NEURONAL ACTIVITIES FROM A DEEP SUBCORTICAL STRUCTURE CAN BE DETECTEDMAGNETICALLY OUTSIDE THE BRAIN IN THE PORCINE PREPARATION

We assessed whether subcortical structures can generate magnetic field s detectable outside the brain by first measuring the somatic evoked m agnetic fields (SEFs) from a decorticated porcine preparation and then from an intact preparation. Strong SEFs were detected a few millimete rs above the corpus callosum after electrical stimulations of the snou t. The waveforms consisted of a large spike (less than or equal to 6 p T) with a peak latency of 11-18 ms depending on the age of the animal, followed by a slow wave. The waveform and latency of the SEF spike we re virtually identical to those of the field potential within the brai n, The SEF topography indicated that the underlying generator of the s pike was in a region contralateral to the stimulation and inferior to the thalamus. The subcortical SEF was strong enough to be detectable e ven above the intact brain, after the cortically generated SEF was rem oved by ablation of the primary cortical area. The results indicate th at a structure deep in the brain can produce remarkably strong magneti c fields detectable outside the brain.