Sensation is the immediate mapping of raw sensory data, such as light patterns, sound waves, or tactile stimulation. Perception is the next step in the process where all that raw data is synthesized into more complex maps. These maps are then linked via memory to similar maps drawn from past experience. This allows us to classify the images into known categories, for example furniture, food, or animal. Perception occurs when the initial sensory information is organized into a sufficiently complete whole (or gestalt) that it can be recognized as an object. At this point, we recognize that the pattern of light waves hitting our retina is actually a chair. Thus sensation works by analysis—by breaking down information into its smallest parts. Perception works by synthesis, by coordinating the parts back into a whole.

Anumber of neurological disorders gives us clues as to the workings of the brain. Visual agnosia, also known as blindsight, is a neurological disorder that sheds light on how the brain makes sense of visual information. People with injuries to the visual association cortex will be blind in the conventional sense of the word. They will not be able to recognize any objects visually. Likewise, they will tell you that they cannot see anything. However, if you put a large object in front of their path, they will walk around it, all the while insisting that they do not see a thing. This suggests that fundamental information about the presence and location of visual stimuli does get through. This preliminary information is processed in the primary visual cortex. However, the coordination of visual information into coherent, recognizable shapes takes place in the association cortices. Therefore, injury in those regions results in visual agnosia. In effect, a visual agnosia is sensation without perception.