Equivalent Dipole Model Research Articles

Identification of the hemisphere activated by hemifield visual stimulation using a single equivalent dipole model

Topographic amplitude distribution of the hemifield pattern visual evoked potential (PVEP) shows substantial intersubject variability. Many subjects have larger P100 amplitudes paradoxically over the hemisphere ipsilateral to the stimulated field, whereas others show larger responses over the stimulated hemisphere. The present study was designed to determine whether a single equivalent dipole model could correctly identify the field of stimulation, and therefore the hemisphere activated, under conditions in which the surface distribution is variable. Under conditions used in the present study, visual examination of the surface amplitude distribution of the P100 peak could not be reliably used to identify the hemifield that was stimulated. Of 28 hemifield PVEPs, obtained from 14 normal subjects, only 13 showed higher amplitude ipsilateral to the field of stimulation. Thus, neither examination of EP wave forms nor topographic maps provided an accurate means for determination of which hemifield was stimulated or which hemisphere was activated. The single equivalent dipole model correctly identified the stimulated hemisphere for 25 28 hemifield PVEPs. Orientation of the equivalent dipole accounted for much of the variability in surface amplitude distribution, with tangential orientations obtained in subjects with ipsilaterally predominant P100 surface topography. Although the dipole model improved identification of the stimulated hemisphere, dipoles were located anterior or inferior to the occipital lobe in some subjects. Results suggest that dipole modeling can provide useful information regarding the source of surface recorded potentials.

Seeing through the skull: Advanced EEGs use MRIs to accurately measure cortical activity from the scalp

There is a vast amount of untapped spatial information in scalp-recorded EEGs. Measuring this information requires use of many electrodes and application of spatial signal enhancing procedures to reduce blur distortion due to transmission through the skull and other tissues. Recordings with 124 electrodes are now routinely made, and spatial signal enhancing techniques have been developed. The most advanced of these techniques uses information from a subject's MRI to correct blur distortion, in effect providing a measure of the actual cortical potential distribution. Examples of these procedures are presented, including a validation from subdural recordings in an epileptic patient. Examples of equivalent dipole modeling of the somatosensory evoked potential are also presented in which two adjacent fingers are clearly separated. These results demonstrate that EEGs can provide images of superficial cortical electrical activity with spatial detail approaching that of O15 PET scans. Additionally, equivalent dipole modeling with EEGs appears to have the same degree of spatial resolution as that reported for MEGs. Considering that EEG technology costs ten to fifty times less than other brain imaging modalities, that it is completely harmless, and that recordings can be made in naturalistic settings for extended periods of time, a greater investment in advancing EEG technology seems very desirable.

Surface dipole densities in lipid monolayers

Surface dipole density differences, μ, between coexisting liquid phases in lipid monolayers have been determined from an analysis of the Brownian motion of trapped lipid domains, as well as from surface potential measurements on each liquid phase. The coexisting liquid phases arise in binary mixtures of dimyristoylphosphatidylcholine (DMPC) with cholesterol or with dihydrocholesterol. Values of μ determined from Brownian motion, surface potential measurements, and measurements of domain electrophoretic mobility provide support for the theoretical equivalent dipole model used previously to describe these monolayers. Air oxidation and ozonolysis of cholesterol in monolayers is detected through the effect of an oxidation product-mediated reduction of interdomain line tension

Localization of the sources of EEG delta, theta, alpha and beta frequency bands using the FFT dipole approximation

FFT dipole approximation and 3-dimensional dipole modelling were used to determine the locations of the equivalent dipole model sources of the delta, theta, alpha, beta-1 and beta-2 frequency bands in 13 normal subjects during resting. From each subject, 2 successive data sets were analysed, each consisting of 10 epochs of 2 sec randomly collected during 30 min. ANOVAs showed that over subjects, the source locations of EEG frequency bands differed significantly in the vertical and antero-posterior dimensions. Results of data set 2 confirmed those of data set 1. The source of delta was deepest and most anterior, theta more posterior and less deep, alpha most posterior and highest on the vertical dimension, beta-1 deeper and slightly more anterior than alpha, and beta-2 again more anterior and deeper than beta-1. Thus, the depth of source location was not linearly related to temporal frequency. The sources of all 5 bands were oriented in the sagittal direction; delta mean fields had steeper gradients anteriorly, alpha and beta-1 posteriorly. The power map for any frequency was well described by a single phase angle. The results indicate that the different EEG frequency bands during a given EEG epoch are generated by neural populations in different brain locations.

Seeing through the skull: Advanced EEGs use MRIs to accurately measure cortical activity from the scalp

There is a vast amount of untapped spatial information in scalp-recorded EEGs. Measuring this information requires use of many electrodes and application of spatial signal enhancing procedures to reduce blur distortion due to transmission through the skull and other tissues. Recordings with 124 electrodes are now routinely made, and spatial signal enhancing techniques have been developed. The most advanced of these techniques uses information from a subject's MRI to correct blur distortion, in effect providing a measure of the actual cortical potential distribution. Examples of these procedures are presented, including a validation from subdural recordings in an epileptic patient. Examples of equivalent dipole modeling of the somatosensory evoked potential are also presented in which two adjacent fingers are clearly separated. These results demonstrate that EEGs can provide images of superficial cortical electrical activity with spatial detail approaching that of O15 PET scans. Additionally, equivalent dipole modeling with EEGs appears to have the same degree of spatial resolution as that reported for MEGs. Considering that EEG technology costs ten to fifty times less than other brain imaging modalities, that it is completely harmless, and that recordings can be made in naturalistic settings for extended periods of time, a greater investment in advancing EEG technology seems very desirable.

The magnetoencephalographic localisation of source-systems in the brain: Early and late components of event related potentials

The application of magnetoencephalography (MEG) to the analysis of sources in the brain responsible for early and late components of evoked potentials is discussed. Representative data are presented and discussed which demonstrate localisation of sources assuming single equivalent dipoles. Distributed systems as sources for some steady-state responses are discussed in relation to the broader issue of the usefulness of equivalent single dipole models. These issues are related to the use of MEG for the analysis of source systems influenced by alcohol and other drugs.

The n.m.r. spectra of porphyrins. 13—A ring current model for the porphyrin and chlorin (7,8‐dihydroporphyrin) rings

AbstractThe equivalent dipole model of the ring current shift in benzene is shown to be equivalent to that of the well‐known two current loop calculation. A network model of the ring current effect in the porphyrin system is described, using the double–dipole approximation, to give a calculation of the ring current shifts in the porphyrin system; this agrees with the observed shifts of protons both in the ring‐plane and above it. A simple modification of the model enables treatment of ring current shifts in the chlorin ring. These models may be used to provide, very simply, good estimates of the ring current shifts of the porphyrin and chlorin rings at points above and outside the current loops; the agreement is sufficiently good to allow assignments of peripheral substituents to be made, and to provide information on their orientation. The model is consistent with a peripheral ring current loop in both the free‐base porphyrins and their metal complexes. The relationship of these results to calculations in polycyclic aromatics and to protonation shifts in porphyrins is discussed.