BEHAVIORAL MEASURES OF CONDUCTION-VELOCITY AND REFRACTORY PERIOD FOR REWARD-RELEVANT AXONS IN THE ANTERIOR LH AND VTA

Psychophysical techniques were used to estimate the conduction velocit ies and refractory periods of reward-relevant fibers in the anterior l ateral hypothalamus (ALH) and ventral tegmental area (VTA). Six male r ats of the Long-Evans strain served as subjects. Stimulation consisted of trains of pulse pairs delivered to the ALH and VTA; each stimulati on site received one pulse from each pair. Collision of antidromic and orthodromic action potentials along the same reward-relevant axons wa s inferred from a decrease in the effectiveness of the stimulation at short intra-pair intervals. Significant collision-like effects were ob tained for 5 of the 6 subjects, supporting the notion that reward-rele vant neurons directly link the ALH and VTA. Estimates of the slowest c onduction velocities ranged from 0.4 to 5.5 m/s. Refractory period est imates, obtained by delivering the trains of pulse pairs to a single s timulation site, ranged from 0.5 to 5.4 ms. The overlap between these psychophysical estimates and the electrophysiological estimates obtain ed in the accompanying paper is consistent with the notion that reward -relevant MFB fibers arise in one or several rostral MFB nuclei.