Elsevier

Experimental Neurology

Volume 7, Issue 2, February 1963, Pages 144-152
Experimental Neurology

Specific impedance of rabbit cerebral cortex

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In rabbits anesthetized with urethane, cortical impedance was measured by applying nonstimulating sinusoidal currents through a 20-μ platinum electrode. Voltage was recorded differentially from two similar electrodes. All three electrodes were within 1 mm of each other. The animal was grounded through a current-monitoring resistor. The EEG was monitored throughout the experiment. The frequency response of the system was flat from 0 to better than 100 kc/sec. A subsidiary experiment which measured the voltage changes in all directions from a 20-μ current source on the cortical surface showed that hemispherical symmetry was true out to 1.3 mm from the current source. The effect of current shunting by pial blood vessels was also determined. Plotted in polar form, the impedance locus was noncircular and left the real axis at 356 ohm-cm at 10 to 20 cycle/sec, had a maximum phase shift of ca 7° at 50 to 100 cycle/sec, and completed the loop at ca 5000 cycle/sec at 256 ohm-cm. At 20,000 to 50,000 cycle/sec another loop started to open. Nothing resembling an inductive component was seen.

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    Measurements were obtained with respect to a reference electrode, 9 mm in diameter, placed over the dorsal surface of the cerebellum. Tissue conductivity was assumed to be isotropic throughout the cerebral grey matter (0.3 Sm−1) and white matter (0.15 Sm−1), and the conductivity of cerebrospinal fluid was set at 1.79 Sm−1 (Ranck, 1963; Baumann et al., 1997; Latikka et al., 2001) (compiled from literature by Horesh (2006)); summarised by Jehl et al. (2015b))). An inversion of the resulting Jacobian matrix was used to solve the inverse problem.

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This investigation was supported by a grant (PHS2-B5082) from the U. S. Public Health Service. Dr. Ranck's present address is: Department of Physiology, University of Michigan School of Medicine, Ann Arbor, Michigan.

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