Anodal block of conduction in the optic tract

¹ Division of Neurology, State University of New York College of Medicine, Brooklyn, New York

Anodal polarization of optic tract fibers just behind the chiasm has been found to produce reversible block of conduction in first and second tract fibers within a sufficiently restricted distance from the tip of the polarizing electrode to suggest that this procedure may be applicable to studies of the function of pathways in the central nervous system. Rushton's expression for the steady current flow across the sheath of an insulated conductor indicates that threshold should vary inversely as the square root of fiber diameter. This expression gives a ratio of 1.4:1 for the threshold current for block in second tract fibers relative to the threshold current for first tract fibers, and our observed ratio of 1:1 was not believed to be significantly different from the theoretical value. The predicted increase in threshold of fibers less than 2.5 µ in diameter was confirmed by our finding that the first and second tract spikes could be depressed by anodal current that did not alter the later third and fourth tract spikes.