ABSOLUTE AND RELATIVE REFRACTORY PERIODS OF THE SUBSTRATES FOR LATERAL HYPOTHALAMIC AND VENTRAL MIDBRAIN SELF-STIMULATION

The pulse-pair paradigm was used to behaviorally assess the absolute and relative refractory periods of neurons subserving brain-stimulation reward in rats. The amplitude of the 2nd pulse was either equal to, 41% greater than, or 73% greater than the amplitude of the 1st pulse. In the equal amplitude condition, recovery from refractoriness began as early as 0.4 ms and did not asymptote until as late as 3.5 ms. A 41% increase in the intensity shortened the time course of recovery in 5 out of 6 cases. In only 1 of these 5 cases did a 73% increase in the intensity of the 2nd pulse produce further changes in time course. Neither increase in the amplitude of the 2nd pulse affected the time course of recovery in 1 subject. The absolute refractory periods of the directly stimulated reward-relevant neurons appear to be less than 1.5 ms and as short as 0.4 ms some of these neurons have relative refractory periods that range from 1.0-3.5 ms.