Aging-related changes of EEG synchronization during a visual working memory task

Differences of EEG synchronization between normal old and young people during a working memory (WM) task were investigated. The synchronization likelihood (SL) is a novel method to assessed synchronization in multivariate time series for non-stationary systems. To evaluate this method to study the mechanisms of WM, we calculated the SL values in brain electrical activity for both resting state and task state. EEG signals were recorded from 14 young adults and 12 old adults during two different states, respectively. SL was used to measure EEG synchronization between 19 electrodes in delta, theta, alpha1, alpha2 and beta frequency bands. Bad task performance and significantly decreased EEG synchronization were found in old group compared to young group in alpha1, alpha2 and beta frequency bands during the WM task. Moreover, significantly decreased EEG synchronization in beta band in the elder was also detected during the resting state. The findings suggested that reduced EEG synchronization may be one of causes for WM capacity decline along with healthy aging.     

Spatial organization of biopotentials and decision-making time during purposeful mental activity in humans

Features of spatial organization of neocortical potentials were studied in subjects with different decision-making time during performing the task of memorizing and subsequently reproducing, on a monitor screen, a sequence of signals. The subjects with a short decision-making time differed from those with a long decision-making time in a higher level of the intra- and interhemispheric coherence in alpha EEG frequency band different neocortical areas during reproduction of a signal sequence (coherence in the frontal, central and parietal areas; coherence between the right central and the left frontal, central, parietal, occipital and temporal areas; coherence between the left occipital and both the frontal areas).     

Temporal Constraints of Behavioral Inhibition: Relevance of Inter-stimulus Interval in a Go-Nogo Task

The capacity to inhibit prepotent and automatic responses is crucial for proper cognitive and social development, and inhibitory impairments have been considered to be key for some neuropsychiatric conditions. One of the most used paradigms to analyze inhibitory processes is the Go-Nogo task (GNG). This task has been widely used in psychophysical and cognitive EEG studies, and more recently in paradigms using fMRI. However, a technical limitation is that the time resolution of fMRI is poorer than that of the EEG technique. In order to compensate for these temporal constraints, it has become common practice in the fMRI field to use longer inter-stimulus intervals (ISI) than those used in EEG protocols. Despite the noticeable temporal differences between these two techniques, it is currently assumed that both approaches assess similar inhibitory processes. We performed an EEG study using a GNG task with both short ISI (fast-condition, FC, as in EEG protocols) and long ISI (slow-condition, SC, as in fMRI protocols). We found that in the FC there was a stronger Nogo-N2 effect than in the SC. Moreover, in the FC, but not in the SC, the number of preceding Go trials correlated positively with the Nogo-P3 amplitude and with the Go trial reaction time; and negatively with commission errors. In addition, we found significant topographical differences for the Go-P3 elicited in FC and SC, which is interpreted in terms of different neurotransmitter dynamics. Taken together, our results provide evidence that frequency of stimulus presentation in the GNG task strongly modulates the behavioral response and the evoked EEG activity. Therefore, it is likely that short-ISI EEG protocols and long-ISI fMRI protocols do not assess equivalent inhibitory processes.     

Correlations between EEG Characteristics and Contents of Some Micro- and Macroelements (Lead,Strontium, and Calcium) in the Organism of 12- to 13-Year-Old Children Living in Ecologically Unfavourable Regions of Ukraine

In 12- to 13-year-old children, inhabitants of regions of Ukraine with high levels of industrial contamination, we found higher contents of lead and calcium, as compared with the upper limits of the norm, in samples of biologically stable tissues (hairs). Concentrations of strontium were within the limits of reference values. Correlation analysis of the EEG frequency components vs levels of the above-mentioned metals demonstrated the existence of significant (or close to significant) relations between normalized spectral powers of the rhythms of background EEG recorded at different functional states (eyes open/closed, solving an arithmetic task) and concentrations of these elements. According to the number of the correlations found and their strength, the order of significance of the effects of the above elements with respect to the pattern of EEG activity was the following: Ca > Sr > Pb. The intensity of significant correlations varied from mild to moderately strong (0.41 < r < 0.70). The tightest correlations with the levels of all elements were observed under conditions of cognitive loading (solving an arithmetic task). High-frequency EEG components (β1 and β2 rhythms) were found to be the EEG components most sensitive to the contents of the mentioned elements.     

Topographic characteristics of cortex activity in delta-, alpha2-, and gamma2- frequency bands related to social creativity

Social creativity-related topographic changes of the delta, alpha2, and gamma2 power were studied using 19-channel EEG. Originality and fluency indices of social thinking were evaluated on basis of specially developed 'divergent' task solution that stimulated different interpretations of social scenes; complex visual stimuli (architectural building) were used as control task. 'Divergent' task performing was characterized by both the greater power of the delta and alpha2 rhythms mostly in the right hemisphere and widespread increase in the gamma2 power as compared with the control task. Positive correlations between delta and gamma2 rhythms in baseline condition were revealed in the fronto-parietal cortex, and this relationship between low- and high-frequency oscillations while 'divergent' task performing was found in the right posterior cortex. Special topographic patterns of delta and gamma2 activity as predictors of social creativity were obtained using multiple regression analysis. These patterns can be interpreted as EEG correlates of a summation of endogenous and exogenous components of social thinking.     

Mental representation of number in different numerical forms

How are numerical operations implemented within the human brain? It has been suggested that there are at least three different codes for representing number: a verbal code that is used to manipulate number words and perform mental numerical operations (e.g., multiplication), a visual code that is used to decode frequently used visual number forms (e.g., Arabic digits), and an abstract analog code that may be used to represent numerical quantities. Furthermore, each of these codes is associated with a different neural substrate. We extend these studies using dense-sensor event-related EEG recording techniques to investigate the temporal pattern of notation-specific effects observed in a parity judgement (odd versus even) task in which single numbers were presented in one of four different numerical notations. Contrasts between different notations demonstrated clear modulations in the visual evoked potentials (VEP) recorded. We observed increased amplitudes for the P1 and N1 components of the VEP that were specific to Arabic numerals and to dot configurations but differed for random and recognizable (die-face) dot configurations. These results demonstrate clear, notation-specific differences in the time course of numerical information processing and provide electrophysiological support for the triple-code model of numerical representation.     

The cortical interactions in short time intervals during the search for verbal associations

Cortical connectivity was studied in tasks of generating the use of words in comparison with reading aloud the same words. These tasks were used earlier in PET and high-density ERP recording studies, which described both the functional anatomy and time course of involvement of cortical areas in word processing. We developed a new method for studying the synchrony of EEG spectral components within the short time intervals compatible with the duration of particular cognitive operations. The wavelet transform of the ERP records and calculation of correlations between the wavelet curves were used to reveal connections between cortical areas. Three stages of intracortical communications developing over the course of task performance were discovered: between the right and left frontal areas (0-200 ms after the stimulus presentation), between the left frontal and left posterior temporo-parietal areas (250-500 ms), and, finally, between the left temporal and right fronto-centro-temporal areas. These findings are in good agreement with the results of the previous PET and ERP studies and supplement them with the circuitry of cortical information transfer. Also, they suggest some differences in information processing during automated reading and performance of more complicated use-generation task.     

Physiological correlates of individual differences in decision-making time during purposeful mental activity in humans

EEG correlates of individual differences in decision-making time were studied in subjects performing the task of memorizing and subsequently reproducing, on a monitor screen, a sequence of signals. Forty-six students were volunteers in the study, carried out with the use of an original computer-aided technique. Pioneering data on the individual specificity of physiological processes underlying human mental activity were obtained. Individual differences in EEG characteristics related to differences in the temporal parameters of the decision-making stage were found. In a situation directly preceding the activity, subjects characterized by a short decision-making time exhibited higher powers of the Δ (in the occipital, parietal, and central cortical areas) and θ-(in both the central and the right frontal and temporal areas) EEG rhythms. The subjects with a short decision-making time differed from those with a long decision-making time in a higher power of the θ rhythm in the right temporal area during memorization and an increased θ rhythm power in the frontal areas during reproduction of a signal sequence.     

Amplitude modulation of an electroencephalogram, connected with initiation and cessation of movement

Amplitude modulations of electroencephalogram were studied under the condition of single voluntary movements of different duration, their mental reproduction and unvoluntary movements (T-reflex). Specific patterns of the averaging integrated electroencephalogram (waves P1, N1, P2) were detected under initiation or termination of the voluntary movements and their mental reproduction. EEG amplitude modulations were analysed with the accompanying movement of different duration. The experiments showed interferation waves P1, N1, P2 under brief movements. These specific patterns of EEG could be represented by the efferent and afferent components of the central motor programs.     

Assessment of EEG frequency dynamics using complex demodulation.

The complex demodulation (CD) approach was applied to human EEG recorded during a cognitive task performance, including voluntary goal-directed movements. The standard CD algorithm was extended by a simple procedure using frequency histograms and power spectra to select the characteristic frequencies of EEG segments around the task performance. In the majority of records, amplitude modulation was found, which decreased or disappeared in the period prior to and at the very beginning of the task performance. It was found that the decrease of modulation in fast beta and gamma components begins approximately one second before that of the alpha components. Frequency modulation appeared in some records at the end of the task in beta and gamma components. The results showed that a cognitive task performance is accompanied by non-linear processes in the frequency components of EEG. These dynamic changes could extend the findings of event-related desynchronization obtained by linear methods.     

Detecting determinism in short time series, with an application to the analysis of a stationary EEG recording

We have developed a new method for detecting determinism in a short time series and used this method to examine whether a stationary EEG is deterministic or stochastic. The method is based on the observation that the trajectory of a time series generated from a differentiable dynamical system behaves smoothly in an embedded phase space. The angles between two successive directional vectors in the trajectory reconstructed from a time series at a minimum embedding dimension were calculated as a function of time. We measured the irregularity of the angle variations obtained from the time series using second-order difference plots and central tendency measures, and compared these values with those from surrogate data. The ability of the proposed method to distinguish between chaotic and stochastic dynamics is demonstrated through a number of simulated time series, including data from Lorenz, R?ssler, and Van der Pol attractors, high-dimensional equations, and 1/f noise. We then applied this method to the analysis of stationary segments of EEG recordings consisting of 750 data points (6-s segments) from five normal subjects. The stationary EEG segments were not found to exhibit deterministic components. This method can be used to analyze determinism in short time series, such as those from physiological recordings, that can be modeled using differentiable dynamical processes.     

Simultaneous fMRI and EEG during the Multi-Source Interference Task

Background

fMRI and EEG are two non-invasive functional imaging techniques within cognitive neuroscience that have complementary advantages to obtain both temporal and spatial information. The multi-source interference task (MSIT) has been shown to generate robust activations of the dorsal anterior cingulate cortex (dACC) on both a single-subject level and in group averages, in fMRI studies. We have now simultaneously acquired fMRI and EEG during a cognitive interference task.

Materials and Methods

Healthy volunteers were tested in an MRI scanner with simultaneous EEG and fMRI recordings during the MSIT.

Results

The interference condition significantly increased the reaction time in the task. The fMRI analyses revealed activation of dACC as expected, in all subjects at the individual level and in group analyses. The posterior cingulate cortex was de-activated. Simultaneous EEG showed the expected anterior distribution of the interference effect, as it was restricted to frontal sites within a time frame of 80–120 ms post response.

Conclusion

The MSIT task is a reliable task for interference evaluation. fMRI shows robust activation of dACC and by adding EEG, an interference effect can be noticed within a temporal interval of 80–120 ms after the response, as a CRN (correct response negativity). This means that EEG could add a more detailed temporal aspect to the fMRI data from an interference task, and that despite the hostile environment within an MRI scanner, EEG data could be used.     

Dynamics of the gamma-responses in an 8-second interval between facial and trigger stimuli as dependent the success of task performance

A cognitive set to facial expression was used as a model with the loading on working memory being increased by increasing the interval between the facial and triggering stimuli to 8 seconds. The aim was to determine whether the intensity of brain potentials evoked in a range of 41–60 Hz (the range 15–60 Hz was used) by facial stimuli is associated with the “success” of task performance (mistake rate). An index of average amplitudes of EEG oscillations was used to measure the response to facial stimuli, and γ responses proved to be associated with the number of mistakes in performing the task. The results make it possible to consider the γ responses to facial stimuli as an EEG correlate of the internal states that correspond to adequate actions of the subject in the test with a 8-s interval between the facial and trigger stimuli.     

Reflection of the Mental Reproduction of Emotional Experiences in the EEG Pattern of 10- to 11-Year-Old Children

We studied the effects of mental reproduction of emotions on the EEG characteristics in 10- to 11-year-old children. Independently of the sign of emotions, their reproduction was related to a rise in the modal alpha frequency and to a decrease in the spectral power of the EEG alpha1 subcomponent. Increases in the power of the beta-rhythm and in the ratio between the beta rhythm power and that of the theta rhythm were more manifested upon mental reproduction of positive emotions.     

Independent EEG sources are dipolar

Independent component analysis (ICA) and blind source separation (BSS) methods are increasingly used to separate individual brain and non-brain source signals mixed by volume conduction in electroencephalographic (EEG) and other electrophysiological recordings. We compared results of decomposing thirteen 71-channel human scalp EEG datasets by 22 ICA and BSS algorithms, assessing the pairwise mutual information (PMI) in scalp channel pairs, the remaining PMI in component pairs, the overall mutual information reduction (MIR) effected by each decomposition, and decomposition 'dipolarity' defined as the number of component scalp maps matching the projection of a single equivalent dipole with less than a given residual variance. The least well-performing algorithm was principal component analysis (PCA); best performing were AMICA and other likelihood/mutual information based ICA methods. Though these and other commonly-used decomposition methods returned many similar components, across 18 ICA/BSS algorithms mean dipolarity varied linearly with both MIR and with PMI remaining between the resulting component time courses, a result compatible with an interpretation of many maximally independent EEG components as being volume-conducted projections of partially-synchronous local cortical field activity within single compact cortical domains. To encourage further method comparisons, the data and software used to prepare the results have been made available (http://sccn.ucsd.edu/wiki/BSSComparison).     

Survival and reproductive rate of mites in relation to resistance of their barn swallow hosts

Parasite resistance may act via a number of different mechanisms that regulate or control the survival and the reproductive rate of parasites. Observations and experiments were used to test for effects of host resistance on parasite survival and rate of reproduction. Natural levels of infestation of barn swallow Hirundo rustica nests by the tropical fowl mite Ornithonyssus bursa were positively related to brood size, inversely related to the length of the outermost tail feathers of male nest owners (a secondary sexual character) and affected by time of reproduction by the host. A mite inoculation experiment, in which 50 adult mites were introduced into nests during the laying period of the host, was used to test for differential survival and reproduction of mites as a function of host resistance. The relationship between survival and reproduction of parasites, male tail length and host resistance was investigated. There was a negative relationship between mite numbers per nest after fledging of nestlings and male tail length. This relationship was mainly caused by a reduction in the number of mites in the first and second nymph stage with increasing tail length of male hosts, implying a reduction in rate of reproduction of mites. The proportion of mites that had recently fed was inversely related to tail length of male hosts. The proportion of nymph stages was positively related to the proportion of mites that had recently had a blood meal. Parasite resistance of barn swallows to the tropical fowl mite thus appeared to act through increased mortality rate of adult and nymph stages of mites, and through reduced reproductive rates of mites on resistant hosts. This is the first study demonstating a direct relationship between fitness components of a parasite and the expression of a secondary sexual character of a host. Received: 11 January 2000 / Accepted: 22 March 2000     

Selection of valid and reliable EEG features for predicting auditory and visual alertness levels

A selection procedure with three rules, high efficiency, low individual variability, and low redundancy, was developed to screen electroencephalogram (EEG) features for predicting behavioral alertness levels. A total of 24 EEG features were derived from temporal, frequency spectral, and statistical analyses. Behavioral alertness levels were quantified by correct rates of performance on an auditory and a visual vigilance task, separately. In the auditory task study, a subset of three EEG features, the relative spectral amplitudes in the alpha (alpha%, 8-13 Hz) and theta (theta%, 4-8 Hz) bands, and the mean frequency of the EEG spectrum (MF), was found to be the best combination for predicting the auditory alertness level. In the visual task study, the mean frequency of the beta band (Fbeta, 13-32 Hz) was the only EEG feature selected. The application of an averaging subwindow procedure within a moving time window to EEG analysis increased the predictive power of EEG features and decreased the disturbing effect of movement artifacts on the EEG data.     

Interval Timing Deficits Assessed by Time Reproduction Dual Tasks as Cognitive Endophenotypes for Attention-Deficit/Hyperactivity Disorder

The literature has suggested timing processing as a potential endophenotype for attention deficit/hyperactivity disorder (ADHD); however, whether the subjective internal clock speed presented by verbal estimation and limited attention capacity presented by time reproduction could be endophenotypes for ADHD is still unknown. We assessed 223 youths with DSM-IV ADHD (age range: 10-17 years), 105 unaffected siblings, and 84 typically developing (TD) youths using psychiatric interviews, intelligence tests, verbal estimation and time reproduction tasks (single task and simple and difficult dual tasks) at 5-second, 12-second, and 17-second intervals. We found that youths with ADHD tended to overestimate time in verbal estimation more than their unaffected siblings and TD youths, implying that fast subjective internal clock speed might be a characteristic of ADHD, rather than an endophenotype for ADHD. Youths with ADHD and their unaffected siblings were less precise in time reproduction dual tasks than TD youths. The magnitude of estimated errors in time reproduction was greater in youths with ADHD and their unaffected siblings than in TD youths, with an increased time interval at the 17-second interval and with increased task demands on both simple and difficult dual tasks versus the single task. Increased impaired time reproduction in dual tasks with increased intervals and task demands were shown in youths with ADHD and their unaffected siblings, suggesting that time reproduction deficits explained by limited attention capacity might be a useful endophenotype of ADHD.     

Late divergence of target and nontarget ERPs in a visual oddball task

Different mental operations were expected in the late phase of intracerebral ERPs obtained in the visual oddball task with mental counting. Therefore we searched for late divergences of target and nontarget ERPs followed by components exceeding the temporal window of the P300 wave. Electrical activity from 152 brain regions of 14 epileptic patients was recorded by means of depth electrodes. Average target and nontarget records from 1800 ms long EEG periods free of epileptic activity were compared. Late divergence preceded by almost identical course of the target and nontarget ERPs was found in 16 brain regions of 6 patients. The mean latency of the divergence point was 570+/-93 ms after the stimulus onset. The target post-divergence section of the ERP differed from the nontarget one by opposite polarity, different latency of the components, or even different number of the components. Generators of post-divergence ERP components were found in the parahippocampal gyrus, superior, middle and inferior temporal gyri, amygdala, and fronto-orbital cortex. Finding of late divergence indicates that functional differences exist even not sooner than during the final phase of the task.     

Improving spatial and temporal resolution in evoked EEG responses using surface Laplacians

Spline generated surface Laplacian temporal wave forms are presented as a method to improve both spatial and temporal resolution of evoked EEG responses. Middle latency and the N1 components of the auditory evoked response were used to compare potential-based methods with surface Laplacian methods in the time domain. Results indicate that surface Laplacians provide better estimates of underlying cortical activity than do potential wave forms. Spatial discrimination among electrode sites was markedly better with surface Laplacian than with potential wave forms. Differences in the number and latencies of peaks, and their topographic distributions, were observed for surface Laplacian, particularly during the time period encompassing the middle latency responses. Focal activities were observed in surface Laplacian wave forms and topographic maps which were in agreement with previous findings from auditory evoked response studies. Methodological issues surrounding the application of spline methods to the time domain are also discussed. Surface Laplacian methods in the time domain appear to provide an improved way for studying evoked EEG responses by increasing temporal and spatial resolution of component characteristics.