Fig. 1. Relation between the measured Bispectral Index (BIS) and propofol plasma concentration (Cp^prop)  versus time (s) for the four groups, in  A –  D . The  dashed line shows the cutoff value for modeling when the two consecutive model approach was used (fixed ≥ 90, sigmoidal < 90), fixed at a BIS of 90.  E –  H show the relation between BIS and Cp, thereby revealing the hysteresis in the relation.