<qa>  What is the conditional discrimination procedure based on?The Davis lab has recently developed a discrimination procedure that avoids these problems and allows for the independent evaluation of excitation and inhibition of fear conditioning. The procedure, referred to as a conditional discrimination abbreviated as AXBX-, is based on a paradigm used in earlier learning theory experiments Rescorla and Wagner, 1972; Bouton and Swartzentruber, 1986. In this experiment, the response to X is conditional upon the presence of either A or B. A becomes excitatory with training as the subject learns that A and X presented together predicts the unconditioned stimulus US. B becomes inhibitory in that B presented with X predicts safety from the US. The presentation of AB results in a reduced response compared to the response to A alone because B is inhibitory. Accordingly, a novel stimulus, C, paired with A results in a greater response compared to AB. X becomes excitatory to the degree that B is inhibitory. Testing of this paradigm with rats in the Davis lab has produced data in good accord with predicted values see Figure 1 in Appendix, and does indeed allow for independent evaluation of excitatory and inhibitory processes within the conditioning test session. To date, this paradigm has not been well studied in human subjects. The translation of this paradigm to human subjects would present an important advance in methodology for studying the neurobiology of fear inhibition in clinical populations with anxiety disorders such as PTSD. 