This study examined threat identification within a perceptual discrimination training paradigm for an x-ray baggage screening task. It explored how manipulations of item overlap (critical contour overlap, non-critical contour overlap, and no overlap) altered detection of actual threat items. The results suggest that threat detection by participants in the overlap groups was superior, but that this may have been due to changes to a more liberal response criterion. Further, participants trained without overlap were superior at determining that no threat was present. The data suggest that this shift in criterion may be due to a varying degree of understanding of what constitutes the critical components of a threat item. The discussion centers on how to develop training interventions which addresses this criterion shift while maintaining higher levels of detection.