Fig. 2.  Memory decay curves and estimates for λ and ψ by drug and dose condition. (A –F ) The power decay curves for each of the drug-dose combinations. The abscissa  represents the duration from the final encoding exposure to the subsequent recognition exposure (the recognition interval). For each curve, recognition data were corrected for false positives, clustered into 13 time points, and then fit to the equation m  ^t=λt  ^−ψusing Marquardt–Levenberg nonlinear regression. The specific equations describing each curve are shown in the upper and lower right corners of panels A–F . (G  and H ) The values for λ and ψ are shown, with error bars  representing the SEM for the estimate derived from the nonlinear regression. However, statistical comparison of the decay functions was performed using a more robust nonlinear mixed effects model.40For λ, independent effects were established for drug (P < 0.0001) and level (P < 0.0001), and significant fixed effects seen for the midazolam:level interaction (P = 0.014), the dexmedetomidine:level interaction (P = 0.008), and the propofol:level interaction (P = 0.008). For ψ, independent effects were established for drug (P = 0.027) and drug level interaction (P < 0.0001), whereas fixed effects were established for midazolam (P = 0.008) and propofol:level interaction (P < 0.0001). DEX = dexmedetomidine; MIDAZ = midazolam; PLAC = placebo; PROP = propofol; THP = thiopental.