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I am currently programming an experiment where participants have to do a divided visual field task. In a paper about the correct methodology about these tasks, it is said that you should use a backward mask (a mask after the target) in order to avoid after-images.

I have no objection of inserting backward masks in my experiment, but I just can't find any rules about how to implement these correctly.

Does it matter if I use a random black/white pixel rectangle where the target was or some symbols (e.g., ### is often used in psycholinguistic experiments) on that location? Or should I put random black/white pixels over the whole screen?

(Location of the target/mask is not important for the participants, so a target-located or a full-screen mask does not change to the goal of my research.)

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Interesting question. I was testing an iPhone app for "memory improvement" - repeat the yellow circle pattern seen on the screen. After each pattern it flashed all dots with green color. Your question makes me understand why the author did that. –  Alex Stone Apr 18 '13 at 2:22

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