"So far, the story of sight has been about what we actually sense: the light and lines detected by the retina and early stages of the visual cortex. These are our feed-forward projections. They represent the external world of reflected photons. And while seeing begins with these impressions, it quickly moves beyond their vague suggestions. After all, the practical human brain is not interested in a camera-like truth; it just wants the scene to make sense. From the earliest levels of visual processing in the brain up to the final polished image, coherence and contrast are stressed, often at the expense of accuracy.
Neuroscientists now know that what we end up seeing is highly influenced by something called top-down processing a term that describes the way cortical brain layers project down and influence (corrupt, some might say) our actual sensations. Why does the mind see everything twice? Because our visual cortex needs help. After the prefrontal cortex receives its imprecise picture, the 'top' of the brain quickly decides what the 'bottom' has seen and begins doctoring the sensory data. Form is imposed onto the formless rubble of the V1; the outside world is forced to conform to our expectations. If these interpretations are removed, our reality becomes unrecognizable. The light just isn´t enough."
[Lehrer, Jonah. "Proust was a neuroscientist", Boston: Ed. Mariner Book. 2008, p.108]
