Under a fixed-ratio schedule, a fixed number of lever-presses are necessary to obtain each injection (e.g., 1 lever press for a fixed-ratio 1, i.e. FR1, schedule). In contrast, under a progressive-ratio schedule, the number of lever-press responses required to obtain a drug injection increases after each injection (47) until the subject fails to emit the required number of responses. The highest ratio that is completed is termed the “breaking point”, and higher breaking points are considered to be an indicator of higher reinforcing effectiveness of the drug.
