Intracranial self-stimulation in rats as a function of various stimulus parameters

Five separate but sequentially planned self-stimulation experiments were performed in rats with monopolar nichrome electrodes implanted in the medial forebrain bundle at the level of the lateral hypothalamus.A total of 272 sessions of 40 min were given and 44 different stimulus parameter combinations were used, i.e. pulse intensities from 0.100–0.250 mA, pulse frequencies from 20–80 pps, pulse widths from 2–10 ms, train durations from 200–800 ms and quantities of charge from 4–64 C.In all experiments the fixed ratio schedule was 21 (2 responses for 1 stimulus) and the 17 rats emitted a total of 508 365 responses. It was found that, besides the individual sensitivity of the rats, the response rate was not a simple function of quantity of charge, and that within each stimulus parameter combination a threshold value for each parameter had to be reached. Selective parameter combinations produced predictable response rates in spite of some contrast effects evoked, within each session, by randomisation of the stimulus parameter combinations according to a latin square design.