Stare at the red dot in the center of the figure for a minute or two. Before long, the green ring will disappear--it simply seems to fade into the white background. There are no tricks: This is a simple, static image file. The effect has been known for more than two centuries and is named for its discoverer, Ignaz Paul Vital Troxler (1780-1866), a Swiss physician and philosopher. "Troxler fading" is actually related to what you experience when you get "dizzy": You become so habituated to a phenomenon (spinning in a circle or seeing a green ring in your peripheral vision) that you stop noticing it's there.
Or, rather, you don't realize that your perceptual system has begun actively ignoring it. It's only when your circumstances change that you see what the phenomenon has done to your perceptual system. When you stop spinning, the world seems to continue, in reverse. When you look away from the green ring, you see a red ring in the same part of your visual field.
This is a perceptual illusion but I like to use it to demonstrate that children with CVI can only focus their attention on one thing. The rest of the world does not seem to exist for them while they attend to the one thing. Note that as you blink it comes back in to focus. The CVI child needs to sort of 'blink' to cause other things to stand out. Make of this what you will it; at least will 'give you pause', I hope.
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