 Two conditioning paradigms would permit the study of fear inhibition, but each has potential confounds complicating the interpretation of results. Extinction paradigms have the problem that the same stimulus causes both excitation of the response and later inhibition of response, so that it is difficult to tease apart whether a given experimental manipulation that enhances extinction does so by increasing inhibition or disrupting excitation. Conditioned inhibition using an AAX- paradigm has the advantage of having two separate stimuli for excitation and inhibition A is excitatory; X is inhibitory, but has the disadvantage of potential second-order conditioning i.e., it is possible that A will second-order condition X, which greatly complicates the separation of excitation and inhibition. 