This study examined the topography of the event-related potential in visual-spatial compared to visual-object target detection. The initial index of target detection in the ERP was an inferior anterior P2a accompanied by a posterior N2b. Single unit studies in the monkey indicate that the detection of task-relevant stimuli requires interaction between prefrontal cortex and perceptual representation areas in the posterior brain. The posterior brain processes the physical features of stimuli while frontal cortex performs higher-order operations, such as evaluating the task-relevance of a stimulus. Target detection requires an interaction between feature representations and relevance representations. We hypothesize that the P2a and N2b ERP indices of target detection reflect this frontal/posterior interaction. Visual-spatial feature information is processed in the dorsal posterior brain (posterior parietal cortex) and visual-object information is processed in the ventral posterior brain (inferior occipito-temporal cortex). We observed that at the peak of the P2a the N2b was located over posterior dorsal leads in visual-spatial target detection and over posterior ventral leads in visual-object target detection. The P2a was largest over inferior prefrontal leads in both tasks. We suggest that this distribution is consistent with interaction between orbitofrontal cortical areas of salience representation and posterior cortical areas of stimulus feature representation.