This study is concerned with three questions about the role of attention in peripheral detection: (1) Is the increased detection rate for spatial locations with high probabilities of target occurrence assigned to them due to sensitivity or criterion effects? (2) Does the effect of spatial cuing (probabilistic priming) require different explanations for letter detection and detection of luminance increments (Shaw, 1984)? (3) Can attention be shared between two separate locations cued to be most likely (Posner, Snyder, & Davidson, 1980)? These questions were investigated in two experiments, both using a signal detection plus localization task (rating method). In Experiment 1 (symbol detection), single or double cues indicating one or two most likely locations (three or two least likely locations) were presented. Introducing the second cued location resulted in a marked sensitivity gain for this position, relative to uncued locations in the single-cue condition. Decision criteria were more liberal for cued and more conservative for uncued locations. In Experiment 2, a luminance increment (single target probe) and two symbol detection (target plus distractors) tasks were compared. For symbol detection, there was a marked priming effect; but for luminance detection, cued locations showed no advantage in sensitivity. However, all tasks showed differential criterion setting for cued and for uncued locations. These results suggest that letter detection is capacity limited, whereas luminance increment detection is not, and furthermore, that decision criteria are largely preset according to a priori target probabilities assigned to particular locations.How does the focusing and dividing of attention influence perception? Recent research on this question has been concerned with two effects: that of "display N," that is, the number of nontargets in the display, and that of "probabilistic priming," that is, the relative frequencies of target occurrence assignedto individuallocations. Experimentswith accuracy as the dependent measure and with high error rates have established that an increase in the number of nontargets, withoutan increase in the number of targets, reduces target detectability (e.g., Estes & Taylor, 1964), and that variations in the probabilitieswith whichthe target occurs at particularlocationsenhancetarget detectability for the more likely positions relative to the less likely locations (e.g., M. L. Shaw & P. Shaw, 1977).Equivalenteffects havebeen demonstratedin reaction time (RT) For the explanation of these effects, it seems useful to assume that there are at least two functional stages between stimulus and response: In the first, "coding," stage, which may consist of a number of substages, each stimulus is converted into an internal representation. In the second, "decision," stage, the internal representation is used to determine a response, for example, a "target present-absent" decision. According to signal detection theory (SDT; Green & Swets, 1966), the internal stimulus representation is characte...