Power Spectral Measurements Research Articles

EEG and coherence responses to monetary gain and loss during a memory task: Effects of attentional impulsivity and learning ability

This study investigated electrophysiological (EEG) measures as a function of individual differences in facets of impulsivity, learning ability, and learning practice during an instrumental learning task. Data analysis (48 women participants) was based on EEG activity within 1-s epoch following visual feedback signals of monetary gain and loss. The EEG spectral power and coherence measures were derived from 30 scalp sites for delta (0.5–3.5 Hz), theta (3.5–7 Hz), lower alpha (7–9.5 Hz), upper alpha (10–12.5 Hz), beta (15–20 Hz), and gammabands (36–44 Hz). High attentional, impulsive individuals displayed a greater error rate than low attentional impulsives. Monetary loss produced greater spectral power than monetary gain for the theta band over frontal scalp sites, while a greater upper alpha power was found for monetary gain across centroparieto-occipital sites. Learning taskpracticewasassociatedwith increased lower alphapowerover centro-parietal sites, increasedupper alphapower over parietal sites, and reduced gamma power over left temporal sites. Good learners as compared to bad learners had (1) enhanced theta on centro-parietal sites and reducedon frontal sites, (2) enhancedbetapower on temporal sites and reduced on parieto-occipital sites, (3) enhanced gamma power on fronto-temporal sites for both monetary loss and gain signals. Good learner/low attentional impulsivity participants, for both monetary loss and gain signals, had a greater gamma power on temporal sites as compared to bad learner/high and low attentional impulsivity groups. The enhanced error rate and the reduced gamma power found in high attentional impulsive subjects are consistent with a number of previous studies indicating frontal lobe dysfunction in association with impulsive behavior and may indicate that impulsive individuals make more effort in activating temporal regions for memory updating. Across theta, upper alpha and gamma frequencybands, good learners had greater fronto-parietal coherences than bad learners, andmonetary loss displayed greater coherences than monetary gain. This indicates a leading role of a fronto-parietal network in outcome processing for memory consolidation and learning. Finally, good learner/high attentional impulsivity subjects, as compared to the other groups, had the highest enhancement of theta, upper alpha, and gamma coherences at far distant cortical regions for monetary loss. This finding outlines the importance of punishment signals in the learning process and suggests that individual differences in attentional impulsivity play an important role in learning ability.

Manifestation of ocular-muscle EMG contamination in human intracranial recordings

It is widely assumed that intracranial recordings from the brain are only minimally affected by contamination due to ocular-muscle electromyogram (oEMG). Here we show that this is not always the case. In intracranial recordings from five surgical epilepsy patients we observed that eye movements caused a transient biphasic potential at the onset of a saccade, resembling the saccadic spike potential commonly seen in scalp EEG, accompanied by an increase in broadband power between 20 and 200Hz. Using concurrently recorded eye movements and high-density intracranial EEG (iEEG) we developed a detailed overview of the spatial distribution and temporal characteristics of the saccade-related oculomotor signal within recordings from ventral, medial and lateral temporal cortex. The occurrence of the saccadic spike was not explained solely by reference contact location, and was observed near the temporal pole for small (<2deg) amplitude saccades and over a broad area for larger saccades. We further examined the influence of saccade-related oEMG contamination on measurements of spectral power and interchannel coherence. Contamination manifested in both spectral power and coherence measurements, in particular, over the anterior half of the ventral and medial temporal lobe. Next, we compared methods for removing the contaminating signal and found that nearest-neighbor bipolar re-referencing and ICA filtering were effective for suppressing oEMG at locations far from the orbits, but tended to leave some residual contamination at the temporal pole. Finally, we show that genuine cortical broadband gamma responses observed in averaged data from ventral temporal cortex can bear a striking similarity in time course and band-width to oEMG contamination recorded at more anterior locations. We conclude that eye movement-related contamination should be ruled out when reporting high gamma responses in human intracranial recordings, especially those obtained near anterior and medial temporal lobe.

EEG Power Spectral Measurements Comparing Normal and “Thatcherized” Faces

ABSTRACTIn this paper we have made a broadband analysis to detect the electroencephalogram (EEG) frequencies that change selectively during the presentation of normal and “thatcherized” faces. Referential recordings to linked ears were obtained in 21 leads in 48 right-handed healthy male volunteers. Increase of delta power (1.75–3.91 Hz) was observed, related to the detection of distortion in faces at bifrontal and left temporoparietal cortex. The other bands had no contribution, when normal and modified faces were compared. These results support our hypothesis that a change in EEG power spectral may be related to discrimination between normal and thatcherized faces.

Voice Activity Detection Based on Discriminative Weight Training Incorporating a Spectral Flatness Measure

In this paper, we present an approach to incorporate discriminative weight training into a statistical model-based voice activity detection (VAD) method. In our approach, the VAD decision rule is derived from the optimally weighted likelihood ratios (LRs) using a minimum classification error (MCE) method. An adaptive on-line means of selecting two kinds of weights based on a power spectral flatness measure (PSFM) is devised for performance improvement. The proposed approach is compared to conventional schemes under various noise conditions, and shows better performance.

Daytime sleepiness, psychomotor performance, waking EEG spectra and evoked potentials in women with severe premenstrual syndrome

We assessed daytime sleepiness using objective and subjective measures in women with severe premenstrual syndrome (PMS) compared with women without significant premenstrual symptoms. Nine women with severe PMS and eight controls (aged 18-40 years) completed a laboratory-based daytime protocol including the maintenance of wakefulness test (MWT), psychomotor vigilance task (PVT), quantitative waking electroencephalogram (EEG), auditory and visual event-related potentials (ERPs), and sleepiness and mood scales during the mid-follicular and late-luteal (premenstrual) phases of the menstrual cycle. In association with increased perceived sleepiness, fatigue and other premenstrual symptoms in the late-luteal phase, women with PMS performed more poorly on the PVT, with increased lapses and slower reaction times (P < 0.05), compared with the follicular phase and controls. However, there were no significant group or menstrual phase differences in latency to sleep on the MWT. Waking spectral EEG power and ERP measures also did not differentiate PMS women when symptomatic. Both groups of women displayed increased spectral power in the delta/theta frequencies (2-6 Hz) and fast alpha frequency (11-12 Hz) in the late-luteal phase relative to the follicular phase. Trait-like differences were apparent in that women with PMS had increased beta1 (12-16 Hz) power and smaller P300 amplitude than controls in both menstrual cycle phases. Our findings indicate that women with severe PMS are subjectively sleepy and fatigued, and show psychomotor slowing when symptomatic compared with when they are symptom-free and compared with controls. However, the ability to maintain wakefulness under soporific conditions, spectral properties of waking EEG and cognitive processing do not vary in synchrony with premenstrual symptoms.

Impact of alcoholism on sleep architecture and EEG power spectra in men and women.

To determine the impact of alcoholism on sleep architecture and sleep EEG power spectra in men and women with uncomplicated alcoholism. 42 alcoholics (27 men) and 42 controls (19 men) screened for medical, psychiatric, and sleep problems participated in a full night of polysomnography following an adaptation night. Data were collected from multiple scalp sites and subjected to power spectral analysis. Sleep architecture and EEG spectral power measures were evaluated for the effects of diagnosis and sex using age as a covariate. Compared with controls, alcoholics had less slow wave sleep and increased proportions of stage 1 and REM sleep. Spectral analysis of NREM sleep showed reduced levels of slow wave activity (SWA, 0.3-4 Hz) and slow theta (theta) power (4-6 Hz) in alcoholics. The differences in SWA extended across the slow band (0.3-1 Hz) and all delta (delta) frequencies and were most prominent over frontal scalp regions. No group differences were seen in the power spectra of REM sleep. Women had more SWA and theta power than men, but there were no sex by diagnosis interactions for any measures, suggesting that alcoholism does not differentially influence men and women. Long-term alcoholism affects sleep even after long periods of abstinence in both men and women. Measures of frontal slow wave activity were particularly sensitive markers of this long-lasting effect. Sleep EEG measures would thus seem to provide a functional correlate of the changes in brain structure seen in frontal cortex of long-term alcoholics.

Properties of dayside outer zone chorus during HILDCAA events: Loss of energetic electrons

The dayside outer zone (DOZ) portion of the magnetosphere is a region where chorus intensities are statistically found to be the most intense. In this study, DOZ chorus have been examined using OGO‐5 plasma wave and GEOTAIL plasma wave, magnetic field and energetic particle data. Dayside chorus is noted to be composed of ∼0.1 to 0.5 s rising‐tone emissions called “elements.” The duration of the elements and their frequency‐time characteristics are repeatable throughout the chorus event (lasting from tens of minutes to hours), but may differ from event to event. Chorus is a right‐hand, circularly polarized electromagnetic plane wave. Waves are detected propagating from along the ambient magnetic field, Bo, to oblique angles near the Gendrin angle, θGendrin. All waves, independent of wave direction of propagation relative to Bo, are found to be circularly polarized, to first order. Chorus rising‐tone elements are composed of coherent “subelements” or “packets” with durations of ∼5 to 10 ms. Consecutive subelements/packets step in frequency with time to form the elements. The peak amplitudes within a packet can be ∼0.2 nT or greater. The subelement or packet amplitudes are at least an order of magnitude larger than previously estimated chorus amplitudes obtained by power spectral measurements. This discrepancy is due to the presence of interspacings between chorus elements, the interspacings between subelements/packets within an element, and the different frequencies of subelements/packets within a rising‐tone. DOZ chorus studied here were found to be consistent with generation via the loss cone instability of substorm‐injected temperature‐anisotropic (/ &gt; 1) E = 5 to 40 keV electrons drifting from the midnight sector to the DOZ region. Using a large amplitude subelement/packet wave magnetic field amplitude of ∼0.2 nT, it is shown that the instantaneous Kennel‐Petschek pitch angle diffusion rate Dαα is ∼5 × 10−2 s−1. This latter quantity is based on incoherent waves. If energetic electrons stay in cyclotron resonance throughout their interaction with a coherent subelement of duration 10 ms, they may be “pitch angle transported” by ∼5°. Therefore electrons within 5° of the loss cone can be lost in a single wave‐particle interaction. Several such interactions as the electrons traverse the wave region can lead to much larger pitch angle transport angles. The similar time‐scales of chorus elements and bremsstrahlung X‐ray microbursts (∼0.5 s) can be explained by the “pitch angle transport” mechanism described above. Increasing and decreasing pitch angle transport via this mechanism will lead to much higher pitch angle diffusion or “super diffusion” rates. Isotropic unpolarized noise of ∼20 pT peak amplitude has also been detected. The noise is well above instrument noise levels and is speculated to be remnants of chorus or hiss.

Dynamics of Long-Wavelength Fluctuations in a Fluid Layer Heated from Above

We report an experimental study of the dynamics of thermal fluctuations in a 4.86 mm thick layer of CS2 heated from above. Stabilizing gradients ranged from 10.3 to 61.7 K/cm. Power spectral measurements were made over the wave vector range 9 cm(-1)<q<100 cm(-1), using a shadowgraph apparatus. Over this range, the spectra change from having a well-resolved oscillatory mode at low q to monotonic decay at higher q. For the smallest gradient, the results for the power spectra compare very well with theory, but slight deviations are seen for larger gradients.

Power Doppler and spectral Doppler measurements of knee-joint synovitis in rheumatoid arthritis patients with superficial pattern signals and in those with deep pattern signals

Power Doppler and spectral Doppler ultrasonography were used to scan 127 knee joints of 72 patients with rheumatoid arthritis (RA). Synovial effusion thickness and synovial proliferation (pannus) thickness, as well as the flow signal diameter, were measured on ultrasonogram prints of the power Doppler using digital calipers. In addition, color-flow signal grades on power Doppler and the resistance index (RI) values on spectral Doppler were evaluated. The values of these five variables were compared among 58 joints with superficial pattern flow signals and 69 joints with deep pattern flow signals. Compared with the joints with deep pattern signals, the joints with superficial pattern signals had significantly higher mean values of effusion thickness (P < 0.0001) and flow signal grades (P < 0.0001), and significantly lower mean RI (P < 0.0001). On the other hand, the joints with deep pattern signals had a significantly higher value of signal diameter (P = 0.0125) and had a trend to higher value of pannus thickness (P = 0.079) as well. Significant correlations were observed between effusion thickness and signal grades (P < 0.0001); effusion thickness and RI (P < 0.0001); signal diameter and pannus thickness (P = 0.0102); signal diameter and RI (P < 0.0001); and signal grades and RI (P < 0.0001). The ultrasonographic measurements of synovitis in RA patients provide valuable information on synovial inflammation.

Power Doppler and spectral Doppler measurements of knee-joint synovitis in rheumatoid arthritis patients with superficial pattern signals and in those with deep pattern signals

Power Doppler and spectral Doppler ultrasonography were used to scan 127 knee joints of 72 patients with rheumatoid arthritis (RA). Synovial effusion thickness and synovial proliferation (pannus) thickness, as well as the flow signal diameter, were measured on ultrasonogram prints of the power Doppler using digital calipers. In addition, color-flow signal grades on power Doppler and the resistance index (RI) values on spectral Doppler were evaluated. The values of these five variables were compared among 58 joints with superficial pattern flow signals and 69 joints with deep pattern flow signals. Compared with the joints with deep pattern signals, the joints with superficial pattern signals had significantly higher mean values of effusion thickness (P < 0.0001) and flow signal grades (P < 0.0001), and significantly lower mean RI (P < 0.0001). On the other hand, the joints with deep pattern signals had a significantly higher value of signal diameter (P = 0.0125) and had a trend to higher value of pannus thickness (P = 0.079) as well. Significant correlations were observed between effusion thickness and signal grades (P < 0.0001); effusion thickness and RI (P < 0.0001); signal diameter and pannus thickness (P = 0.0102); signal diameter and RI (P < 0.0001); and signal grades and RI (P < 0.0001). The ultrasonographic measurements of synovitis in RA patients provide valuable information on synovial inflammation.

Calibration of optical tweezers with positional detection in the back focal plane

We explain and demonstrate a new method of force and position calibrations for optical tweezers with back-focal-plane photodetection. The method combines power spectral measurements of thermal motion and the response to a sinusoidal motion of a translation stage. It consequently does not use the drag coefficient of the trapped object as an input. Thus, neither the viscosity, nor the size of the trapped object, nor its distance to nearby surfaces needs to be known. The method requires only a low level of instrumentation and can be applied in situ in all spatial dimensions. It is both accurate and precise: true values are returned, with small error bars. We tested this experimentally, near and far from surfaces in the lateral directions. Both position and force calibrations were accurate to within 3%. To calibrate, we moved the sample with a piezoelectric translation stage, but the laser beam could be moved instead, e.g., by acousto-optic deflectors. Near surfaces, this precision requires an improved formula for the hydrodynamical interaction between an infinite plane and a microsphere in nonconstant motion parallel to it. We give such a formula.

The role of the neck and trunk in facilitating head stability during walking

An apparent goal of the human postural system is to maintain head stability during walking. Although much is known about sensory-motor stabilising mechanisms associated with the head and neck, less is known about how the postural system attenuates motion between the trunk and neck segments in order to regulate head motion. Therefore the purpose of this study was to determine the role that the neck and the trunk play in stabilising the head at a range of walking speeds. Eight healthy male subjects (age: 23+/-4 years) performed self-selected slow, preferred, and fast walking speed trials along a 30 m walkway. Four custom-designed wireless triaxial accelerometers were attached to the head, upper trunk, lower trunk, and shank of each subject to measure vertical (VT), anterior-posterior (AP), and mediolateral (ML) accelerations. Acceleration data were examined in each direction using RMS, power spectral, harmonic, and regularity measures. Signal regularity was increased from the lower to upper trunk for all walking speeds and directions with the exception of the slow speed in the AP direction. Evidence from analysis of power spectral and amplitude characteristics of acceleration signals was suggestive that accelerations are also attenuated from the lower to upper trunk by dynamics of the intervening trunk segment. Differences in selected power spectral and amplitude characteristics between the accelerations of the upper trunk and head due to the intervening neck segment were only detected in the AP direction at preferred and fast walking speeds. Overall the findings of the present study suggest that the trunk segment plays a critical role in regulating gait-related oscillations in all directions. Only accelerations in the direction of travel at preferred and fast speeds required additional control from the neck segment in order to enhance head stability during walking.

Modulation of spectral power and of phase resetting of EEG contributes differentially to the generation of auditory event-related potentials

Nowadays, the mechanisms involved in the genesis of event-related potentials (ERPs) are a matter of debate among neuroscientists. Specifically, the debate lies in whether ERPs arise due to the contribution of a fixed-polarity and fixed-latency superimposed neuronal activity to background electroencephalographic oscillations (evoked model) and/or due to a partial phase synchronization of the ongoing EEG (oscillatory model). The participation of the two mechanisms can be explored by the spectral power modulation and phase coherence of scalp EEG rhythms, respectively. However, an important limitation underlies their measurement: the fact that an added neural activity will be relatively phase-locked to stimulus, thus enhancing both spectral power and phase synchrony measures and making the contribution of each mechanism less clear-cut. This would not be relevant in the case that an increase in phase concentration was not accompanied by any concurrent spectral power modulation, thus opening the way to an oscillatory-based explanation. We computed event-related spectral power modulations and phase coherence to an auditory repeated-stimulus presentation paradigm with tone intensity far from threshold (90 dB SPL), in which N1 decreases its amplitude (N1 gating) as an attenuation brain process. Our data indicate that evoked and oscillatory activity could contribute together to the non-attenuated N1, while N1 to repeated stimuli could be explained by partial phase concentration of scalp EEG activity without concurrent power increase. Therefore, our results show that both increased spectral power and partial phase resetting contribute differentially to different ERPs. Moreover, they show that certain ERPs could arise through reorganization of the phase of ongoing scalp EEG activity only.

DSP-Enabled Radio Astronomy: Towards IIIZW35 Reconquest

In radio astronomy, the radio spectrum is used to detect weak emission from celestial sources. By spectral averaging, observation noise is reduced and weak sources can be detected. However, more and more observations are polluted by man-made radio frequency interferences (RFI). The impact of these RFIs on power spectral measurement ranges from total saturation to subtle distortions of the data. To some extent, elimination of artefacts can be achieved by blanking polluted channels in real time. With this aim in view, a complete real-time digital system has been implemented on a set of FPGA and DSP. The current functionalities of the digital system have high dynamic range of 70 dB, bandwidth selection facilities ranging from 875 kHz to 14 MHz, high spectral resolution through a polyphase filter bank with up to 8192 channels with 49 152 coefficients and real-time time-frequency blanking with a robust threshold detector. This receiver has been used to reobserve the IIIWZ35 astronomical source which has been scrambled by a strong satellite RFI for several years.

Anxiety and synchrony of alpha oscillations

The associations between state anxiety and individually adjusted alpha sub-bands mean spectral power and coherence measures registered in resting condition and during experimental settings were investigated in 30 males aged 18–25 years. Averaged across all cortical sites, spectral power was treated as a complex measure dependent on both the number of active alpha oscillators and the degree of their synchrony. Averaged across all electrode pairs, squared coherence was treated as a global measure of alpha synchrony. The relationship between state anxiety and baseline alpha power was mostly mediated by alpha synchrony. During transition through experimental settings, the dynamics of alpha2 and alpha3 power was mostly accounted for by changes of these sub-bands' synchrony. The synchrony-independent power component demonstrated linear trends implying that, during the experiment, the number of active alpha oscillators linearly decreased in subjects with low state anxiety and linearly increased in subjects with high state anxiety. These findings are discussed in terms of alpha synchronization as an active process, allowing dynamic regulation of alpha system preparedness for processing of external stimuli.

Coordination of head and trunk accelerations during walking

The purpose of this study was to investigate the relationship between oscillatory dynamics of the head and trunk in each plane of motion during walking. Head and trunk accelerations of ten healthy subjects (age: 23 +/- 4 years) were measured in the vertical (VT), anterior-posterior (AP) and mediolateral (ML) directions using a pair of tri-axial accelerometers. All subjects performed five walking trials along a level 20 m walkway at their preferred gait velocity (1.30 +/- 0.15 m s(-1)). Acceleration data were analysed using power spectral, harmonic and regularity measures. Results indicated that: (1) oscillations of the head were smoother, with a greater proportion of power at lower frequencies than oscillations of the trunk, (2) differences in power spectral properties between the head and trunk were most pronounced in the ML direction, (3) coupling between VT-AP, VT-ML, and AP-ML accelerations were greater for the head than trunk, and (4) for both segments, the weakest coupling was observed for AP-ML acceleration relations. Overall, the results of this study suggest that accelerations of the head are significantly attenuated, and more tightly controlled, compared to accelerations of the lower trunk. This attenuation process was particularly evident for the ML direction, whereby head accelerations showed the greatest differences compared to ML accelerations at the trunk.

Non-linear analysis of the electroencephalogram for detecting effects of low-level electromagnetic fields

The study compared traditional spectral analysis and a new scale-invariant method, the analysis of the length distribution of low-variability periods (LDLVPs), to distinguish between electro-encephalogram (EEG) signals with and without a weak stressor, a low-level modulated microwave field. During the experiment, 23 healthy volunteers were exposed to a microwave (450 MHz) of 7 Hz frequency on-off modulation. The field power density at the scalp was 0.16 mW cm(-2). The experimental protocol consisted of ten cycles of repetitive microwave exposure. Signals from frontal EEG channels FP1 and FP2 were analysed. Smooth power spectrum and length distribution curves of low-variability periods, as well as probability distribution close to normal, confirmed that stationarity of the EEG signal during recordings was achieved. The quantitative measure of LDLVPs provided a significant detection of the effect of the stressor for the six subjects exposed to the microwave field but for none of the sham recordings. The spectral analysis revealed a significant result for one subject only. A significant effect of the exposure to the EEG signal was detected in 25% of subjects, with microwave exposure increasing EEG variability. The effect was not detectable by power spectral measures.

Human mediotemporal EEG characteristics during propofol anesthesia

Evidence for a response-control-related kind of declarative memory during deep propofol anesthesia has recently been reported. Connectivity within the mediotemporal lobe (MTL), and in particular rhinal-hippocampal synchronization within the gamma band, has been shown to be crucial for declarative memory formation. Thus, we analyzed EEG recordings obtained from the scalp, as well as directly from within the hippocampus and from the anterior parahippocampal gyrus, which is covered by rhinal cortex, in patients with unilateral temporal lobe epilepsy during propofol anesthesia, which preceded electrode explantation. For the gamma band a power decrease starting with induction of anesthesia was observed at scalp position Cz, but a power increase was detected at MTL locations. In contrast to prior results for sleep recordings, rhinal-hippocampal coherence did not decrease within the gamma band at deeper levels of anesthesia. These findings may represent an indirect electrophysiological correlate of partially intact declarative memory formation during deep propofol sedation. Furthermore, we investigated how well the plasma propofol level, as well as different stages of anesthesia including the burst suppression phase, could be monitored by different spectral as well as by nonlinear EEG measures. We observed that conventional spectral power measures, most prominently those recorded from mediotemporal locations, are most closely correlated with the plasma propofol level, whereas different stages of anesthesia can be distinguished best by nonconventional spectral as well as nonlinear measures.

Vortex avalanche phenomena in MgB2 superconducting film studied by current noise measurements

Vortex avalanche phenomenon enhanced by thermomagnetic instabilities suppresses critical current in MgB2 films. This effect is investigated at 4.2K by spectral noise power measurements up to 1kHz with a dc superconducting quantum interference device apparatus. 1∕fγ behavior is shown at low frequencies in magnetic fields and feeding currents below jc. γ increases from 1 to 2 at increasing magnetic field, typical of fluxon avalanche processes. A peak in the power spectrum frequency behavior, observed only in magnetic field, is attributed to vortex–antivortex annihilations. Thermal instabilities disappear when an efficient thermal link to the He bath is attained and no vortex avalanche extra-noise is observed.

Alpha oscillations as a correlate of trait anxiety

The associations among psychometric measures of anxiety and depression and individually adjusted electroencephalogram (EEG) spectral power measures registered in resting condition and during experimental settings were investigated in 30 males aged 18–25 years. During all stages of registration, Taylor Manifest Anxiety and Spielberger state anxiety (SA) and trait anxiety (TA) scores were positively related to alpha and negatively to delta relative power with these relations being independent of cortical site. Within-subject estimate of the strength of reciprocal relationship between alpha and delta oscillations (alpha–delta anticorrelation, or ADA) was positively related to trait anxiety and depression. Three minutes after an alarming event (unexpected loud sound), a further increase of alpha power was observed. In low-anxiety subjects, this increase was mostly associated with fast alpha (alpha 3), whereas in high-anxiety ones, it was mainly linked to slow alpha (alpha 2). SA mediated relationship between TA and EEG power, while ADA and alpha band reactivity showed trait-like features being associated with TA even after accounting for SA. These findings are interpreted as an indication of higher vigilance and higher reactivity of alpha system in anxious individuals.