(a) Cluttered displays containing a variable number of short line segments were presented. Display elements continuously changed color from green to red (or vice versa) at random moments and locations. A short tone pip could be presented simultaneously with the color change of the target element. Participants were required to detect and report the orientation of the target element, consisting of a horizontal or vertical line amongst ±45° tilted line distractors. (b) In the absence of a sound, search times increased linearly with the number of distractor items in the display (white squares). In contrast, when the sound was present, search times became much shorter and independent of set size, indicating that the target stimulus popped out of the background (black squares). (c). Search times as a function of the relative stimulus onset asynchrony (SOA) between color change of the target, and onset of the sound. Negative SOAs indicate that the tone preceded the visual target event, and positive SOAs indicate that the target event preceded the tone. (Data from condition with set size fixed at 48 elements). Adapted, with permission, from ref [24].