Estimation of Epileptic Source Location by Means of the Dipole Tracing Method

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The Dipole Tracing (abbreviated as DT) system which we have developed in these ten years is aimed at estimating locations, as a function of time, of active neurons in the human brain in terms of the scalp potentials. This method is based on a realistic head model with uniform electric conductivity. Difficulty inherent in this method is in removal of the effect of skull which has electric conductivity as low as 1/80 times that of the brain tissue. A systematic displacement of the estimated current dipoles towards the center of the skull by 30% was observed as a result of the experiment at University Hospital, Uppsala, Sweden, and this number was used mostly successfully. However, when the head is very remote from a sphere, it is difficult to find the center for the correction, and moreover when electric sources in the brain are separated in space we also have a difficulty in the skull correction. We have obtained a new idea for skull correction technique which can be used automatically … More on computer work. The basic idea is the following. The observed scalp potentials generated by an artificial current dipole on the cortical surface at a know position are compared with scalp potentials which would be obtained without the skull. A linear transformation between these should be numerically derived. If algorithm of finding this transformation matrix is found, the correct position of a dipole is automatically obtained. The detail mathematical processes are under investigation. The skull correction in case of two simultaneous dipoles is not clear as yet. The Uppsala group has their own data acquisition system which is compatible to ours and are sending us data magnetic tape routinely for us to make a data base for the skull correction and tracing abnormal electric activity during seizure.Cognition processes of single letters and characters were examined with the DT system. When a single letter was presented on the right visual field, the first dipole appeared in the primary visual cortex and then moved to the left visual association area, left angular gyrus, and to left inferior temporo-occipital cortex. Part of our result agrees with PET data. When, on the other hand, a letter was presented on the left visual field, a dipole in the right visual association area moved to the left hemisphere and followed the same path as described above. Less