Electroencephalographic evidence of correlated event-related signals between the brains of spatially and sensory isolated human subjects

OBJECTIVE: To determine whether correlated event-related potentials (ERPs) can be detected between the brains of spatially and sensory isolated human subjects. DESIGN AND SETTING: Simultaneous digitized electroencephalograms (EEGs) were recorded from the occipital area in pairs of human subjects placed in sound attenuated rooms separated by 10 meters. One person relaxed in one of the rooms while the other received visual stimulation while in the other room. Prior to each experiment, members of the pair were randomly designated as sender and receiver. Sessions were subsequently repeated with subjects reversing their roles. Previous to each session, the sender was instructed "to attempt sending an image/thought." The receiver was instructed "to remain open to receive any image/thought from his/her partner." Alternating stimulus-on/stimulus-off conditions were presented throughout the session to the sender, while a stimulus-off condition was presented to the receiver. SUBJECTS: Thirty-seven (37) female, and 23 male subjects (n = 60; 30 pairs) participated in the study. Subjects knew each other well and claimed to have previous experience of being emotionally/psychologically connected to one another. OUTCOME MEASURES: A Runs test was applied to compare EEG "hits" in the receiver's EEG during the sender' stimulus-on condition versus sender's stimulus-off conditions. Test results at p < 0.01 were considered evidence of correlated brain signals. Pairs in whom at least one member had significant results were invited back for replication. RESULTS: Of the 60 subjects tested, 5 (4 women/1 man) showed significantly higher brain activation (p < 0.01) during their sending partner's stimulus-on condition as compared to stimulus-off condition. Using the Stouffer z meta-analytic method all receiver EEG results across all 60 subjects were combined by transforming the individual session p values into z scores. Data analyses showed overall significant results for EEG data recorded during the flickering condition (z =-3.28, p = 0.0005) as well as nonsignificant results for data recorded during the static condition (z = 0.35, p = 0.64). Four pairs participated in a replication experiment during which one pair replicated the effect. CONCLUSIONS: These results indicate that in some pairs of human subjects a signal may be detected in the brain of a distant member of the pair when the brain of the other member is visually stimulated. These data support the findings of similar studies performed in seven laboratories reported in the peer-reviewed literature since 1963. Research in this area should now proceed with investigation of its physical and biologic mechanism, its generalizability to varying populations and relationships, and its clinical application.     

Integration of fMRI and simultaneous EEG: towards a comprehensive understanding of localization and time-course of brain activity in target detection

fMRI and EEG are complimentary methods for the analysis of brain activity since each method has its strength where the other one has limits: The spatial resolution is thus in the range of millimeters with fMRI and the time resolution is in the range of milliseconds with EEG. For a comprehensive understanding of brain activity in target detection, nine healthy subjects (age 24.2 +/- 2.9) were investigated with simultaneous EEG (27 electrodes) and fMRI using an auditory oddball paradigm. As a first step, event-related potentials, measured inside the scanner, have been compared with the potentials recorded in a directly preceding session in front of the scanner. Attenuated amplitudes were found inside the scanner for the earlier N1/P2 component but not for the late P300 component. Second, an independent analysis of the localizations of the fMRI activations and the current source density as revealed by low resolution electromagnetic tomography (LORETA) has been done. Concordant activations were found in most regions, including the temporoparietal junction (TPJ), the supplementary motor area (SMA)/anterior cingulate cortex (ACC), the insula, and the middle frontal gyrus, with a mean Euclidean distance of 16.0 +/- 6.6 mm between the BOLD centers of gravity and the LORETA-maxima. Finally, a time-course analysis based on the current source density maxima was done. It revealed different time-course patterns in the left and right hemisphere with earlier activations in frontal and parietal regions in the right hemisphere. The results suggest that the combination of EEG and fMRI permits an improved understanding of the spatiotemporal dynamics of brain activity.     

Novelty and target processing during an auditory novelty oddball: a simultaneous event-related potential and functional magnetic resonance imaging study

Recent evidence suggests that both spatiotemporally distinct and overlapping brain regions are involved in bottom-up- and top-down-driven attentional processing. However, existing studies are based on a variety of different approaches, including electroencephalography (EEG) and functional magnetic resonance imaging (fMRI), raising the question of how EEG and fMRI findings in this field are related to each other. The present study aimed at disentangling common from specific regions underlying bottom-up novelty-processing and top-down target-processing. Simultaneous EEG and fMRI recordings were employed to investigate how fMRI-identified brain regions contribute to event-related potential (ERP) signatures of novelty- and target-processing. Fourteen subjects performed a modified novelty oddball task in which either rare tones or novel sounds served as targets in different blocks, allowing us to separate novelty-related from mere distractor-related effects. ERP signatures of novelty- and target-processing could be identified, confirming previous research based on recordings outside the scanner. fMRI analyses revealed that, despite considerable overlap of regions activated during novelty- and target-processing, bilateral superior temporal and right inferior frontal areas showed pronounced activation related to novelty-processing. fMRI-informed ERP dipole seeding was used to integrate both signals. The source modeling results further implicated temporal and inferior frontal sources in novelty-processing. Target-related fMRI activation on the other hand was confirmed in a network comprising distributed frontoparietal regions as well as bilateral caudate nucleus and cerebellum. Most regions identified by fMRI showed a contribution to target-related ERP signatures. This pattern of findings underscores the potential of simultaneous EEG/fMRI recordings for the spatiotemporal characterization of target- and novelty-processing.     

fMRI and EEG responses to periodic visual stimulation.

EEG/VEP and fMRI responses to periodic visual stimulation are reported. The purpose of these experiments was to look for similar patterns in the time series produced by each method to help understand the relationship between the two. The stimulation protocol was the same for both sets of experiments and consisted of five complete cycles of checkerboard pattern reversal at 1.87 Hz for 30 s followed by 30 s of a stationary checkerboard. The fMRI data was analyzed using standard methods, while the EEG was analyzed with a new measurement of activation-the VEPEG. Both VEPEG and fMRI time series contain the fundamental frequency of the stimulus and quasi harmonic components-an unexplained double frequency commonly found in fMRI data. These results have prompted a reappraisal of the methods for analyzing fMRI data and have suggested a connection between our findings and much older published invasive electrophysiological measurements of blood flow and the partial pressures of oxygen and carbon dioxide. Overall our new analysis suggests that fMRI signals are strongly dependant on hydraulic blood flow effects. We distinguish three categories of fMRI signal corresponding to: focal activated regions of brain tissue; diffuse nonspecific regions of steal; and major cerebral vessels of arterial supply or venous drainage. Each category of signal has its own finger print in frequency, amplitude, and phase. Finally, we put forward the hypothesis that modulations in blood flow are not only the consequence but are also the cause of modulations in functional activity.     

Complete artifact removal for EEG recorded during continuous fMRI using independent component analysis

The simultaneous recording of EEG and fMRI is a promising method for combining the electrophysiological and hemodynamic information on cerebral dynamics. However, EEG recordings performed in the MRI scanner are contaminated by imaging, ballistocardiographic (BCG) and ocular artifacts. A number of processing techniques for the cancellation of fMRI environment disturbances exist: the most popular is averaged artifact subtraction (AAS), which performs well for the imaging artifact, but has some limitations in removing the BCG artifact, due to the variability in cardiac wave duration and shape; furthermore, no processing method to attenuate ocular artifact is currently used in EEG/fMRI, and contaminated epochs are simply rejected before signal analysis. In this work, we present a comprehensive method based on independent component analysis (ICA) for simultaneously removing BCG and ocular artifacts from the EEG recordings, as well as residual MRI contamination left by AAS. The ICA method has been tested on event-related potentials (ERPs) obtained from a visual oddball paradigm: it is very effective in attenuating artifacts in order to reconstruct clear brain signals from EEG acquired in the MRI scanner. It performs significantly better than the AAS method in removing the BCG artifact. Furthermore, since ocular artifacts can be completely suppressed, a larger number of trials is available for analysis. A comparison of ERPs inside the magnetic environment with those obtained out of the MRI scanner confirms that no systematic bias in the ERP waveform is produced by the ICA method.     

Event-related electroencephalographic correlations between isolated human subjects

OBJECTIVE: To examine electroencephalograms (EEG) in pairs of people to see if event-related potentials evoked in one person's brain are correlated with concurrent responses in the brain of a distant, isolated person. DESIGN: Simultaneously record EEGs using independent physiologic monitoring systems. One person relaxes in a double steel-walled, electromagnetically and acoustically shielded room while a second, located in a dimly lit room 20 meters away, is stimulated at random times by the live video image of the first person. SUBJECTS: Thirteen (13) pairs of volunteers. Eleven (11) pairs of adult friends and 2 mother-daughter pairs. OUTCOME MEASURES: Epochs of interest were the moments of stimulus onset and offset, +/- 5 seconds, in both participants' EEGs. A positive correlation was postulated to appear between the ensemble variance of the stimulated subjects' EEGs versus an identical measure in the nonstimulated subjects. Control data using the same equipment and test conditions, but without humans present, was collected to check for equipment and analytical artifacts. Nonparametric bootstrap methods were used to assess statistical significance of the observed correlations. RESULTS: The control test resulted in a correlation of r =-0.03, p = 0.61; the experimental test resulted in r = 0.20, p = 0.0005. Three (3) of the 13 pairs of participants showed independently significant correlations. Examination of the stimulated subjects' event-related potentials showed that the stronger their responses, the larger the corresponding responses in the nonstimulated subjects (p = 0.0008). CONCLUSION: Under certain conditions, the EEG of a sensorially isolated human subject can become correlated with event-related potentials in a distant person's EEG. This suggests the presence of an unknown form of energetic or informational interaction.     

Negative covariation between task-related responses in alpha/beta-band activity and BOLD in human sensorimotor cortex: An EEG and fMRI study of motor imagery and movements

Similar to the occipital alpha rhythm, electroencephalographic (EEG) signals in the alpha- and beta-frequency bands can be suppressed by movement or motor imagery and have thus been thought to represent the “idling state” of the sensorimotor cortex. A negative correlation between spontaneous alpha EEG and blood-oxygen-level-dependent (BOLD) signals has been reported in combined EEG and fMRI (functional Magnetic Resonance Imaging) experiments when subjects stayed at the resting state or alternated between the resting state and a task. However, the precise nature of the task-induced alpha modulation remains elusive. It was not clear whether alpha/beta rhythm suppressions may co-vary with BOLD when conducting tasks involving varying activations of the cortex. Here, we quantified the task-evoked responses of BOLD and alpha/beta-band power of EEG directly in the cortical source domain, by using source imaging technology, and examined their covariation across task conditions in a mixed block and event-related design. In this study, 13 subjects performed tasks of right-hand, right-foot or left-hand movement and motor imagery when EEG and fMRI data were separately collected. Task-induced increase of BOLD signal and decrease of EEG amplitudes in alpha and beta bands were shown to be co-localized at the somatotopic sensorimotor cortex. At the corresponding regions, the reciprocal changes of the two signals co-varied in the magnitudes across imagination and movement conditions. The spatial correspondence and negative covariation between the two measurements were further shown to exist at somatotopic brain regions associated with different body parts. These results suggest an inverse functional coupling relationship between task-induced changes of BOLD and low-frequency EEG signals.     

Single trial variability of EEG and fMRI responses to visual stimuli

Recent EEG-fMRI studies have suggested a novel method of data fusion which uses single trial (ST) estimates of event-related potentials in the fMRI analysis. This is potentially very powerful, but rests on the assumption that the ST variability observed in EEG is reflected in the fMRI signal. The current study investigated this assumption and compared two different data processing strategies for each modality. Five subjects underwent separate EEG and fMRI sessions with checkerboard stimuli at two contrasts. EEG data were preprocessed using wavelet denoising and independent component analysis (ICA), whilst the general linear model and ICA were used for fMRI. Amplitudes and latencies of the P1 and N2 components of the visual evoked potential (VEP) were calculated for each trial. For fMRI, the amplitudes and latencies of the ST haemodynamic responses (HR) were calculated. Within modality, the results for the two processing methods were significantly correlated in the majority of data sets. Across modality, the average amplitudes of the VEPs and HRs were also significantly correlated. Examination of ST variability demonstrated that the amplitudes of the mean VEPs and HRs are both influenced by the latency variability of the ST responses to a greater extent than the amplitude variability. For high contrast stimuli the latency variability in EEG and fMRI was significantly correlated, with a similar trend seen for the low contrast stimuli. The results confirm the validity of examining both the EEG and fMRI signals on an ST basis and suggest an underlying neuronal origin in both modalities.     

Acquisition of typical EEG waveforms during fMRI: SSVEP, LRP, and frontal theta

Recent work has demonstrated the feasibility of simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). Virtually no systematic comparisons between EEG recorded inside and outside the MR scanner have been conducted, and it is unknown if different kinds of frequency mix, topography, and domain-specific processing are uniformly recordable within the scanner environment. The aim of the study was to investigate several typical EEG waveforms in the same subjects inside the magnet during fMRI and outside the MR examination room. We examined whether uniform artifact subtraction allows the extraction of these different EEG waveforms inside the scanner during EPI scanning to the same extent as outside the scanner. Three well-established experiments were conducted, eliciting steady state visual evoked potentials (SSVEP), lateralized readiness potentials (LRP), and frontal theta enhancement induced by mental addition. All waveforms could be extracted from the EEG recorded during fMRI. Substantially no differences in these waveforms of interest were found between gradient-switching and intermediate epochs during fMRI (only the SSVEP-experiment was designed for a comparison of gradient-with intermediate epochs), or between waveforms recorded inside the scanner during EPI scanning and outside the MR examination room (all experiments). However, non-specific amplitude differences were found between inside and outside recorded EEG at lateral electrodes, which were not in any interaction with the effects of interest. The source of these differences requires further exploration. The high concordance of activation patterns with published results demonstrates that EPI-images could be acquired during EEG recording without significant distortion.     

EEG-BOLD correlations during (post-)adolescent brain maturation

The transition from adolescence to adulthood is a critical stage in the human lifespan during which the brain still undergoes substantial structural and functional change. The changing frequency composition of the resting state EEG reflects maturation of brain function. This study investigated (post)adolescent brain maturation captured by two independently but simultaneously recorded neuronal signals: EEG and fMRI. Data were collected in a 20 min eyes-open/eyes-closed resting state paradigm. EEG, fMRI-BOLD signal and EEG-BOLD correlations were compared between groups of adults, age 25 (n=18), and adolescents, age 15 (n=18). A typical developmental decrease of low-frequency EEG power was observed even at this late stage of brain maturation. Frequency and condition specific EEG-fMRI correlations proved robust for multiple brain regions. However, no consistent change in the EEG-BOLD correlations was identified that would correspond to the neuronal maturation captured by the EEG. This result indicates that the EEG-BOLD correlation measures a distinct aspect of neurophysiological activity that presumably matures earlier, since it is less sensitive to late maturation than the neuronal activity captured by low-frequency EEG.     

A method for removing imaging artifact from continuous EEG recorded during functional MRI

Combined EEG/fMRI recording has been used to localize the generators of EEG events and to identify subject state in cognitive studies and is of increasing interest. However, the large EEG artifacts induced during fMRI have precluded simultaneous EEG and fMRI recording, restricting study design. Removing this artifact is difficult, as it normally exceeds EEG significantly and contains components in the EEG frequency range. We have developed a recording system and an artifact reduction method that reduce this artifact effectively. The recording system has large dynamic range to capture both low-amplitude EEG and large imaging artifact without distortion (resolution 2 microV, range 33.3 mV), 5-kHz sampling, and low-pass filtering prior to the main gain stage. Imaging artifact is reduced by subtracting an averaged artifact waveform, followed by adaptive noise cancellation to reduce any residual artifact. This method was validated in recordings from five subjects using periodic and continuous fMRI sequences. Spectral analysis revealed differences of only 10 to 18% between EEG recorded in the scanner without fMRI and the corrected EEG. Ninety-nine percent of spike waves (median 74 microV) added to the recordings were identified in the corrected EEG compared to 12% in the uncorrected EEG. The median noise after artifact reduction was 8 microV. All these measures indicate that most of the artifact was removed, with minimal EEG distortion. Using this recording system and artifact reduction method, we have demonstrated that simultaneous EEG/fMRI studies are for the first time possible, extending the scope of EEG/fMRI studies considerably.     

fMRI-acoustic noise alters brain activation during working memory tasks

Scanner noise during functional magnetic resonance imaging (fMRI) may interfere with brain function and change blood oxygenation level dependent (BOLD) signals, a problem that generally worsens at the higher field strengths. Therefore, we studied the effect of increased acoustic noise on fMRI during verbal working memory (WM) processing. The sound pressure level of scanner noise was increased by 12 dBA from "Quiet" to "Loud" echo planar imaging (EPI) scans by utilizing resonant vibration modes of the gradient coil. A WM paradigm with graded levels of task difficulty was used to further access WM load. Increased scanner noise produced increased BOLD responses (percent signal change) bilaterally in the cerebellum, inferior (IFG), medial (medFG), and superior (SFG) frontal, fusiform (FusG), and the lingual (LG) gyri, and decreased BOLD responses bilaterally in the anterior cingulate gyrus (ACG) and the putamen. This finding suggests greater recruitment of attention resources in these brain regions, probably to compensate for interference due to louder scanner noise. Increased working memory load increased the BOLD signals in IFG and the cerebellum, but decreased the BOLD signals in the putamen and the LG. These findings also support the idea that brain function requires additional attention resources under noisier conditions. Load- and acoustic-noise-related changes in BOLD responses correlated negatively in the WM network. This study demonstrates that MR noise affects brain activation pattern. Future comparisons between studies performed under different acoustic conditions (due to differing magnetic field strengths, pulse sequences, or scanner manufacturers) might require knowledge of the sound pressure level of acoustic noise during fMRI.     

High-frequency (600 Hz) population spikes in human EEG delineate thalamic and cortical fMRI activation sites

Functional magnetic resonance imaging (fMRI) measures neural activity indirectly via its slow vascular/metabolic consequences. At a temporal resolution on the order of seconds, fMRI does not reveal the real 'language of neurons', spelt out by fast electrical discharges ('spikes') which occur on a time scale of milliseconds. In animal studies, these limitations have been addressed by adding invasive electrode measurements to fMRI. Here, we propose to circumvent this 'inverse problem of fMRI' by deriving a noninvasive spike measure from recordings of ultrafast electroencephalography (EEG) signals during fMRI. We demonstrate how in response to median nerve stimulation 600 Hz oscillatory EEG signals can be measured reliably during fMRI. These high-frequency bursts (HFBs) are supposed to reflect population spikes in the thalamus and the somatosensory cortex, respectively. We show that distinct fMRI activations in these two generator structures can be attributed to spontaneous HFB fluctuations. Thus, our approach allowed the noninvasive identification of neural processes along the thalamocortical pathway unfolding at a millisecond time scale.     

The Human Connectome Project: a data acquisition perspective

The Human Connectome Project (HCP) is an ambitious 5-year effort to characterize brain connectivity and function and their variability in healthy adults. This review summarizes the data acquisition plans being implemented by a consortium of HCP investigators who will study a population of 1200 subjects (twins and their non-twin siblings) using multiple imaging modalities along with extensive behavioral and genetic data. The imaging modalities will include diffusion imaging (dMRI), resting-state fMRI (R-fMRI), task-evoked fMRI (T-fMRI), T1- and T2-weighted MRI for structural and myelin mapping, plus combined magnetoencephalography and electroencephalography (MEG/EEG). Given the importance of obtaining the best possible data quality, we discuss the efforts underway during the first two years of the grant (Phase I) to refine and optimize many aspects of HCP data acquisition, including a new 7T scanner, a customized 3T scanner, and improved MR pulse sequences.     

A mini-cap for simultaneous EEG and fMRI recording in rodents

Simultaneous recording of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) is now widely accepted as a prevailing tool to study brain functions. For over a decade, EEG caps with high-dense arrays of electrodes for EEG-fMRI studies in humans have been commercially available. However, simultaneous EEG and fMRI recording in rodents has been limited to only a few electrodes due mainly to two technical reasons, i.e. a small available scalp area and the proximity of the electrodes to the brain tissue. In this paper, we introduce both a new EEG mini-cap and a protocol to obtain whole scalp EEG recordings simultaneously with 7 T fMRI signals in rodents. We provide methodological protocol to evaluate a number of problems emerging from the particulars of using rodents in simultaneous EEG and fMRI recording. The quality and reproducibility of both EEG and fMRI signals were demonstrated using a conventional forepaw stimulation paradigm in Wistar rats. Based on this quantitative analysis, we conclude that simultaneous EEG-fMRI recordings are achievable in rodents without significant signal loss. In light of the contemporary transgenic models and advanced drug administration protocols in rodents, the proposed methodology could be remarkable as a futurist experimental platform.     

Geometrical interpretation of fMRI-guided MEG/EEG inverse estimates

Magneto- and electroencephalography (MEG/EEG) and functional magnetic resonance imaging (fMRI) provide complementary information about the functional organization of the human brain. An important advantage of MEG/EEG is the millisecond time resolution in detecting electrical activity in the cerebral cortex. The interpretation of MEG/EEG signals, however, is limited by the difficulty of determining the spatial distribution of the neural activity. Functional MRI can help in the MEG/EEG source analysis by suggesting likely locations of activity. We present a geometric interpretation of fMRI-guided inverse solutions in which the MEG/EEG source estimate minimizes a distance to a subspace defined by the fMRI data. In this subspace regularization (SSR) approach, the fMRI bias does not assume preferred amplitudes for MEG/EEG sources, only locations. Characteristic dependence of the source estimates on the regularization parameters is illustrated with simulations. When the fMRI locations match the true MEG/EEG source locations, they serve to bias the underdetermined MEG/EEG inverse solution toward the fMRI loci. Importantly, when the fMRI loci do not match the true MEG/EEG loci, the solution is insensitive to those fMRI loci.     

The hemodynamic response of the alpha rhythm: an EEG/fMRI study

EEG was recorded during fMRI scanning of 16 normal controls in resting condition with eyes closed. Time variations of the occipital alpha band amplitudes were correlated to the fMRI signal variations to obtain insight into the hemodynamic correlates of the EEG alpha activity. Contrary to earlier studies, no a priori assumptions were made on the expected shape of the alpha band response function (ARF). The ARF of different brain regions and subjects were explored and compared. It was found that: (1) the ARF of the thalamus is mainly positive. (2) The ARFs at the occipital and left and right parietal points are similar in amplitude and timing. (3) The peak time of the thalamus is a few seconds earlier than that of occipital and parietal cortex. (4) No systematic BOLD activity was found preceding the alpha band activity, although in the two subjects with the strongest alpha band power such correlation was present. (5) There is a strong and immediate positive correlation at the eyeball, and a strong negative correlation at the back of the eye. Furthermore, it was found that in one subject the cortical ARF was positive, contrary to the other subjects. Finally, a cluster analysis of the observed ARF, in combination with a Modulated Sine Model (MSM) fit to the estimated ARF, revealed that within the cortex the ARF peak time shows a spatial pattern that may be interpreted as a traveling wave. The spatial pattern of alpha band response function represents the combined effect of local differences in electrical alpha band activity and local differences in the hemodynamic response function (HRF) onto these electrical activities. To disentangle the contributions of both factors, more advanced integration of EEG inverse modeling and hemodynamic response modeling is required in future studies.     

Measurement and reduction of motion and ballistocardiogram artefacts from simultaneous EEG and fMRI recordings

Recording the electroencephalogram (EEG) during functional magnetic resonance imaging (fMRI) permits the identification of haemodynamic changes associated with EEG events. However, subject motion within the MR scanner can cause unpredictable and frustrating artefacts on the EEG that may appear focally, bilaterally or unilaterally and can sometimes be confused for epileptiform activity. Motion may arise from a number of sources: small involuntary cardiac-related body movements (ballistocardiogram); acoustic vibrations due to the scanner machinery; and voluntary subject movements. Here we describe a new real-time technique for removing ballistocardiogram (BCG) and movement artefact from EEG recordings in the MR scanner using a novel method for recording subject motion. We record the current induced in a number of wire loops, attached to a cap worn by the subject, due to motion in the static magnetic field of the scanner (Faraday's Law). This is the same process that leads to the motion artefacts on the EEG, and hence these signals are ideally suited to filtering these artefacts from the EEG. Our filter uses a linear adaptive technique based upon the Recursive Least Squares (RLS) algorithm. We demonstrate in both simulations and real EEG recordings from epilepsy patients that our filter significantly reduces the artefact power whilst preserving the underlying EEG signal.     

EEG-correlated fMRI of human alpha activity

Electroencephalography-correlated functional magnetic resonance imaging (EEG/fMRI) can be used to identify blood oxygen level-dependent (BOLD) signal changes associated with both physiological and pathological EEG events. Here, we implemented continuous and simultaneous EEG/fMRI to identify BOLD signal changes related to spontaneous power fluctuations in the alpha rhythm (8-12 Hz), the dominant EEG pattern during relaxed wakefulness. Thirty-two channels of EEG were recorded in 10 subjects during eyes-closed rest inside a 1.5-T magnet resonance (MR) scanner using an MR-compatible EEG recording system. Functional scanning by echoplanar imaging covered almost the entire cerebrum every 4 s. Off-line MRI artifact subtraction software was applied to obtain continuous EEG data during fMRI acquisition. The average alpha power over 1-s epochs was derived at several electrode positions using a Fast Fourier Transform. The power time course was then convolved with a canonical hemodynamic response function, down-sampled, and used for statistical parametric mapping of associated signal changes in the image time series. At all electrode positions studied, a strong negative correlation of parietal and frontal cortical activity with alpha power was found. Conversely, only sparse and nonsystematic positive correlation was detected. The relevance of these findings is discussed in view of the current theories on the generation and significance of the alpha rhythm and the related functional neuroimaging findings.