Compared sensitivity for auditory and visual signals in a simple detection task with 8 undergraduates and in a related task in which 6 undergraduates were also imaging mental pictures and sounds. Sensitivity (d') was reduced during imagery; within the imaging conditions, it was smaller when image and signal were both auditory or both visual than for cross-modal conditions and smaller with unfamiliar than familiar images. Likelihood ratio was also smaller in the isomodal imaging conditions, as there were more visual false alarms during visual imagery and more auditory false alarms during the auditory imagery. Data are not consistent with the assumption that d' is lower during imagery due to distraction; they do not entirely fit a channel competition model, but suggest that imagery functions as an internal signal which is confused with the external signal. (PsycINFO Database Record (c) 2006 APA, all rights reserved)