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  • 1
    Electronic Resource
    Electronic Resource
    Implications of recording strategy for estimates of neocortical dynamics with electroencephalography (1993)
    Woodbury, NY : American Institute of Physics (AIP)
    Chaos 3 (1993), S. 257-266 
    Details
    ISSN: 1089-7682
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Neocortical dynamics evidently involves very complex, nonlinear processes including top-down and bottom-up interactions across spatial scales. The dynamics may also be strongly influenced by global (periodic) boundary conditions. The primary experimental measure of human neocortical dynamics at short time scales (∼few ms) is the scalp electroencephalogram (EEG). It is shown that different recording and data analysis strategies are sensitive to different parts of the spatial spectrum. Thus experimental measures of system dynamics (e.g., correlation dimension estimates) can generally be expected to depend on experimental method. These ideas are illustrated in two ways: a large scale, quasilinear theory of neocortical dynamics in which standing wave phenomenon occur with predicted frequencies in the general range of EEG, and a relatively simple nonlinear physical system consisting of a linear string with attached nonlinear springs. The string/springs system is integrated numerically to illustrate transitions from periodic to chaotic behavior as mesoscopic nonlinear influences dominate macroscopic linear effects. The implications of these results for new theories of neocortical dynamics, experimental estimates of dynamic properties, and cognitive EEG studies are considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.165989
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Estimation of large scale neocortical source activity with EEG surface Laplacians (1989)
    Springer
    Brain topography 2 (1989), S. 141-154 
    Details
    ISSN: 1573-6792
    Keywords: Spatial scale ; Laplacian ; Source ; Forward problem ; Inverse problem ; Evoked potential ; P300
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Relationships between neural current sources recorded at different spatial and temporal scales are considered. A number of computer simulations using a three-concentric spheres model of the head are used to illustrate relationships between distributed neocortical sources at the macrocolumn scale and resulting scalp potentials. Computer simulations also illustrate the advantages of calculating surface Laplacians which are more sensitive than potentials to local sources. Surface Laplacians of interictal, epileptic spikes and auditory evoked potentials (including P300) are presented as examples of this sensitivity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01128851
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    A Visual study of surface potentials and Laplacians due to distributed neocortical sources: Computer simulations and evoked potentials (1991)
    Springer
    Brain topography 4 (1991), S. 151-168 
    Details
    ISSN: 1573-6792
    Keywords: Spline ; Laplacian ; Sampling ; Reference electrode ; SEP ; P300
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary A "picture book" of surface potentials, Laplacians, and magnetic fields due to distributed, neocortical sources is presented. The mathematically simulated data is based on 4200 current sources at the macrocolumn scale. Estimated scalp surface maps are based on the three-concentic spheres model of the head. Emphasis is placed on the effects of sampling with a limited number of electrodes, the choice of reference electrode, and the use of the spline Laplacian to improve spatial resolution. The spline Laplacian is applied to median and ulnar nerve somatosensory evoked potentials and to auditory evoked potentials including P300. Substantial improvement in spatial resolution over conventional methods is obtained. The implementation of practical high resolution EEG systems based on the spline Laplacian is considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01132772
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Spatial sampling and filtering of EEG with spline Laplacians to estimate cortical potentials (1996)
    Springer
    Brain topography 8 (1996), S. 355-366 
    Details
    ISSN: 1573-6792
    Keywords: Spatial nyquist ; Laplacian ; Splines
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01186911
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Generation of human EEG by a combination of long and short range neocortical interactions (1989)
    Springer
    Brain topography 1 (1989), S. 199-215 
    Details
    ISSN: 1573-6792
    Keywords: EEG theory ; neocortical dynamics ; EEG dispersion relations ; alpha rhythm ; standing waves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary A theory of neocortical interactions is developed involving both local delays (PSP rise and decay times) and global delays due to finite velocity of action potentials in corticocortical fibers. The theory is based on plausible assumptions regarding input/output relations in neocortical columns and realistic neural parameters. The simultaneous existence of short wavelength waves propagating away from multiple epicenters and long wavelength standing waves due to global boundary conditions is predicted. Both phenomena appear to have dominant oscillation frequencies in the general range of observed EEG phenomena in humans. A mechanism by which removal of diffuse input from the reticular formation may cause an abrupt drop in EEG frequency (as in the transition from the awake to sleeping state) is postulated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01129583
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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    Quantitative representation of the upper surface of the human head (1991)
    Springer
    Brain topography 3 (1991), S. 365-371 
    Details
    ISSN: 1573-6792
    Keywords: Head shape ; Laplacian ; Current source density ; Inverse problem, Ellipsoid
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary A three dimensional digitizer was used to locate 62 positions on an electrode cap placed on the heads of nine subjects. Nonlinear regression methods were used to obtain the best fit sphere, ellipsoid, and perturbed ellipsoid. RMS errors varied from approximately 1 cm to 2 mm, depending on subject and surface chosen. The spherical coordinates of the standard 10/20 electrodes on best fit spheres were obtained. A method to estimate ellipsoid axes using only three standard EEG head dimensions with an ordinary tape measure is derived from the data. The results have at least two potential applications: the forward/inverse problem in EEG and MEG, and the estimation of surface Laplacians from EEG data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01129639
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    Paper (German National Licenses)
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  • 7
    Electronic Resource
    Electronic Resource
    On the Relationship of Synaptic Activity to Macroscopic Measurements: Does Co-Registration of EEG with fMRI Make Sense? (2000)
    Springer
    Brain topography 13 (2000), S. 79-96 
    Details
    ISSN: 1573-6792
    Keywords: EEG ; ERP ; fMRI ; PET ; Co-registration ; Generators ; Current sources ; Dipole localization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1026683200895
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    Paper (German National Licenses)
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  • 8
    Electronic Resource
    Electronic Resource
    The surface laplacian, high resolution EEG and controversies (1994)
    Springer
    Brain topography 6 (1994), S. 221-226 
    Details
    ISSN: 1573-6792
    Keywords: Spline-Laplacian ; Cortical imaging ; Source localization ; EEG ; Spatial resolution
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The surface Laplacian estimate (i.e., current source density) as obtained with spline functions is evaluated in the context of some recent controversies concerning high resolution EEG and source localization. In simulation studies, the spline-Laplacian provides much better estimates of cortical surface potential than is obtained from raw scalp potential, provided dense electrode arrays (e.g., 64 or more electrodes) are used. Spline-Laplacians (which are relatively independent of volume conductor model) provide estimates of cortical potential distribution which are quite similar to those obtained with a cortical imaging algorithm based on a four sphere model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01187712
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    Paper (German National Licenses)
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