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Dynamic localisation of evoked potential generators
$227 cance of differences between event-related potential (ERP) scalp distributions for different experimental conditions, subject groups, or ERP components. However, there is a fundamental incompatibility between the additive model upon which A N O V A s are based and the multiplicative effect of ERP voltages produced by differences in source strength. Using potential distributions generated by dipole sources in spherical volume conductor models, we demonstrate that highly significant interactions involving electrode location can be obtained between scalp distributions with identical shapes generated by the same source. Therefore, such interactions cannot be used as unambiguous indications of shape differences between distributions and hence of differences in source configuration. This ambiguity can be circumvented by scaling the data to eliminate overall amplitude differences between experimental conditions before A N O V A is performed. Such analyses retain sensitivity to genuine differences in distributional shape, but do not confuse amplitude and shape differences.
DPWI.08 DYNAMIC LOCALISATION POTENTIAL GENERATORS.
OF
EVOKED
V. lsgum, B. Barac and T.S. Prevec
(Zagreb. Yugoslavia) The dynamic following of sensory information flow through the cerebral sensory pathways and the determination of the time and locus of successive electrically activated pathways enables a functional investigation and comparison of the computed electrical activity of activated structures with scalp-recorded evoked potential distributions. These distributions provide the basic information for the inverse determination of source generators. Besides the classical single dipole model for computing the electrical activity of parallel and simultaneously active neural populalions, the model of two independent dipoles for approximation of two such morphologically and functionally similar structures is used. Because neither of these models can represent adequately the electrical activity dispersed in the brain, (e.g. in the case of the optic radiation and visual cortex) a model of circular dipolar surface is introduced. The parameters of the model are: locus, orientation, radius and intensity of elementary dipole moments. Such models and their time-variable locus and surfaces, for the last case, can describe the pathways and the volume of the analysed information channel. This methodology is demonstrated for the visual system and shows good correlation between computed models and the time of activation of known anatomic pathways.
D P W I . 0 9 DATA R E D U C T I O N O F VEP U S I N G A D I P O L E MODEL. H. T.M. Haenen
(Groningen, Holland) Topographic display of VEPs showed large variability of the P1 (P100) distribution over the posterior scalp in twenty normal subjects. This necessitates a relatively large number of recording sites. We were able to reduce this large amount of data while maintaining the essential features by using a dipole model. Measured potentials from up to eight electrodes could be estimated accurately by the model in the whole normal population. In order to obtain reliable dipole Iocalisation, a special iteration method was developed based on the linear relationship between dipole moment and electrode potentials and constraints on the position of the dipole. The outputs of the model are the three dipole moment components over time. A further reduction is possible by rotating the dipole moment axis such that an optimal signal to noise criterion is fulfilled. The relevant dynamic behaviour of the dipole moment can then be mapped to a single channel. An evaluation of this single channel over the normal population reveals an enhancement of all peaks in the N I P 1 / N 2 P 2 complex, while some peaks which are missing in the original tracings, may appear in the single dipole channel.
D P W I . 1 0 SCALP P O T E N T I A L S AND C U R R E N T DENSITIES M A P P I N G BY S U R F A C E S P L I N E S INTERPOLATION: C O M P A R I S O N W I T H O T H E R M E T H O D S , F. Perrin, O. Bertrand, M.H. Giard and J. Pernier
(Bron, France) In scalp evoked potential mapping, potentials are only measured at the electrode sites. To represent the potential on the whole scalp it is necessary to interpolate in-between these known values. Surface splines are mathematical tools for interpolating functions of two variables. They can be obtained by minimizing the bending energy of an infinite plate constrained to pass through known points. The use of these functions to compute scalp potentials presents several advantages: the coordinates of the electrodes location need not be located in a rectangular array - the maxima and minima are not necessarily displayed at an electrode location - the degree of smoothness of the interpolated surface may be varied -mathematically these functions are easily differentiated making possible the representation of (the 'reference free') scalp radial current densities. We have compared several methods of the spline type and two methods of interpolation based on linear combination of the potentials at the four nearest electrodes.
DPWI.08 DYNAMIC LOCALISATION POTENTIAL GENERATORS.
OF
EVOKED
V. lsgum, B. Barac and T.S. Prevec
(Zagreb. Yugoslavia) The dynamic following of sensory information flow through the cerebral sensory pathways and the determination of the time and locus of successive electrically activated pathways enables a functional investigation and comparison of the computed electrical activity of activated structures with scalp-recorded evoked potential distributions. These distributions provide the basic information for the inverse determination of source generators. Besides the classical single dipole model for computing the electrical activity of parallel and simultaneously active neural populalions, the model of two independent dipoles for approximation of two such morphologically and functionally similar structures is used. Because neither of these models can represent adequately the electrical activity dispersed in the brain, (e.g. in the case of the optic radiation and visual cortex) a model of circular dipolar surface is introduced. The parameters of the model are: locus, orientation, radius and intensity of elementary dipole moments. Such models and their time-variable locus and surfaces, for the last case, can describe the pathways and the volume of the analysed information channel. This methodology is demonstrated for the visual system and shows good correlation between computed models and the time of activation of known anatomic pathways.
D P W I . 0 9 DATA R E D U C T I O N O F VEP U S I N G A D I P O L E MODEL. H. T.M. Haenen
(Groningen, Holland) Topographic display of VEPs showed large variability of the P1 (P100) distribution over the posterior scalp in twenty normal subjects. This necessitates a relatively large number of recording sites. We were able to reduce this large amount of data while maintaining the essential features by using a dipole model. Measured potentials from up to eight electrodes could be estimated accurately by the model in the whole normal population. In order to obtain reliable dipole Iocalisation, a special iteration method was developed based on the linear relationship between dipole moment and electrode potentials and constraints on the position of the dipole. The outputs of the model are the three dipole moment components over time. A further reduction is possible by rotating the dipole moment axis such that an optimal signal to noise criterion is fulfilled. The relevant dynamic behaviour of the dipole moment can then be mapped to a single channel. An evaluation of this single channel over the normal population reveals an enhancement of all peaks in the N I P 1 / N 2 P 2 complex, while some peaks which are missing in the original tracings, may appear in the single dipole channel.
D P W I . 1 0 SCALP P O T E N T I A L S AND C U R R E N T DENSITIES M A P P I N G BY S U R F A C E S P L I N E S INTERPOLATION: C O M P A R I S O N W I T H O T H E R M E T H O D S , F. Perrin, O. Bertrand, M.H. Giard and J. Pernier
(Bron, France) In scalp evoked potential mapping, potentials are only measured at the electrode sites. To represent the potential on the whole scalp it is necessary to interpolate in-between these known values. Surface splines are mathematical tools for interpolating functions of two variables. They can be obtained by minimizing the bending energy of an infinite plate constrained to pass through known points. The use of these functions to compute scalp potentials presents several advantages: the coordinates of the electrodes location need not be located in a rectangular array - the maxima and minima are not necessarily displayed at an electrode location - the degree of smoothness of the interpolated surface may be varied -mathematically these functions are easily differentiated making possible the representation of (the 'reference free') scalp radial current densities. We have compared several methods of the spline type and two methods of interpolation based on linear combination of the potentials at the four nearest electrodes.