Fig. 3.
Ratio–response function for cocaine intravenous self-administration in animals with high and low cocaine intake. Results are expressed as number of responses (A) and reinforcements (number of infusions) earned (B) over ratios. HI animals reacted at the increase in ratio (the response requirement necessary to obtain one drug infusion) with a proportional increase in responding, whereas, after FR24 (24 responses for one infusion), responding did not increase any more in LI animals. As a consequence, the intake of cocaine remained stable for HI but rapidly decreased in LI at the increase in ratio.