Higher areas of the brain can also send visual input back to neurons in lower areas of the visual cortex. [...] As humans, we have the ability to see with the mind's eye – to have a perceptual experience in the absence of visual input. For example, PET scans have shown that when subjects, seated in a room, imagine they are at their front door starting to walk either to the left or right, activation begins in the visual association cortex, the parietal cortex, and the prefrontal cortex - all higher cognitive processing centers of the brain.[7]