Richard B. Silberstein

Spatial-temporal structures of human alpha rhythms: theory, microcurrent sources, multiscale measurements, and global binding of local networks

A theoretical framework supporting experimental measures of dynamic properties of human EEG is proposed with emphasis on distinct alpha rhythms. Robust relationships between measured dynamics and cognitive or behavioral conditions are reviewed, and proposed physiological bases for EEG at cellular levels are considered. Classical EEG data are interpreted in the context of a conceptual framework that distinguishes between locally and globally dominated dynamic processes, as estimated with coherence or other measures of phase synchronization. Macroscopic (scalp) potentials generated by cortical current sources are described at three spatial scales, taking advantage of the columnar structure of neocortex. New EEG data demonstrate that both globally coherent and locally dominated behavior can occur within the alpha band, depending on narrow band frequency, spatial measurement scale, and brain state. Quasi‐stable alpha phase structures consistent with global standing waves are observed. At the same time, alpha and theta phase locking between cortical regions during mental calculations is demonstrated, consistent with neural network formation. The brain‐binding problem is considered in the context of EEG dynamic behavior that generally exhibits both of these local and global aspects. But specific experimental designs and data analysis methods may severely bias physiological interpretations in either local or global directions. Hum. Brain Mapping 13:125–164, 2001.

A theoretical and experimental study of high resolution EEG based on surface Laplacians and cortical imaging

Two different methods to improve the spatial resolution of EEG are discussed: the surface Laplacian (e.g., current source density) and cortical imaging (e.g., spatial deconvolution). The former methods tend to be independent of head volume conductor model, whereas the latter methods are more model-dependent. Computer simulation of scalp potentials due to either a few isolated sources or 4200 distributed cortical sources and studies of actual EEG data both indicate that the two methods provide similar estimates of cortical potential distribution. Typical correlation coefficients between either spline-Laplacian or cortical image and simulated (calculated) cortical potential are in the 0.8-0.95 range, depending partly on CSF thickness. By contrast, correlation coefficients between simulated scalp and cortical potential are in the 0.4-0.5 range, suggesting that high resolution methods provide much better estimates of cortical potential than is obtained with conventional EEG. The two methods are also applied to steady-state visually evoked potentials and spontaneous EEG. Correlation coefficients obtained from real EEG data are in the same general ranges as correlations obtained from simulations. The new high resolution methods can provide a dramatic increase in the information content of EEG and appear to have widespread application in both clinical and cognitive studies.

On the Relationship of Synaptic Activity to Macroscopic Measurements: Does Co-Registration of EEG with fMRI Make Sense?

A two-scale theoretical description outlines relationships between brain current sources and the resulting extracranial electric field, recorded as EEG. Finding unknown sources of EEG, the so-calledg “inverse problem”, is discussed in general terms, with emphasis on the fundamental non-uniqueness of inverse solutions. Hemodynamic signatures, measured with fMRI, are expressed as voxel integrals to facilitate comparisons with EEG. Two generally distinct cell groups (1 and 2), generating EEG and fMRI signals respectively, are embedded within the much broader class of synaptic action fields. Cell groups 1 and 2 may or may not overlap in specific experiments. Implications of this incomplete overlap for co-registration studies are considered. Each experimental measure of brain function is generally sensitive to a different kind of source activity and to different spatial and temporal scales. Failure to appreciate such distinctions can exacerbate conflicting views of brain function that emphasize either global integration or functional localization.

Spatial filtering and neocortical dynamics: estimates of EEG coherence

The spatial statistics of scalp electroencephalogram (EEG) are usually presented as coherence in individual frequency bands. These coherences result both from correlations among neocortical sources and volume conduction through the tissues of the head. The scalp EEG is spatially low-pass filtered by the poorly conducting skull, introducing artificial correlation between the electrodes. A four concentric spheres (brain, CSF, skull, and scalp) model of the head and stochastic field theory are used here to derive an analytic estimate of the coherence at scalp electrodes due to volume conduction of uncorrelated source activity, predicting that electrodes within 10-12 cm can appear correlated. The surface Laplacian estimate of cortical surface potentials spatially bandpass filters the scalp potentials reducing this artificial coherence due to volume conduction. Examination of EEG data confirms that the coherence estimates from raw scalp potentials and Laplacians are sensitive to different spatial bandwidths and should be used in parallel in studies of neocortical dynamic function.

Spatial sampling and filtering of EEG with spline Laplacians to estimate cortical potentials

SummaryThe electroencephalogram (EEG) is recorded by sensors physically separated from the cortex by resistive skull tissue that smooths the potential field recorded at the scalp. This smoothing acts as a low-pass spatial filter that determines the spatial bandwidth, and thus the required spatial sampling density, of the scalp EEG. Although it is better appreciated in the time domain, the Nyquist frequency for adequate discrete sampling is evident in the spatial domain as well. A mathematical model of the low-pass spatial filtering of scalp potentials is developed, using a four concentric spheres (brain, CSF, skull, and scalp) model of the head and plausible estimates of the conductivity of each tissue layer. The surface Laplacian estimate of radial skull current density or cortical surface potential counteracts the low-pass filtering of scalp potentials by shifting the spatial spectrum of the EEG, producing a band-passed spatial signal that emphasizes local current sources. Simulations with the four spheres model and dense sensor arrays demonstrate that progressively more detail about cortical potential distribution is obtained as sampling is increased beyond 128 channels.

Brain-imaging detection of visual scene encoding in long-term memory for TV commercials

ABSTRACT The authors report on experimental research using a new brain imaging technique (steady-state probe topography or SSPT) to investigate whether patterns of brain activity in the left or right frontal hemispheres could identify which frames from new TV commercials would be recognised by consumers one week later. The research revealed that video scenes held on screen for 1.5 seconds or longer were better recognised, and that scenes that elicited the fastest brain activity in the left frontal hemisphere were better recognised. The authors conclude that visual content that stimulates left-brain activity would create memorable advertising - and suggest a new method of pre-testing commercials.

Gender differences in the cortical electrophysiological processing of visual emotional stimuli

The processing of visual emotional stimuli has been investigated previously; however, gender differences in the processing of emotional stimuli remain to be clarified. The aim of the current study was to use steady-state probe topography (SSPT) to examine steady-state visually evoked potentials (SSVEPs) during the processing of pleasant and unpleasant images relative to neutral images, and to determine whether this processing differs between males and females. Thirty participants (15 males and 15 females) viewed 75 images low on the arousal dimension (categorised as pleasant, neutral or unpleasant) selected from the International Affective Picture System (IAPS), whilst a 13-Hz sinusoidal white visual flicker was superimposed over the visual field and brain electrical activity was recorded from 64 electrode sites. Results suggest that pleasant and unpleasant images relative to neutral images are associated with reductions in frontal latency and occipital amplitude. In addition, electrophysiological gender differences were observed despite there being no differences found between males and females on subjective mood or behavioural ratings of presented images (valence and arousal dimensions). The main gender difference reported in the current study related to the processing of unpleasant images (relative to neutral images) which is associated with widespread frontal latency reductions (predominantly right sided) in females but not in males. Our results suggest that gender differences do exist in the processing of visual emotional stimuli, and illustrate the importance of taking these differences into account during investigations of emotional processing. Finally, these gender differences may have implications for the pathophysiology of mood disorders such as depression.

Steady-state visual evoked potentials and travelling waves

OBJECTIVE The amplitude and phase of the steady-state visual evoked potential (SSVEP) is sensitive to cognition and attention but the underlying mechanism is not well understood. This study examines stimulus evoked changes in the SSVEP phase topography and the putative role of travelling waves. METHODS Eighteen subjects viewed a central-field checkerboard and full-field flicker stimulus temporally modulated at the peak alpha rhythm frequency. EEG was recorded from 10 midline scalp sites and the bipolar SSVEP obtained from differences between adjacent electrodes. RESULTS The SSVEP phase comprised either progressive variations consistent with travelling waves or a phase reversal consistent with standing waves. The checkerboard pattern elicited travelling wave patterns in 14 subjects with estimated phase velocities ranging from 7 to 11 m/s after correcting for folded cortex. The flicker stimulus elicited phase reversals in 9 subjects, suggesting standing waves. Six subjects demonstrated a phase topography specific to the stimulus with travelling wave patterns associated with the checkerboard and standing wave patterns associated with the flicker. CONCLUSIONS These differences suggest the emergence of travelling and standing waves under different spatial configurations of visual input to the cortex and that wave phenomena contribute to the spatiotemporal dynamics of the SSVEP.

Steady-State Visually Evoked Potential topography associated with a visual vigilance task

SummaryThis paper describes data which demonstrate a correlation between the magnitude of the Steady-State Visually Evoked Potential (SSVEP) and visual vigilance. The SSVEP was recorded from 64 scalp sites and elicited by a 13Hz uniform visual flicker presented continuously while subjects undertook a visual vigilance task. Fifteen right-handed males were required to view three times a series of 180 geometrical shapes comprising a sequence of 60 squares, 60 circles and a further 60 squares. Each viewing of the 180 shapes constituted a trial. Trials 1 and 2 were identical while trial 3 differed from the first two in that one of the circles was modified. Subjects were ignorant as to the location of the modified circle and prior to the third trial, were challenged to identify the modified circle. A comparison of trials 2 and 3 indicated that the appearance of the modified circle was associated with an attenuation of the SSVEP in the occipito/parietal region. The same comparison indicated a pronounced SSVEP attenuation in the centro/parietal region during the interval that subjects were anticipating the appearance of the modified circle. These results suggest a distinction between the cortical activation patterns occurring during different phases of a visual vigilance task.

Steady state visually evoked potential (SSVEP) topography in a graded working memory task

The steady state visually evoked potential (SSVEP) elicited by a diffuse 13-Hz visual flicker was recorded from 64 scalp sites in 30 subjects performing a low and high demand version of an object working memory task. During the perceptual component of the task, the SSVEP amplitude was reduced at left and right parieto-occipital sites. During the hold or memory component of the task, the SSVEP amplitude exhibited a load-dependent increase at frontal and occipito-parietal sites, while the SSVEP latency exhibited a load-dependent reduction at central and left frontal sites. We suggest that SSVEP amplitude changes index cortical information processing modes in that perceptual processes are associated with an SSVEP amplitude reduction, while holding information in active short-term or working memory is associated with an SSVEP amplitude increase. We also discuss changes in SSVEP amplitude and latency in terms of changes in the behavior of cortico-cortico and thalamo-cortico loops that utilize cortical layer I. Such cortico-cortico and thalamo-cortical loops are also proposed to constitute a neurophysiological mechanism for holding information in working memory.