In object substitution masking (OSM) a surrounding mask (typically comprising of 4 dots) onsets with a target but lingers after offset; under such conditions, the ability to perceive the target can be significantly reduced. OSM was originally claimed to occur only when a target was not the focus of attention, for instance, when embedded in an array of distractors (Di Lollo, Enns, & Rensink, 2000). It was argued that the distractors influenced the time taken for focal attention to reach the target. Some recent work, however, failed to find any such distractor influence; the effect of mask duration was found to be independent of set size when steps were taken to avoid ceiling effects in the smallest set size condition (Argyropoulos, Gellatly, Pilling, & Carter, 2013; Filmer, Mattingley, & Dux, 2014). In 3 experiments, we repeatedly found that set size manipulations can interact with mask duration (in which neither ceiling nor floor effects are evident), with the effect of the mask on target perceptibility being amplified according to the number of distractor items. However, a further experiment (Experiment 4) showed that crowding by nearby distractors was actually responsible for this "set size" effect. When decoupled from crowding, set size alone did not interact with masking, though it did influence overall accuracy. Thus, the presence of distractors does influence OSM, but not in the way originally assumed by Di Lollo and colleagues in their model. The Crowding × OSM interaction suggests that the 2 phenomena involve partly overlapping mechanisms.
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