Hi, I thought you may like this Mentalrobics exercise from Braingle.com:
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Afterimages

This exercise improves your ability to create vivid and accurate visual images.

Find a nearby object and stare at it for a few seconds. Now close your eyes and notice that there is an afterimage of the object that fades after a few seconds. Try to turn this afterimage into a visualization and retain it in your mind when the afterimage fades. Open your eyes and look at the object again. You will notice things that you didn't see before. Close your eyes and try to visualize the object again and incorporate the details that you missed the first time. Repeat this process until you are able to retain a clear image of the object in your mind for several seconds after the afterimage fades. With practice you will be able to create detailed images in your mind from any object that you see.
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For more mind exercises: http://www.braingle.com/mind/