Contributions of Stereotactic EEG Electrodes in Grey and White Matter to Speech Activity Detection.

Recent studies have shown it is possible to decode and synthesize speech directly using brain activity recorded from implanted electrodes. While this activity has been extensively examined using electrocorticographic (ECoG) recordings from cortical surface grey matter, stereotactic electroen-cephalography (sEEG) provides comparatively broader coverage and access to deeper brain structures including both grey and white matter. The present study examines the relative and joint contributions of grey and white matter electrodes for speech activity detection in a brain-computer interface.

Prefrontal High Gamma in ECoG Tags Periodicity of Musical Rhythms in Perception and Imagination.

Rhythmic auditory stimuli are known to elicit matching activity patterns in neural populations. Furthermore, recent research has established the particular importance of high-gamma brain activity in auditory processing by showing its involvement in auditory phrase segmentation and envelope tracking. Here, we use electrocorticographic (ECoG) recordings from eight human listeners to see whether periodicities in high-gamma activity track the periodicities in the envelope of musical rhythms during rhythm perception and imagination. Rhythm imagination was elicited by instructing participants to imagine the rhythm to continue during pauses of several repetitions. To identify electrodes whose periodicities in high-gamma activity track the periodicities in the musical rhythms, we compute the correlation between the autocorrelations (ACCs) of both the musical rhythms and the neural signals. A condition in which participants listened to white noise was used to establish a baseline. High-gamma autocorrelations in auditory areas in the superior temporal gyrus and in frontal areas on both hemispheres significantly matched the autocorrelations of the musical rhythms. Overall, numerous significant electrodes are observed on the right hemisphere. Of particular interest is a large cluster of electrodes in the right prefrontal cortex that is active during both rhythm perception and imagination. This indicates conscious processing of the rhythms' structure as opposed to mere auditory phenomena. The autocorrelation approach clearly highlights that high-gamma activity measured from cortical electrodes tracks both attended and imagined rhythms.

Control of a brain-computer interface using stereotactic depth electrodes in and adjacent to the hippocampus.

A brain-computer interface (BCI) is a device that enables severely disabled people to communicate and interact with their environments using their brain waves. Most research investigating BCI in humans has used scalp-recorded electroencephalography or intracranial electrocorticography. The use of brain signals obtained directly from stereotactic depth electrodes to control a BCI has not previously been explored. In this study, event-related potentials (ERPs) recorded from bilateral stereotactic depth electrodes implanted in and adjacent to the hippocampus were used to control a P300 Speller paradigm. The ERPs were preprocessed and used to train a linear classifier to subsequently predict the intended target letters. The classifier was able to predict the intended target character at or near 100% accuracy using fewer than 15 stimulation sequences in the two subjects tested. Our results demonstrate that ERPs from hippocampal and hippocampal adjacent depth electrodes can be used to reliably control the P300 Speller BCI paradigm.

Commonalities and differences among vectorized beamformers in electromagnetic source imaging.

A number of beamformers have been introduced to localize neuronal activity using magnetoencephalography (MEG) and electroencephalography (EEG). However, currently available information about the major aspects of existing beamformers is incomplete. In the present study, detailed analyses are performed to study the commonalities and differences among vectorized versions of existing beamformers in both theory and practice. In addition, a novel beamformer based on higher-order covariance analysis is introduced. Theoretical formulas are provided on all major aspects of each beamformer; to examine their performance, computer simulations with different levels of correlation and signal-to-noise ratio are studied. Then, an empirical data set of human MEG median-nerve responses with a large number of neuronal generators is analyzed using the different beamformers. The results show substantial differences among existing MEG/EEG beamformers in their ways of describing the spatial map of neuronal activity. Differences in performance are observed among existing beamformers in terms of their spatial resolution, false-positive background activity, and robustness to highly correlated signals. Superior performance is obtained using our novel beamformer with higher-order covariance analysis in simulated data. Excellent agreement is also found between the results of our beamformer and the known neurophysiology of the median-nerve MEG response.

A model for frequency dependence of conductivities of the live human skull.

A mathematical model (sigma(omega) approximately equal to A omega alpha, where, sigma is identical with conductivity, omega = 2 pi f is identical with applied frequency (Hz), A (amplitude) and alpha (unit less) is identical with search parameters) was used to fit the frequency dependence of electrical conductivities of compact, spongiosum, and bulk layers of the live and, subsequently, dead human skull samples. The results indicate that the fit of this model to the experimental data is excellent. The ranges of values of A and alpha were, spongiform (12.0-36.5, 0.0083-0.0549), the top compact (5.02-7.76, -0.137-0.0144), the lower compact (2.31-10.6, 0.0267-0.0452), and the bulk (7.46-10.6, 0.0133-0.0239). The respective values A and alpha for the respective layers of the dead skull samples were (40.1-89.7, -0.0017-0.0287), (5.53-14.5, -0.0296 - -0.0061), (4.58-15.9, -0.0226-0.0268), and (12.7-25.3, -0.0158-0.0132).

Conductivities of three-layer live human skull.

Electrical conductivities of compact, spongiosum, and bulk layers of the live human skull were determined at varying frequencies and electric fields at room temperature using the four-electrode method. Current, at higher densities that occur in human cranium, was applied and withdrawn over the top and bottom surfaces of each sample and potential drop across different layers was measured. We used a model that considers variations in skull thicknesses to determine the conductivity of the tri-layer skull and its individual anatomical structures. The results indicate that the conductivities of the spongiform (16.2-41.1 milliS/m), the top compact (5.4-7.2 milliS/m) and lower compact (2.8-10.2 milliS/m) layers of the skull have significantly different and inhomogeneous conductivities. The conductivities of the skull layers are frequency dependent in the 10-90 Hz region and are non-ohmic in the 0.45-2.07 A/m2 region. These current densities are much higher than those occurring in human brain.

Conductivities of three-layer human skull.

In this study, electrical conductivities of compact, spongiosum, and bulk layers of cadaver skull were determined at varying electric fields at room temperature. Current was applied and withdrawn over the top and bottom surfaces of each sample and potential drop across different layers was measured using the four-electrode method. We developed a model, which considers of variations in skull thicknesses, to determine the conductivity of the tri-layer skull and its individual anatomical structures. The results indicate that the spongiform and the two compact layers of the skull have significantly different and inhomogeneous conductivities ranging from 0.76 +/- .14 to 11.5 +/- 1.8 milliS/m.

Magnetoencephalographic characterization of sleep spindles in humans.

Sleep spindles in EEG recordings of adults are most prominent over the central and frontal midline regions. Early magnetoencephalographic recordings agreed with conventional EEG findings. However, more recent small-array magnetoencephalography and quantitative EEG studies suggest that the source areas for spindles are more widespread. We used a whole-head 122-channel biomagnetometer to characterize the sources of sleep spindles in four normal volunteers. Parallel interactive and automated multiple dipole spatiotemporal source modeling was conducted on the data sets of 10 spindles from each subject. Principal component analysis was used to estimate the number of sources in interactive source modeling, and singular value decomposition was used in automated dipole modeling. Spectral analysis of the epochs containing sleep spindles was performed. Principal component analysis and singular value decomposition suggested that all sleep spindles were made up of activity from multiple sources. Similarly, interactive and automated multiple dipole source modeling showed that three or more sources were present in 75% of spindle bursts. The sources for sleep spindles localized to all four cerebral lobes. Parietal and frontal lobes were the areas most frequently involved. Interactive source modeling resulted in more frequent temporal lobe than occipital dipole localizations; automated source modeling showed more frequent occipital than temporal sources. Spindle source localizations varied across subjects and across different spindles within subjects. Our results indicate that individual sleep spindles are generated by multiple cortical sources that are widespread within and across individuals.

Frequency of bitemporal independent interictal epileptiform discharges in temporal lobe epilepsy.

PURPOSE: Bitemporal interictal epileptiform discharges (IEDs) occur in < or =42% of scalp EEGs in patients with temporal lobe epilepsy (TLE) studied with routine EEGs or partial analysis of long-term recordings. METHODS: Twenty-eight patients with TLE demonstrating exclusively unilateral temporal IEDs on routine EEGs underwent 24-h continuous recording. The entire record was visually inspected for epileptiform discharges. We used continuous EEG to assess the significance of long-term recording in detecting bilateral IEDs. RESULTS: Twenty-two patients had left temporal IEDs; 21 had right temporal IEDs. Seventeen (61%) patients had IEDs originating from both the right and left temporal lobes. The probability of detecting bilateral independent IEDs was correlated with the duration of continuous EEG recording. There was no correlation between the number of IEDs originating from one side and the probability of detecting independent IEDs on the other side. The frequencies of IEDs were not correlated with the length of time since onset of epilepsy. CONCLUSIONS: The findings suggest that when long-term recordings are performed, the incidence of bilateral discharges in TLE is higher than previously reported in the literature and supports the view that TLE is commonly a bilateral disease.

Lambda waves: incidence and relationship to photic driving.

The earliest studies on lambda waves in the 1950s postulated a relationship with the photic driving response. Intracranial depth electrode studies from the 1960s showed generators for lambda waves and the occipital driving response to be different but both originating from the posterior hemisphere. Use of computer averaging techniques in the 1980s found these two cerebral responses to be similarly affected by certain diseases. We examined the incidence of lambda over a wider range of ages than previously studied, and also evaluated for a statistical correlation between lambda and occipital driving. Three hundred sixty-seven conventional EEGs were prospectively collected and analyzed. Each record was interpreted for the presence of lambda waves, photic driving, and epileptiform discharges. The incidence of lambda waves, photic driving and epileptiform discharges was 76%, 85%, and 23%, respectively. The incidence of lambda and photic driving exhibited age-dependent characteristics. The amplitude and duration of lambda waves also exhibited age-dependent characteristics. A strong correlation existed between the presence of lambda waves and photic driving (p< 0.001). No significant correlation existed between epileptiform discharges and either lambda waves or photic driving. The strong correlation between the presence of lambda waves and the occipital photic driving response suggests the possibility a common region in the brain needs to be functionally activated in order to express these waveforms.

Global brain dysfunction in an infant with pyridoxine dependency: evaluation with EEG, evoked potentials, MRI, and PET.

Pyridoxine dependency often presents with intractable seizures in neonates and infants. We describe an infant with pyridoxine-dependent seizures and report the first PET and evoked potential results and EEG and MRI findings. All studies show either diffuse structural or functional abnormality. Our data suggest this coenzyme deficiency causes a generalized disease process.