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.)