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CHAPTER 8 190 by Lovibond, Mitchell, Minard, Brady, and Menzies (2009), which may be explained by conditional inhibition (Krypotos, Effting, Kindt, & Beckers, 2015). A conditional inhibitor is a stimulus that directly predicts the nonoccurrence of the US (see Figure 1; Treanor & Barry, 2017). When a conditional inhibitor is present during an extinction procedure (i.e., exposure), the expected outcome becomes similar to the actual outcome (i.e., no threat), which prevents inhibitory learning. An example is quickly leaving the supermarket when feeling dizzy to prevent fainting in panic disorder. Explaining the negative effects of safety behavior on extinction learning in terms of conditional inhibition seems similar to explaining them by a misattribution of safety to the behavior (“I did not faint, because I left the supermarket”; Salkovskis, 1991). Figure 1. A conditional inhibitor predicts the nonoccurrence of a US. It thereby prevents the CS from acquiring an inhibitory association with the US (Treanor & Barry, 2017). The circle with CS indicates the conditional stimulus, the circle with CI the conditional inhibitor (e.g., safety behavior), and the circle with US the unconditional stimulus. The arrow indicates an excitatory association, and the line with a bar at the end an inhibitory association. Second, the finding that safety behavior that minimized the severity of threat, but did not preclude its occurrence prevented extinction in chapter 2 (van Uijen, Dalmaijer, et al., 2017) may be explained by negative occasion setting. An occasion setter is a cue that does not directly influence the occurrence of threat, but provides information about whether a CS will be followed by a US (i.e., it “sets the occasion” for whether a CS will be followed by a US; Treanor & Barry, 2017). A negative occasion setter is a cue that inhibits the association between the CS and US (Bouton, 2016; see Figure 2). Individuals may learn that in the presence of this cue, the CS does CS US CI