When your brain processes visual inputs, some information is ignored / discarded. This is pretty well known, and most of us have had experiences where we've failed to notice something that was right in front of us (for example, the "where's Waldo?" books).
As a (slightly weak, but really cool) example, consider the pictures on this website. The first picture shows a man standing in front of a shelf in a supermarket aisle. It's not hard to imagine that, if you didn't know he was there, and looked pretty quickly at the scene, you might miss him.
Now, the question that I want to know the answer to (and that Freeman and Simoncelli have helped answer), is "what information is used, and what information is discarded?".
To help answer this question, they hypothesized a certain set of statistics that might characterize an image. The details of these statistics are technical, and are based on a model of visual cortex.
Then, they took real images, and for each image they computed their statistics, and then generated synthetic images that had all of their statistics correct, but were otherwise as random as possible.
They then had human subjects perform a discrimination task, where they were shown one picture (a real one), then another one shortly after, and were asked whether the two images were the same or not.
What they found was there were certain (pretty severe!) image manipulations for which their subjects couldn't tell the difference between synthetic and real images, thus performing at chance levels (50%) on the discrimination task.
The structure of the un-noticeable manipulations they performed let them infer several properties of the visual system, which agree well with what others have measured by using invasive electrophysiology techniques.
So, next time you look out your window, and think you are seeing all the "stuff" that's out there, think again! You're actually only seeing a (very!) impoverished fraction of the available information.
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