The perceptual process involves brain structuring, and how it organizes incoming stimuli, determines what we experience.
Although we can easily see this figure, it doesn't seem to stay put. I see three prongs and then all of a sudden there appears to be only two!
The figure, of course, hasn't changed on the page. What we are experiencing is the perceptual process--brain reorganization. As the brain finds different ways structuring the stimuli, we experience different images!
Structuring is far more comprehensive than you or I could have imagined.
It is true that the images from the retina are carried pretty much in tact to the back of the head (the outer surfaces of the occipital lobes). But then, as they are brought deeper into the cerebral tissue, an amazing thing happens. The image is totally dismantled and disseminated to unknown areas of the brain. Horizontal and vertical lines, colors, shades and even motion itself is dissected out for distribution.