Robert D. Sidman

A reliable method for localizing deep intracranial sources of the EEG

We have demonstrated the reliability of a noninvasive method for successfully localizing the intracranial origin of the EEG. The dipole localization method (DLM) is a computer-assisted, mathematical method based on electrical field theory and is similar to localization methods currently used by electrocardiologists. In 12 patients with intractable epilepsy who were being evaluated for surgery, a known current source was introduced between two adjacent depth electrodes. Using scalprecorded EEG only, DLM accurately and reliably localized the source to within 2 cm of the known origin in all instances where a discrete source was present. We conclude that DLM is a valid and reliable noninvasive method for localizing the intracranial source of some scalp-recorded EEG potentials, and that in some patients, use of this method may obviate the need for depth electrode implantation.

Age-related features of the resting and P300 auditory evoked responses using the dipole localization method and cortical imaging technique

Two mathematical techniques, the dipole localization method (DLM) and the cortical imaging technique (CIT), are used to analyze the resting and P300 auditory responses in young and old normal volunteers. These methods identify certain age-related features of these evoked responses that are not found by standard topographic methods. These features include the orientation of the P200 resting response, and the laterality of the N120 response, and the eccentricity of the P300 response in the P300 stimulus condition. Theoretical dipole sources and simulated cortical surface maps are also constructed for one normal subject and one psychiatric inpatient and compared. These mathematical methods appear to enhance the discriminating power of traditional electrophysiological measures.

Numerical tests of a method for simulating electrical potentials on the cortical surface

The cortical imaging technique (CIT) based on solving a harmonic inward continuation problem is validated by applying it to artificially derived data. Pairs of dipolar sources with different depths and separations are introduced into a spherical conducting medium simulating the head. Scalp potential maps are constructed by interpolating the simulated data between 28 scalp electrode positions. Noise is added to the data to approximate the variability in measured potentials that would be observed in practice. CIT is used in each case to construct potential maps on layers concentric to and within the layer representing the scalp. In several instances when the dipole pair is deep and closely spaced, the sources cannot be separated by the scalp topographical maps but are easily separated by the cortical topographical maps. CIT is also applied to scalp-recorded potentials evoked by bilateral median nerve stimulation and pattern-reversal visual stimulation. Applications of CIT to scalp-recorded potentials suggest that it might be possible to draw inferences about the neural generators of these potentials.<<ETX>>

A method for simulating intracerebral potential fields: the cortical imaging technique.

Source localization techniques such as the dipole localization method (DLM) have been used to elucidate the neural origins of scalp-recorded potentials. The type of source assumed in such techniques is usually suggested by the distribution of voltage maximums and minimums in scalp topographical contour maps. Unfortunately, the physical layers between the neural generators and scalp recording sites tend to smear and attenuate the potential fields, making it impossible, in some cases, to distinguish between single and multiple sources or extended layers. In this report, a mathematical (noninvasive) technique is described for simulating the potential fields that could be recorded directly on the surface of the brain. Such “cortical” potential fields exhibit details that are not apparent in the scalp topography. In several recent publications, this cortical imaging technique (CIT) has been tested on artificial and experimental data. After describing these results, some possible applications of CIT to clinical data will be presented.

Scalp and depth recordings of induced deep cerebral potentials

A balanced square wave was introduced between two adjacent depth electrodes implanted in the course of studying patients with intractable epilepsy and who were being considered for surgery. The stimulus current was designed so that charge density loading was well within limits of safety to avoid tissue damage. No neuronal activation was seen, and the stimulus intensity was significantly less than that used in subsequent stimulation session for the purpose of eliciting a clinical response and after-discharges. Averaging techniques were used to record the stimulus at distant electrodes both within the cerebrum and on the scalp. The recorded voltage decrement from the source was nearly identical with the theoretical voltage decrement predicted using principles of electric field theory in which the brain was assumed to be a homogeneous conductive medium. When the voltage recorded on the scalp was compared with the voltages recorded from depth electrodes, it was found that the effect of the highly resistive skull on voltage decrement was relatively less the more centric the source. This result also confirmed predictions based on electric field theory. Most significantly, voltages well within the physiologic range introduced in deep mesial temporal lobe structures were recorded from the scalp.

The Time-Dependent Equivalent Dipole Source for the Response to Median Nerve Stimulation

A single current dipole source, varying in time, is used to characterize the neural generators of a component of the response evoked by median nerve stimulation. By allowing the equivalent source to move in time, it may be possible to model the movement of currents along sensory pathways.

Spatio-Temporal Progression of the AEP P300 Component Using the Cortical Imaging Technique

SummaryThe cortical imaging technique (CIT), a mathematical method for simulating the potential fields on the surface of the brain, was used to analyze the spatio-temporal progression of the AEP P300 component (as well as the preceding and subsequent N2a and N3 components) from thirty normal adult subjects recorded in a standard “oddball” paradigm. Comparisons were made between the progressions of the endogenous event-related cognitive potentials and the exogenous stimulus-dependent potentials (Nl component). Cortical imaging results suggest that different and multiple generator sites are involved in the production of exogenous and endogenous evoked responses. We particularly note the asymmetric development of the P300 component and the apparent anterior generator sites for the N2a component. This last result is interesting because the N2a precedes the P300 component and supports an earlier frontal contribution.

Visualization of the Cortical Potential Field by Medical Imaging Data Fusion

We describe the visualization of the potential field on the scalp and on the cortical surface. The surfaces are derived from magnetic resonance imaging data and the potential fields are reconstructed from electroencephalography data. The visualization tool is validated with clinical and cognitive application studies.