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. The amplitude of the second pulse was either equal to, 41% greater than, or 73% greater than the amplitude of the first pulse. In the equal amplitude condition, recovery from refractoriness began as early as 0.4 msec and did not asymptote until as late as 3.5 msec. A 41% increase in the intensity shortened the time course of recovery in five out of six cases. In only one of these five cases did a 73% increase in the intensity of the second pulse produce further changes in time course. Neither increase in the amplitude of the second pulse affected the time course of recovery in one subject. The absolute refractory periods of the directly stimulated reward-relevant neurons appear to be less than 1.5 msec and as short as 0.4 msec; some of these neurons have relative refractory periods that range between 1.0 and 3.5 msec.