biological foundation of mental imagery is not fully understood. Studies using fMRI have shown that the lateral geniculate nucleus and the V1 area of the visual cortex are activated during mental imagery tasks. Ratey writes:

The visual pathway is not a one-way street. 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.