Rate-frequency (R/F) functions taken from two representative animals. R/F functions for the five intracranial self-stimulation tests from one control and one quinpirole (QNP)-treated animal. Each plot represents data from one test session under pre-drug (open circles) and post-drug (filled circles) condition. R/F functions were obtained by logarithmically changing the frequency of the stimulation pulses in 0.4 s stimulation train from a value that sustained maximum lever pressing to one that failed to sustain lever pressing. Each data point represents the mean of four trials. Both animals demonstrate a curve-shift to the left after high amphetamine (highAmph) treatment and to the right after an effective dose of haloperidol (highHalo). The application of haloperidol or d-amphetamine at a low dose had only minor effects on stimulation thresholds.