Estimation of the relative value of cocaine.

Curves represent (A) choice between cocaine and water sweetened with saccharin and (B) percent of completed trials as a function of the relative cost of saccharin. The cost of saccharin was gradually increased either between sessions (open circles) or within sessions (closed circles). In the former case, each cost level was tested at least 5 times consecutively until stabilization of behavior. Data points represent the means (± s.e.m.) of the last 3 stable testing sessions. For other details, see Materials and Methods, and legend of Figure 3. *, different from the indifference level (P<0.05, t-test).