ANALYSIS OF THE FORM AND DISTRIBUTION OF EVOKED CORTICAL POTENTIALS UNDER THE INFLUENCE OF POLARIZING CURRENTS

Abstract

Effects of polarizing currents applied at the surface and at various depths beneath the surface are described for a variety of evoked cortical potentials in the cat. In general results can be interpreted according to a model of a bundle of axons hyperpolarized at one end and depolarized at the other, accordingly distorting the extracellular current flow of the transient response. Thus cathodal current applied at the surface augments surface positive components of evoked responses and anodal current diminishes them; the reverse is true for surface negative components. An exception to the rule is the spike of afferent radiation axons which is increased by anodal and decreased by cathodal current. This is consistent with the inference that the radiation terminal does not support an active spike but is passively depolarized by the approaching impulse (killed end effect). Polarization of sensorimotor cortex with simultaneous recording from pyramidal tract demonstrates significant changes in the number of units responding to a cortical shock. Slow wave spindles in sigmoid gyrus may be triggered by antidromic pyramidal volleys when all afferent paths from the brainstem are severed.