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  • 1
    Electronic Resource
    Electronic Resource
    Origin of P16 median nerve SEP component identified by dipole source analysis — subthalamic or within the thalamo-cortical radiation? (1995)
    Springer
    Experimental brain research 104 (1995), S. 511-518 
    Details
    ISSN: 1432-1106
    Keywords: Early SEPs ; P16 ; Source analysis ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Following median nerve stimulation, several monophasic peaks were recorded at the scalp in the 15–18 ms time range. Source analysis, using three different methods, modelled a source near the centre of the head with an orientation towards the activated hemisphere and a peak activity at 16 ms post stimulus. Magnetic recordings detected no signal in this time range, which confirmed a subcortical location of the source. From dipole localization it was not possible to assign the exact origin of the P16 source to either the subthalamic level or the thalamo-cortical radiation, because of the limited spatial resolution at the centre of the spherical head model. An estimate of the conduction velocity of the medial lemniscus pointed towards a subthalamic origin. The P16 source was preserved in two patients with a lesion of the thalamo-cortical radiation and the ventral thalamus. Further evidence for a subthalamic location of P16 was derived from the physical mechanisms generating far-field potentials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00231985
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Comparing Regularized and Non-Regularized Nonlinear Dipole Fit Methods: A Study in a Simulated Sulcus Structure (1999)
    Springer
    Brain topography 12 (1999), S. 3-18 
    Details
    ISSN: 1573-6792
    Keywords: EEG/MEG ; Nonlinear dipole fit ; Simulated annealing ; Regularization ; Truncated singular value decomposition ; Finite element method
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The inverse problem arising from EEG and MEG is largely underdetermined. One strategy to alleviate this problem is the restriction to a limited number of point-like sources, the focal source model. Although the singular value decomposition of the spatio-temporal data gives an estimate of the minimal number of dipoles contributing to the measurement, the exact number is unknown in advance and noise complicates the reconstruction. Classical non-regularized nonlinear dipole fit algorithms do not give an estimate for the correct number because they are not stable with regard to an overestimation of this parameter. Too many sources may only describe noise but can still attain a large magnitude during the inverse procedure and may be indiscernible from the true sources. This paper describes a nonlinear dipole fit reconstruction algorithm with a new regularization approach for the embedded linear problem, automatically controlled by the noise in the data and the condition of the occuring least square problems. The algorithm is stable with regard to source components which “nearly” lie in the kernel of the projection or lead field operator and it thus gives an estimate of the unknown number parameter. EEG simulation studies in a simulated sulcus structure are carried out for an instantaneous dipole model and spatial resolution in the sulcus and stability of the new method are compared with a classical reconstruction algorithm without regularization.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1022281005608
    Library Location Call Number Volume/Issue/Year Availability
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    Paper (German National Licenses)
    Fulltext
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