Capacity limits during perceptual encoding.

When a unique stimulus is embedded in an otherwise homogenous display, it is thought to "pop-out" due to its relative increase in salience. We investigated whether the visual system has the ability to equally prioritize multiple salient pop-out items competing for awareness or whether this early stage of visual processing is constrained by capacity limits. We used signal detection (d') methods to determine if sensitivity to a salient pop-out item decreases as function of the number of total salient items present in the visual display. Participants engaged in a signal detection task where they had to report the presence or absence of a simple display change that involved either a pop-out or a non-salient distractor stimulus. Results across four experiments consistently showed that sensitivity to changes involving pop-out stimuli was significantly reduced after more than one of these items was present in the visual array. Results are discussed in terms of neural models of visual encoding and other known capacity limits during visual processing.