Figure 3. Isobolographic plot for the interaction between intrathecal clonidine (CLON) and MK-801 (MK). The ED⁵⁰values for each agent are plotted on the Y and X axis, respectively, and the thick lines present the SEM of the ED⁵⁰. The dotted line connecting these two points is the theoretical line of additivity. The theoretical additive ED⁵⁰point (A) is calculated from the ED⁵⁰values and 95% confidence intervals (CI) of each agent. The experimental ED⁵⁰point (B) for the combinations lies within the space defined by the theoretical line of additivity. The confidence intervals do not overlap, indicating a supra-additive interaction.