Figure 1. The task and stimuli. A. Example trial sequence.

Each trial began with the presentation of a symbolic cue that was both predictive and instructive regarding the upcoming object category: On 80% of trials (valid trials) the cue was followed by a composite image of two superimposed objects, while on 20% of the trials (invalid trials), the cue was followed by a composite image of an uncued object and a checkerboard. The composite image followed the cue after an anticipation period (cue-to-target period) varying from 1.0 to 2.5 s. Participants were required to discriminate whether the cued object image was blurry or not-blurry (valid trials), or whether the uncued object image was blurry or not-blurry (invalid trials), and to respond as fast as they could using a button response. B. Example stimulus images in the attention task. In the set of example valid trial stimuli shown (left panels), Face is the target object category to be identified as in-focus or blurry, and the overlaid Tool or Scene images are the distractor images. For each stimulus image, both the target and distractor could be blurry or in-focus, independently of each other. Example invalid trial stimuli are also provided to illustrate that both the uncued target image and the overlaid checkerboard could be blurry or not-blurry, independently of one another. In the invalid trial condition, participants were still required to discriminate and respond to the uncued target image with the same blurry/not-blurry distinction, using the same response buttons as for valid trials.