Far-field potentials generated by action potentials of isolated frog sciatic nerves in a spherical volume.

Previous results in cylindrical volumes have shown that action potentials generate far-field potentials when experimental conditions are such that quadrupolar components of the action potential are reduced to an equivalent dipole. We now show that the same conclusions are also reached within a spherical volume, again recording far-field potentials from isolated bullfrog nerves. A mathematical proof is given that shows that in a sphere, antipodal electrodes primarily detect far-field potentials from dipole generators and not quadrupole generators. A revised conception of the 'far-field' in evoked responses is discussed which equates far-field recordings with dipole detection.