Magnetic evoked field associated with transcortical currents in turtle cerebellum

The neural basis of magnetic evoked fields of the brain was studied with an isolated turtle cerebellum as a model preparation. The turtle cerebellum is a nearly flat tissue with neural processes arranged along three orthogonal axes of symmetry. According to theoretical results, this geometry should enable us to selectively measure the magnetic field due to a subpopulation of nerve cells whose longitudinal axes are perpendicular to the cerebellar surface, by simply placing the cerebellum vertically in the bath so that these cells are horizontal and by measuring the field along the rostrocaudal axis perpendicular to the longitudinal axis of these nerve cells. To elicit neural activity in these cells the dorsal midline was electrically stimulated with a bipolar electrode. Consistent with our expectations, the one-dimensional profile of the field normal to bath surface (Bz) was antisymmetrical along the rostrocaudal axis, implying that the underlying currents were transcortical. Also, the Bz field at a field extremum varied as a cosine of the orientation of the cerebellum when it was rotated about its rostrocaudal axis with the amplitude being zero when the cerebellum was horizontal. The Bz field was dipolar as judged by statistically excellent fits of the dipolar field to the one-dimensional field profile and to the distance function relating the field magnitude at an extremum to measuring distance. This was the case even for the initial component thought to be due to antidromic action currents invading the soma and dendrites of Purkinje cells. We also showed that the dipolar term of the source could be localized within 1 mm of the actual source location in most cases.