Temporal integration in self-stimulation: a paradox lost?

Seven self-stimulating rats with electrodes located along the medial forebrain bundle were used to study how excitation dissipates at the end of a train of rewarding electrical pulses. On a 5-s, fixed-interval schedule, the rats pressed a lever to obtain 2 trains of pulses separated by gaps of up to 2 s; the first train was fixed at a just-subthreshold number of pulses, whereas the second train was used to scale the number of pulses needed to just support consistent responding. The number of pulses needed grew with increasing gaps between the 2 trains, rapidly at first and then decelerating to an asymptote, with time constants of a few tenths of a second. These results support C. R. Gallistel's (1974, 1978) model of leaky integration of rewarding brain stimulation.