Following initial shaping of lever-press responding, rats were trained under a fixed-ratio 1 (FR 1) schedule of brain stimulation. During behavioral sessions, each lever press resulted in the delivery of a 0.5 s train of square wave cathodal pulses (0.1 ms pulse duration) and illumination of the stimulus lights over the lever. Stimulation intensity and frequency were set at 150 μA and 126 Hz, respectively, during initial 60-min training sessions. Stimulation intensity was then individually adjusted for each rat until ICSS rates > 30 stimulations/min were observed. This intensity was then held constant, and frequency manipulations were introduced. Sessions involving frequency manipulations consisted of three sequential 10 min components. During each component, a descending series of 10 frequencies (158 to 56 Hz in 0.05 log increments) was presented, with each frequency available for a 1 min trial. Each frequency trial consisted of a 10 s time-out, during which five non-contingent “priming” stimulations were delivered at the frequency of stimulation that would be available during that trial, followed by a 50 s “response” period, during which responding produced electrical stimulation under a FR 1 schedule as described above. The chamber remained dark until the response period, during which the house light would illuminate as a discriminative stimulus that electrical brain stimulation could be received contingent upon pressing the response lever. The stimulation intensity was again adjusted to yield baseline frequency-rate curves that had high rates of reinforcement for the upper frequencies and minimal responding at the remaining frequencies (see Figure 1B). This intensity (120–340 μA across rats) was then held constant throughout the study. Training continued until frequency-rate curves were not statistically different over three days of training as indicated by lack of a significant effect of “day” in a two-way analysis of variance (ANOVA) with frequency and day as the two variables (see data analysis).

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