Transcranial alternating current stimulation enhances individual alpha activity in human EEG

Non-invasive electrical stimulation of the human cortex by means of transcranial direct current stimulation (tDCS) has been instrumental in a number of important discoveries in the field of human cortical function and has become a well-established method for evaluating brain function in healthy human participants. Recently, transcranial alternating current stimulation (tACS) has been introduced to directly modulate the ongoing rhythmic brain activity by the application of oscillatory currents on the human scalp. Until now the efficiency of tACS in modulating rhythmic brain activity has been indicated only by inference from perceptual and behavioural consequences of electrical stimulation. No direct electrophysiological evidence of tACS has been reported. We delivered tACS over the occipital cortex of 10 healthy participants to entrain the neuronal oscillatory activity in their individual alpha frequency range and compared results with those from a separate group of participants receiving sham stimulation. The tACS but not the sham stimulation elevated the endogenous alpha power in parieto-central electrodes of the electroencephalogram. Additionally, in a network of spiking neurons, we simulated how tACS can be affected even after the end of stimulation. The results show that spike-timing-dependent plasticity (STDP) selectively modulates synapses depending on the resonance frequencies of the neural circuits that they belong to. Thus, tACS influences STDP which in turn results in aftereffects upon neural activity.The present findings are the first direct electrophysiological evidence of an interaction of tACS and ongoing oscillatory activity in the human cortex. The data demonstrate the ability of tACS to specifically modulate oscillatory brain activity and show its potential both at fostering knowledge on the functional significance of brain oscillations and for therapeutic application.     

Dynamic characteristics of the human resonance EEG responses to rhythmic photostimulation

The development of the resonance EEG responses of the left and right occipital areas was studied in right-handed men during prolonged (12 or 120 s) rhythmic, photostimulation with the intensity of 0.7 J and frequencies of 6, 10, and 16 Hz. Analysis of the EEG fine spectral structure was applied to compare the accumulated baseline EEG spectra and EEG spectra during photostimulation, to observe the dynamics of the short-term spectra and to detect power changes in the EEG narrow spectral band sharply coincident with the stimulation frequency. The more pronounced EEG responses to photostimulation were observed in subjects with the initially low EEG baseline, α-rhythm. Two-minute flash trains produced a substantial increase in the EEG power within the stimulation frequency with superposed oscillatory processes with different periods. These fluctuations are considered a reflection of intricate interaction between the adaptive and resonance EEG responses to the presented intermittent stimulation. Under 12-s stimulation the resonance EEG responses are steadily recorded within the first 3 s of stimulation and immediately after the flash cessation EEG power at the stimulation frequency returns to the initial level. The resonance EEG responses were more pronounced in the right hemisphere than in the left one, especially, at the stimulation frequencies of 6 and 16 Hz. With increasing the stimulation frequency, the maximum of resonance EEG responses was reached earlier. Under the stimulation frequency of 6 Hz, the maximal response was recorded 9–12 s after the beginning of flashes, at the frequencies of 10 and 16 Hz, it was recorded within 3–6 and 3 s, respectively.     

Dissociation of subjectively reported and behaviorally indexed mind wandering by EEG rhythmic activity

Inattention to current activity is ubiquitous in everyday situations. Mind wandering is an example of such a state, and its related brain areas have been examined in the literature. However, there is no clear evidence regarding neural rhythmic activities linked to mind wandering. Using a vigilance task with thought sampling and electroencephalography recording, the current study simultaneously examined neural oscillatory activities related to subjectively reported and behaviorally indexed mind wandering. By implementing time-frequency analysis, we found that subjectively reported mind wandering, relative to behaviorally indexed, showed increased gamma band activity at bilateral frontal-central areas. By means of beamformer source imaging, we found subjectively reported mind wandering within the gamma band to be characterized by increased activation in bilateral frontal cortices, supplemental motor area, paracentral cortex and right inferior temporal cortex in comparison to behaviorally indexed mind wandering. These findings dissociate subjectively reported and behaviorally indexed mind wandering and suggest that a higher degree of executive control processes are engaged in subjectively reported mind wandering.     

Changes in human EEG alpha activity following exposure to two different pulsed magnetic field sequences

The present study investigates the effects of a weak (+/-200 microT(pk)), pulsed, extremely low frequency magnetic field (ELF MF) upon the human electroencephalogram (EEG). We have previously determined that exposure to pulsed ELF MFs can affect the EEG, notably the alpha frequency (8-13 Hz) over the occipital-parietal region of the scalp. In the present study, subjects (n = 32) were exposed to two different pulsed MF sequences (1 and 2, used previously) that differed in presentation rate, in order to examine the effects upon the alpha frequency of the human EEG. Results suggest that compared to sham exposure, alpha activity was lowered over the occipital-parietal regions of the brain during exposure to Sequence 1, while alpha activity over the same regions was higher after Sequence 2 exposure. These effects occurred after approximately 5 min of pulsed MF exposure. The results also suggest that a previous exposure to the pulsed MF sequence determined subjects' responses in the present experiment. This study supports our previous observation of EEG changes after 5 min pulsed ELF MF exposure. The results of this study are also consistent with existing EEG experiments of ELF MF and mobile phone effects upon the brain.     

The rhythmic structure of the human EEG: the current state and trends of the research

During the past decade, spectral analysis has become indispensable instrument for different kinds of EEG processing. With the development of dedicated computer system, investigation of shifts in human EEG rhythm under various conditions has improved considerably. However, it is difficult to make general conclusions from this line of research, since a large number of studies are carried out using the ambiguous experimental approaches and different methods. Present paper aims to evaluate a modern state of the art in the field of human EEG rhythmical structure investigation. The results from recent relevant articles are briefly reviewed according to the universal scheme (EEG rhythm--experimental condition--observed effect). Due to such presentation, the obtained results have been summarized and some tendencies of modern investigations have been revealed. The extension of studied frequency range of rhythmical EEG components to both high (> 35 Hz) and low (< 1 Hz) frequencies, the shift to a more detailed spectral structure analysis simultaneously with ignoring the fixed boundaries of traditional EEG rhythms, the growing attempts to reveal EEG rhythmical structure correlates of cognitive activity, and a wide utilization of dynamic approaches for the analysis of brain electrical activity are discussed in some detail. The observed data are indicate of high functional significance and perspectives of human EEG rhythmical structure investigation.     

Resonance phenomena to rhythmic light stimulation with various intensity and frequency in human EEG

The photoinduced resonance EEG response in the occipital area (O1 and O2) of right-handed men during 12-s intermittent photic stimulation was studied as a function of flash frequency (6, 10, or 16 Hz) and intensity (5 levels from 0.05 to 0.7 J). The EEG power in the narrow band coinciding with stimulation frequency was FFT-extracted in 3-s intervals before, during, and after each stimulation. It was found that increase in flash intensity was accompanied by an enhancement of the resonance EEG response and decrease in time of reaching its maximal value. These changes were to a greater extent characteristic for the right hemisphere. The low-intensity stimulation induced more pronounced resonance effects in the left hemisphere, whereas the high-intensity flashes to a greater extent involved the right hemisphere. The asymmetry of the EEG response to stimulation of the middle intensity was slight, and the time of reaching the maximal level of the resonance activation was about 6-8 s. A relatively high level of the resonance EEG response was observed during stimulation with the frequency of 10 Hz, even in case of its minimal intensity. The most pronounced resonance EEG response was induced in the right occipital area by the high-intensity 16-Hz stimulation. The enhanced sensitivity of the right hemisphere to intensity of flashes is interpreted as an indication of interhemispheric differences in nonspecific adaptive mechanisms of the brain.     

Modifications in the human EEG during extralong physical activity

In a single-case study, the effects of very long-lasting physical exercise, namely a 24-h-long ultramarathon, on the human electroencephalogram (EEG) were evaluated. While only effects of relatively short exercise have been reported earlier, we focused on the changes induced by these long-lasting physical requires. EEG was recorded repeatedly using an auditory oddball paradigm, and event-related potentials (ERPs), as well as changes in the current oscillatory brain activity (in particular, event-related desynchronization, ERD), were repeatedly monitored. While an increase in several attention-related ERP parameters was reported for shorter exercises, the results of our study show that cognitive performance-related EEG phenomena slowly decreased throughout the race. The P300 amplitude decreased, and the P300 latency increased with ongoing exercise duration. In addition, the difference between standard and target tones at N200b, as well as the difference in the lower alpha ERD, decreased with time, indicating a reduced automatic stimulus evaluation. Neirofiziologiya/Neurophysiology, Vol. 39, No. 1, pp. 81–87, January–February, 2007.     

Segmental structure of the EEG alpha activity in adolescents with disorders of schizophrenic spectrum

The EEG records of 39 healthy adolescents and 45 age-matched schizophrenic patients were analyzed. The broad-band EEG spectral analysis and segmental analysis of the alpha-activity revealed significant differences between the groups. Schizophrenics differed in a decreased spectral power for the alpha2 and betal frequency bands and increased power for the delta and theta bands. Also, in schizophrenic adolescents, quasi-stationary alpha-rhythm segments were longer, and within-segmental EEG amplitudes were higher than in the healthy subjects; the amplitude variability and the steepness of transitions between neighbor segments were increased. The results of the EEG segmental analysis suggest a disintegration of local cortical neuronal ensembles in schizophrenia.     

Possible contributions of CPG activity to the control of rhythmic human arm movement

There is extensive modulation of cutaneous and H-reflexes during rhythmic leg movement in humans. Mechanisms controlling reflex modulation (e.g., phase- and task-dependent modulation, and reflex reversal) during leg movements have been ascribed to the activity of spinal central pattern generating (CPG) networks and peripheral feedback. Our working hypothesis has been that neural mechanisms (i.e., CPGs) controlling rhythmic movement are conserved between the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that for rhythmic leg movement. This hypothesis has been tested by studying the regulation of reflexes in arm muscles during rhythmic arm cycling and treadmill walking. This paper reviews recent studies that have revealed that reflexes in arm muscles show modulation within the movement cycle (e.g., phase-dependency and reflex reversal) and between static and rhythmic motor tasks (e.g., task-dependency). It is concluded that reflexes are modulated similarly during rhythmic movement of the upper and lower limbs, suggesting similar motor control mechanisms. One notable exception to this pattern is a failure of contralateral arm movement to modulate reflex amplitude, which contrasts directly with observations from the leg. Overall, the data support the hypothesis that CPG activity contributes to the neural control of rhythmic arm movement.     

Parental generalized EEG alpha activity predisposes to spike wave discharges in offspring

Familial and twin studies have shown that the individual variability of the normal human electroencephalogram (EEG) is largely genetically determined. In epileptology, these genetic parameters of the EEG background activity are almost totally neglected. The aim of the present study has been to investigate whether a special genetic type of background activity might be related to the pathogenesis of epilepsy. EEG recordings of parents of 257 epileptic children were evaluated retrospectively. Some 156 healthy adults served as controls. Special attention was paid to alpha activity extending to the frontal region, both in bipolar and in referential recordings (Alpha I). Alpha I was found significantly more often in parents of children with primary generalized epilepsy (18%) compared with parents of children with focal epilepsy (8%) or controls (9%). In a second step, parental EEGs of children with different EEG patterns associated with epilepsy were studied. Alpha I was found significantly more often in parents of children with focal sharp waves and generalized spikes and waves (26%) than in parents of probands with focal sharp waves without additional generalized spikes and waves (8%) or in controls (9%). Parents of probands with theta rhythms and spikes and waves had alpha I significantly more often (18%) than parents of probands with theta rhythms without additional spikes and waves (8%) or controls (9%). The findings reveal a clear correlation between the type of EEG background activity in parents and the EEG characteristics in their children, thus pointing to common mechanisms.