Experimental tests of the cortical imaging technique-applications to the response to median nerve stimulation and the localization of epileptiform discharges

In a previous paper a method for simulating the electric potentials on the surface of the brain was introduced (see ibid., vol.38, p.294-9, 1991). This method consisted of the construction of a layer of radially oriented current dipoles in a conducting sphere that simulated the head so that the voltages generated by the layer would take the values measured on the surface of the medium (the scalp). The harmonic potential function for this layer was then evaluated in the interior of the medium in an attempt to approximate the potentials that would be generated by the actual neural sources but which could not be observed without recourse to invasive recording techniques. The method is tested here by applying it to the scalp-recorded potentials evoked by right median nerve stimulation, where direct cortical recordings are available for comparison, and to the scalp-recorded epileptiform discharges from two patients where the spike foci were well defined. The effects of varying the ´noise ratio´, an input parameter in CIT which allows one to account for noise in scalp-recorded data, is discussed.