EXPERIMENTAL NEUROLOGY 75, 5 14-5 19 (1982) RESEARCH NOTE Antidromic Activation: Measuring the Refractory Period at the Site of Axonal Stimulation HARVEY A. SWADLOW Department of Psychology, University of Connecticut, Stows, Connecticut 06.268 Received September 15, I981 The validity of collision tests for the identification of antidromic acti- vation [see e.g., (1, 3)] and the derivatives of such tests which are used in studies of branched axons [see e.g., (5)] depends on an accurate measure of the refractory period at the site of electrical stimulation. Collision tests are based on the notion that a stimulus presented during an interval after a spontaneous spike must fail to activate the unit due to a collision between the orthodromic and antidromic impulses. The period of refractoriness (the collision interval) will be equal to the conduction time from the recording site (usually at the soma) to the site of stimulation, plus the refractory period at the site of stimulation. ’ In practice, a measure of the refractory period is usually obtained by determining the least interval between two pulses applied to the site of stimulation which generate two spikes at the recording site. Because the refractory period at or near the recording site (i.e., soma or axon hillock) may be significantly longer than the refractory period at the site of stimulation, this method is subject to considerable error. Thus, for example, if the refractory period at the site of stimulation were 0.5 ms and the refractory period at the recording site were 1.0 ms, two pulses applied to the stimulation site at intervals of 0.5 to 0.9 ms might result in only a single spike at the recording site. This would result in a Abbreviation: LGNd-dorsal lateral geniculate nucleus. ’ The contribution of utilization time to the determination of the collision interval has recently been clarified (3); also see below. 514 0014-4886/82/020514-06sO2.00/0 Copyright 0 1982 by Academic Press. Inc. All rights of reproduction in any form reserved.