RetGen

In the case of sight, the primary visual cortex is responsible for taking in information, and also for forming remembered or imagined images. This dual function, Dr. Ramachandran and other experts say, suggests that normal vision is in fact a fusion of incoming sensory information with internally generated sensory input, the brain filling in the visual field with what it is used to seeing or expects to see. If you expect the person sitting next to you to be wearing a blue shirt, for example, you might, in a quick sideways glance, mistakenly perceive a red shirt as blue. A more direct gaze allows for more external information to correct the misperception.

"In a sense, we are all hallucinating all the time," Dr. Ramachandran said. "What we call normal vision is our selecting the hallucination that best fits reality."

With extensive vision loss, less external information is available to adjust and guide the brain's tendency to fill in sensory gaps. The results may be Thai dancers or monsters from a children's book.
The New York Times