Experimental design and stimuli. Four Gabor stimuli were presented in four nearby locations in the periphery of the upper right quadrant as (a) pop-out displays, in which a single item differed in color and orientation from the others, or (b) heterogeneous displays, in which all four stimuli differed in color and orientation. These stimuli were presented either (c) sequentially or (d) simultaneously. (c-d) A stimulation period of 1 s, which was repeated in blocks of 18 s, is shown for a heterogeneous display. Stimuli were presented for 250 ms, followed by a blank period of 750 ms, on average, in each location. During all conditions, subjects detected target letters at fixation (illustrated in lower left corner of each display).