As bizarre as the effect is, the science behind it is fairly straightforward. Because reading subtle changes in faces is so important to our survival ("Is that the murderous Oggoth or his gentle brother, Hogar? Does that look on Oggoth's face mean that he is scared or angry?"), we as a species developed a dedicated part of the brain solely for the task of processing faces, treating it as its own category of object. However, if that part of the brain is damaged or fails to develop for whatever reason, you process faces the same way you do a regular, inanimate object.
"So what?" you might be saying. "That doesn't sound so bad. It's not like stacks of LEGOs are any more difficult to see than a person's face." I will forgive your spectacular wrongness if you will perform a small exercise: Look at this random picture of LEGOs for five seconds ...
... then cover it and tell me exactly how many and what type of each piece is in the picture, and exactly where each one was positioned. That's what faces are like to me. I can see them perfectly fine, but I don't have the ability to process them as whole faces, rather than just piles of different face parts. Which means ...
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