Previewing distractors reduces efficiency of visual processing at previewed locations.

Visual marking refers to the phenomenon in which old items in a visual search are excluded from the search when new items appear in the visual field. Visual marking may result from inhibition of irrelevant information at the location of old items before new items appear. Moreover, sensitivity to increments in contrast at the old locations has been shown to be lower than that to increments at the new locations. We used equivalent noise analysis to examine whether the reduction in sensitivity is the result of an increase in internal noise or a decrease in calculation efficiency. Following a search in which reaction time was measured, participants were asked to indicate whether a Gaussian luminance blob was present. Parameters estimated from the threshold-versus-noise contrast function indicated that calculation efficiency at old locations was lower than that at new locations, and internal noise did not increase at old locations but rather decreased slightly. Thus, the reduction in sensitivity at old locations is attributable a decrease in calculation efficiency. These data suggest that an inhibitory template for visual marking may benefit visual search by diverting limited attentional resources, such as time and resolution, away from previewed locations and reserving them for the target search.