The daunting complexity of the visual system is functional as well as structural, as is
shown in Figure 2–3. The pathways and their many ramifications are not one-way
streets. Most visual areas that send output to another area also receive input from that
area; that is, they have reciprocal connections—for example, LGN provides input to V1
and V1 provides other input to LGN. This dynamic arrangement reflects an important
principle of visual perception: visual perception—in fact, all perception—is the product
of bottom-up and top-down processes. Bottom-up processes are driven by sensory information from the physical world. Top-down processes actively seek and extract sensory information and are driven by our knowledge, beliefs, expectations, and goals.
Almost every act of perception involves both bottom-up and top-down processing.
One way to experience the distinction consciously is to slow part of the topdown contribution. Look at Figure 2–4. There is certainly something there to be
seen: bottom-up processes show you lines and define regions. But if you play with
the image mentally and consider what the regions might signify, you can feel a topdown contribution at work. The image could be … a bear climbing up the other side
of a tree! Whether or not you came up with this solution yourself, your appreciation
of it depends on top-down knowledge: your knowledge of what a tree and a bear’s
paws look like, your knowledge of how bears climb trees. This kind of knowledge
not only organizes what you see, but also can even modulate the processes that created the representations of the lines and regions.”