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Our eyes are the main information input for most of us. They send immense amounts of data streaming into our brains all day, every day. So much of it, that our brain can’t possibly handle all, so it throws some of it away. And even the information that we do get from our eyes can’t always be trusted.
There are two phenomena that are particularly deceptive: change blindness and afterimages.
These two eye tricks occur for two different reasons, but they both result in us not being able to always trust what we see.
Change blindness is when we cannot find the differences in a scene when there was a discontinuity that interrupted our view of the scene. The image below is an example showing how change blindness can trick us. How many things can you count that change from before the gray to after the gray?
Now how many changes can you count in the version below?
There are at least six, depending on how you count. Without the gray in the middle, the changes are easier to spot. This is because the changes now cause “motion” which is processed pre-attentively. When there are interruptions in our visual field, we can’t easily see changes that happened during the interruption. The phenomenon is due to our visual system inputting far too much information for our brain to process all of it. We don’t remember the scene exactly. Instead our brain focuses on what it perceives as important in the scene and remembers that information to a fair degree of accuracy. This is good enough for most things, but there are occasions that we miss things.
Afterimages are almost the opposite of change blindness. They occur when we stare at one thing for too long, and then look away. The image below is a good examples of an image that intentionally exploit this effect.
If you can see the Visually prism in the correct colors afterwards, you did it correctly. (Sometimes it helps to blink repeatedly.) This phenomenon happens because the rods (and cones to a degree) in your eye are trying to optimize themselves constantly. They are being overstimulated for those colors when you stare at the image, so they reduce sensitivity to those colors. When you look away, they have over-optimized, so you see a negative of the image.
Change blindness and afterimages are just two illusions that can affect data visualization. There are many others, and it is important to be aware of them when you are designing visualizations where precise and accurate communication is necessary.