In an experiment by Most et al. (2000), participants' inattentional blindness was tested by having them observe a computer screen with numerous black and white shapes moving across it. They were instructed to focus on the shapes of one color while ignoring the other. During this task, a red cross passed across the screen. Despite its clear visibility, about one-third of the participants failed to notice the red cross, demonstrating how intense focus on a specific activity can prevent the perception of other unexpected stimuli.