Most important is the tissue conductivity, which differs depending on the tissue type. The spread of extracellular currents caused by cellular electric activity is mainly confined to the inside of the skull. The brain tissue inside the skull may be compared to a compartment with similar conductivity within (gray matter ca. 0.3 S m-1, white matter 0.14 S m-1) surrounded by a compartment with high conductivity such as liquor (ca. 1.54 S m-1). The symmetry of extracellular currents is distorted by these conductivity barriers. Tissue permeability can be considered constant in living tissue. Studies of the heads of piglets, which have a gyrencephalic brain similar to the human brain, show that the bone is of almost no importance in the distribution of the magnetic field. A detailed analysis provides evidence that only small changes in the magnetic field occur if the current dipole is located deep within the brain, causing weakening of the amplitude if the bone is removed (Okada et al., 1999).
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