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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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Your question was a loooooong time ago, but I just ran across a couple of good references explaining what backward masking does and how to choose one. This(1) is a great paper examining the neural mechanisms and timing of visual backward masking; according to this (2) 2000 review of masking theory, there are four subtypes of backward masking.

Backward masking can be achieved by

  • light, when the target is masked by a larger and spatially overlapping uniform flash of light.

  • metacontrast, when the mask does not overlap the mask spatially.

  • structure, when the mask overlaps the target as well as shares many structural features of the target.

  • noise, when the mask consists of random-dot noise over the target.

(There are some papers from the 1960s & 1970s talking about the transient vs sustained activation theory of backward masking function, but they are hard to find online because of their age so I have not included them.)

It's probably a good idea to go and read the 2000 review paper--the full paper is publicly available at that URL--because what kind of backward mask you choose depends on the hypothesis of the experiment.

References
(1) Journal of Cognitive Neuroscience, May 1999, Vol. 11, No. 3, Pages 300-311. doi: 10.1162/089892999563409
(2) Perception & Psychophysics, December 2000, Volume 62, Issue 8, pp 1572-1595.

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