapping components in ERPs without loosing their temporal relationships. The application of a four-octave quadratic B-spline wavelet transform at the level of single-sweep data allowed us to obtain new information and revealed the presence of separate events during P300 development. Several delta, theta, and alpha frequency components in the P300 latency range differed between target and non-target processing. These findings indicate that P300 is composed of multiple functional components and that the WT method is of use for the study of P300 functional correlates more precisely.

P300 and alpha event‐related desynchronization (ERD)

In the present study we evaluated the relationships between the P300 event-related potential and event-related desynchronization (ERD) of electroencephalographic alpha activity by simultaneously analyzing P300 as well as 7-10- and 10-14-Hz alpha ERD responses from auditory passive and active oddball conditions. We compared the effects of task (target vs. nontarget) and electrode (Fz, Cz, Pz) on P300 and ERD, and correlated P300 amplitude/latency with ERD maximal amplitude/latency across individuals. The major findings were that P300 as well as slow and fast alpha ERD manifested similar task and electrode effects. P300 preceded ERD and predicted individual variance of both slow and fast alpha ERD. The relationships of P300 with alpha ERD were different for the slow and fast alpha frequencies. These findings indicate that P300 and ERD are related such that slow and fast alpha ERDs are specifically guided or modified by the internal event(s) indexed by P300.

Time-frequency analysis of single-sweep event-related potentials by means of fast wavelet transform

A time-frequency decomposition was applied to the event-related potentials (ERPs) elicited in an auditory oddball condition to assess differences in cognitive information processing. Analysis in the time domain has revealed that cognitive processes are reflected by various ERP components such as N1, P2, N2, P300, and late positive complex. However, the heterogeneous nature of these components has been strongly emphasized due to simultaneously occurring processes. The wavelet transform (WT), which decomposes the signal onto the time-frequency plane, allows the time-dependent and frequency-related information in ERPs to be captured and precisely measured. A four-octave quadratic B-spline wavelet transform was applied to single-sweep ERPs recorded in an auditory oddball paradigm. Frequency components in delta, theta, and alpha ranges reflected specific aspects of cognitive information processing. Furthermore, the temporal position of these components was related to specific cognitive processes.

On the relation of movement-related potentials to the go/no-go effect on P3

According to Simson et al. [Simson, R., Vaughan, H.G., Jr., Ritter, W., 1977. The scalp topography of potentials in auditory and visual go/nogo tasks. Electroencephalography and Clinical Neurophysiology 43, 864-875], the difference between no-go P3 and go-P3 (the go/no-go effect) is due to overlap of P3 onto the return of the preceding contingent negative variation (CNV) in no-go trials and onto the continuing CNV in go trials. Similarly, according to Kok [Kok, A., 1986. Effects of degradation of visual stimuli on components of the event-related potential (ERP) in go/nogo reaction tasks. Biological Psychology 23, 21-38], the go/no-go effect is due to movement-related negative potentials, in particular contralateral negativity, adding with P3 in go trials. To investigate these notions, we studied how CNV, go-P3 and no-go P3 are lateralized at fronto-central sites when the side of the response varies across trials, comparing these effects between hand movements and eye movements and delineating them more precisely for hand movements with multichannel recordings. The go/no-go effect was larger and contralaterally lateralized with hand movements than with eye movements. Dipole analysis dissected its components into a large contribution of the medial cingulate gyrus, into activity of motor areas contralateral to the cued hand and a left-frontal source. Motor-related portions of the effect seemed to build upon and extend motor-related components included in CNV. Results provide support for the notion that the go/no-go effect is related to movement-related potentials. We suggest that go-P3 and no-go P3 are characterized by addition and reduction of motor-related activation to the core P3.

ANALYSIS OF PHASE-LOCKING IS INFORMATIVE FOR STUDYING EVENT-RELATED EEG ACTIVITY

Abstract. A new method is presented for quantitative evaluation of single-sweep phase and amplitude electroencephalogram (EEG) characteristics that is a more informative approach in comparison with conventional signal averaging. In the averaged potential, phase-locking and amplitude effects of the EEG response cannot be separated. To overcome this problem, single-trial EEG sweeps are decomposed into separate presentations of their phase relationships and amplitude characteristics. The stability of the phase-coupling to stimulus is then evaluated independently by analyzing the single-sweep phase presentations. The method has the following advantages: information about stability of the phase-locking can be used to assess event-related oscillatory activity; the method permits evaluation of the timing of event-related phase-locking; and a global assessment and comparison of the phase-locking of ensembles of single sweeps elicited in different processing conditions is possible. The method was employed to study auditory alpha and theta responses in young and middle-aged adults. The results showed that whereas amplitudes of frequency responses tended to decrease, the phase-locking increased significantly with age. The synchronization with stimulus (phase-locking) was the only parameter reliably to differentiate the brain responses of the two age groups, as well as to reveal specific age-related changes in frontal evoked alpha activity. Thus, the present approach can be used to evaluate dynamic brain processes more precisely.

Age effects on visual EEG responses reveal distinct frontal alpha networks

OBJECTIVES The present study aimed to describe the effect of aging on single-trial visual alpha responses. METHODS Visual evoked potentials were recorded at F3, Cz, P3, and O1 in 12 young (20-30-year-olds) and in 10 middle-aged adults (50-55-year-olds). Slow (7-10 Hz) and fast (10-15 Hz) alpha frequency bands were analyzed. Three parameters of single alpha responses were assessed for the 0-300 ms period after stimulus: (i) maximal single-sweep amplitude; (ii) phase-locking with stimulus, and (iii) enhancement of post-stimulus relative to pre-stimulus alpha activity. RESULTS Ongoing alpha activity at anterior sites was larger in middle-aged subjects. Age differences in response amplitude depended on the anterior shift of ongoing alpha activity. Over fronto-central areas, the phase-locking of fast alpha responses was significantly increased, whereas the phase-locking of slow alpha responses was decreased in middle-aged compared to young adults, independently of amplitude. In contrast to slow alpha responses, frontal and occipital fast alpha responses were interrelated. CONCLUSIONS These observations are in accordance with previous findings from the auditory modality implying that the age-related changes in frontal alpha oscillations are modality-independent. Slow and fast frontal alpha responses were affected differentially by the age, which might reflect the activations of functionally distinct alpha networks.