Ipsiversive rotation elicited by electrical stimulation of the mesencephalon: An analysis of intracranial electrical stimulation parameters and refractory periods

Abstract

The effects of pulse amplitude, width, and frequency on ipsiversive rotation elicited by electrical stimulation of the mesencephalon were investigated in rats. The net refractory periods at these sites were determined by an adaptation of Helmholtz's paired-pulse technique. Over a wide range of effective charges (0.15 to 6.25 μc/sec), it was demonstrated that total effective charge is a relatively poor predictor of the vigor of rotation. At high charges the most vigorous rotation was produced if the charge was increased by increasing pulse frequency. At low charges the most vigorous rotation was produced if the charge was decreased by decreasing pulse width. Changes in pulse amplitude were generally of intermediate efficacy in modulating rotation. These data suggest that neurons are very sensitive to the spatio-temporal characteristics of electrical stimulation and are not the crude integrating devices that they have traditionally been claimed to be. Possible mechanisms of action of the parametric manipulations are discussed and it is concluded that only frequency changes provide a quantitative as opposed to qualitative modulation of neural function. The paired-pulse technique indicated that the neuron pools subserving ipsiversive rotation are characterized by net refractory periods of approximately 2.3 msec.