Conductivity in the somatosensory cortex of the cat -- evidence for cortical anisotropy.

Orthogonal conductivity components were determined for 3 depths in the somatosensory cortex of cats and relative vertical conductivities were determined for all depths. (2) For cortical layers II--III, the conductivity was nearly twice as large (1.7 times) in the anteroposterior direction as it was in the mediolateral direction, whereas in layer IV the conductivity in the mediolateral direction was about 1.4 times greater than it was in the anteroposterior direction. (3) With the exception of the anteroposterior direction of layers II--III and the mediolateral direction of layer IV, the vertical conductivity of the cortex was always greater than either of the horizontal conductivities. (4) Vertical conductivities varied with cortical depth. The lowest vertical conductivity occurred in layer I. It increased in layers II--III, dropped in layer IV, and increased again in layer VI to a value comparable to layers II--III. (5) Adjacent determinations of conductivity indicated that over short distances (1--2 mm) the cortex was electrically homogeneous. (6) These data suggest that the cellular organization of the somatosensory cortex changes markedly and abruptly with cortical depth. Furthermore, they suggest that a significant portion of the coritcal neuropile in layers II--III and in layer IV is highly polarized. The possible anatomical basis for this polarization is discussed as are the effects of cortical anisotropy upon conductivity measurements.