Realistic modeling of VEP topography

Effect of brain damage and source location on left-right asymmetry of visual evoked potentials in a realistic model of the head

The left-right asymmetry of visual evoked potentials (VEP) was studied using a realistic three-dimensional model of the head, constructed from CT scans. The integral equation describing the potential distribution due to an inner current-dipole (simulating the VEP source), inside the biological volume conductor (the head) was solved numerically using the finite volume method. The effect of several structural parameters, such as the natural head geometry, the location and orientation of the current source, and conductivity changes (simulating a brain damage) on the VEP asymmetry was examined. The results revealed that the natural anatomical asymmetry presented in a normal head induces some degree of VEP asymmetry (up to 0.42% at the occipital electrodes), yet the major cause of left-right asymmetry is due to asymmetrical location of the source: up to 6.53% in the O(2)-O(1) electrodes for an angular shift of 3 degrees to the left. It was also found that conductivity changes inside the head have a smaller effect on the VEP asymmetry (up to 3.35% in hemorrhaged brain compared to 1.96% in the normal brain at the C(4)-C(3) electrodes). These findings may help in better understanding VEP sources of asymmetry, originating not only due to cerebral functionality, but from structural parameters as well.

Asymmetric scalp distribution of pattern visual evoked potentials during interictal phases in migraine

The N70 and P100 components of transient pattern visual evoked potentials (P-VEPs) were measured in migraine patients, with and without aura, and in normal subjects in order to evaluate their latency, amplitude and occipital scalp distribution. The aim was to find any typical electrophysiological abnormalities in migraine. P-VEP N70 and P100 were analyzed in 59 patients without any known visual field defect. Mean latency and amplitude values were within normal ranges for either N70 and P100 all over the occipital scalp; the only significant abnormality we found was related to the absolute right-left amplitude ratio either for N70 and P100 waves, providing an asymmetry in P-VEP scalp distribution; this finding was detected in 78.9% of patients with aura and 72.5% without aura. Our results show that in migraine patients, both P-VEP waves N70 and P100, have an asymmetric topographic distribution, even during interictal phases, that can be explained by a cortical disturbance in agreement with the neural hypothesis of headache.

The influence of skull-conductivity misspecification on inverse source localization in realistically shaped finite element head models

The electric conductivities of different tissues are important parameters of the head model and their precise knowledge appears to be a prerequisite for the localization of electric sources within the brain. To estimate the error in source localization due to errors in assumed conductivity values, parameter variations on skull conductivities are examined. The skull conductivity was varied in a wide range and, in a second part of this paper, the effect of a nonhomogeneous skull conductivity was examined. An error in conductivity of lower than 20% appears to be acceptable for fine finite element head models with average discretization errors down to 3 mm. Nonhomogeneous skull conductivities, e.g., sutures, yield important mislocalizations especially in the vincinty of electrodes and should be modeled.

Inverse localization of electric dipole current sources in finite element models of the human head

The paper describes finite element related procedures for inverse localization of multiple sources in realistically shaped head models. Dipole sources are modeled by placing proper monopole sources on neighboring nodes. Lead field operators are established for dipole sources. Two different strategies for the solution of inverse problems, namely combinatorial optimization techniques and regularization methods are discussed and applied to visually evoked potentials, for which exemplary results are shown. Most of the procedures described are fully automatic and require only proper input preparation. The overall work for the example presented (from EEG recording to visual inspection of the results) can be performed in roughly a week, most of which is waiting time for the computation of the lead field matrix or inverse calculations on a standard and affordable engineering workstation.

Estimating cortical activity from VEPS with the shrinking ellipsoid inverse

An iterative inverse method using Tikhonov regularization (the shrinking ellipsoid method) previously tested in a model system is used to invert the sequence of bioelectric scalp fields evoked by the onset of a checkerboard pattern in either the right or left lower hemifield. The shrinking ellipsoid method is modified from its original description to accommodate simultaneously inverting a sequence of thirteen VEP scalp fields measured from 65 to 125 ms after stimulus onset. This allows the evoked cortical activity to be tracked in 5-ms intervals without distortion due to occasional VEP scalp fields in the sequence that have too low a signal-to-noise ratio to be reliably inverted in isolation. A new method is described to identify the surface of the cortex from MRI data. This is required to implement the shrinking ellipsoid inverse. Results from two subjects studied in detail are presented. The earliest cortical activity occurs either in area MT (the middle temporal area) or simultaneously in MT and striate cortex (V1). However when it does occur in both areas, the activity in V1 is relatively weak and quickly subsides. Seventy-five ms after stimulus onset activity is seen mainly near MT corresponding to a region identified from PET studies as one that subserves motion processing. Activity moves to V1 by 90-100 ms after stimulus onset. Near 120 ms after stimulus onset, cortical activity returns to the region near MT. Virtually all activity identified in this time epoch occurs in the cortical hemisphere contralateral to the location of the stimulus in the visual field.

The electrical conductivity of human cerebrospinal fluid at body temperature

The electrical conductivity of human cerebrospinal fluid (CSF) from seven patients was measured at both room temperature (25 degrees C) and body temperature (37 degrees C). Across the frequency range of 10 Hz-10 kHz, room temperature conductivity was 1.45 S/m, but body temperature conductivity was 1.79 S/m, approximately 23% higher. Modelers of electrical sources in the human brain have underestimated human CSF conductivity by as much as 44% for nearly two decades, and this should be corrected to increase the accuracy of source localization models.

Bilateral comparison of occipital lobe sulci: a sulcus identifying algorithm

A procedure for automatic identification of sulci of cerebral hemispheres in magnetic resonance images is presented. The algorithm departs from information of the spatial location of a given feature (e.g., sulcal contour) in one hemisphere and automatically defines its course in the other hemisphere. The automatic tracing of sulci was successfully performed in the mesial aspect of both hemispheres in 18 normal individuals. The sulcal location in the target hemisphere was first approximated by assuming an affine transformation between hemispheres, and then refined by local edge analysis. The method produced reliable results in comparing the intersulcal areas (the cuneus), sulcal length, their complexity, and the angle between the pariteooccipital and retrocalcarine fissures.

Iterative refinement of the minimum norm solution of the bioelectric inverse problem

Functional brain imaging based on the bioelectric scalp field depends on the solution of the inverse problem. The object is to estimate the current distribution in the cortex from a measured noisy scalp potential field. A minimum norm least squares solution is often used for this purpose. Although this approach leads to a unique solution, it represents only one of a set of feasible solutions. In particular, it leads to a solution in which current is found to be generated widely in the cortex. There are other feasible minimum norm solutions for which cortical current is generated only in a restricted region, and it is likely that in many cases, these solutions are of physiological importance. This study presents a method to uncover these solutions. It is based on the idea that a minimum norm solution can be used to define a region of interest, an ellipsoid, within which it is worthwhile to search for another feasible solution. The minimum norm approach is used iteratively and the ellipsoid shrinks. As it shrinks it provides evidence pointing to the location of the active cortical region. The method explicitly recognizes the role of measurement noise in defining feasible solutions. It is tested in a model of the human head that incorporates a realistic cortex in a three-shell sphere. The method improves the localization of cortical activity mimicking physiological processing.

Functional brain imaging: dipole localization and Laplacian methods

The performance of two methods, used to localize brain activity from evoked potential fields measured on the scalp, was assessed in a tank model of the human head. This physical model contained a human skull encased in a polymer simulating the resistivity and geometry of brain and scalp. The dipole localization method mislocalized the positions of known dipole sources by several centimeters. The mislocalization was systematic. The dipoles were localized too deeply in the head. The Laplacian method yielded a field resembling the brain surface field (epicortical potential field) provided that the iso-potential contours of the scalp field closed within the measurement range. Clipping resulted in a serious mislocalization of the position of the peak of the epicortical potential field.

A short latency cortical component of the foveal VEP is revealed by hemifield stimulation

Transient evoked potentials were recorded simultaneously over 5 electrodes placed in a horizontal row across the occiput. A range of spatial frequencies were presented as either full-field or hemifield stimuli. Subjects were 11 normal observers and 5 patients with lesions causing a homonymous hemianopic field defect. The shortest latency peak response was at approximately 70 msec, a negative potential (N70). For all spatial frequencies, full-field stimuli evoked a lower amplitude N70 at the midline than the sum of N70 amplitudes to two hemifield stimuli, suggesting partial cancellation. The latency and amplitude of N70 increased as spatial frequency increased. N70 and P100 differed in respect to their response to spatial frequency and field size, further suggesting that they may not be subsets of a unitary response. For hemifield stimulation, N70 had an ipsilateral maximum and attenuated or completely reversed in polarity across the midline. Consistent with the data of normals using hemifield stimuli, in 5 patients a full-field stimulus elicited an N70 lateralized contralaterally to the homonymous hemianopia, i.e., the ipsilateral N70 was absent. The absolute amplitude difference between the left and right electrodes was significant for hemifield stimulation in normals and full-field stimulation in the patients, but not for full-field stimulation in normals. Our results imply that the evaluation of N70 hemispheric distribution is useful for the evaluation of paramacular visual field defects.