Sight is the primary sense through which we perceive the world around us. This is the case as well for most of the animals on earth. Even though our sight as humans not nearly as powerful as that of a hawk or a spider, a large portion of our brains -- some say as much as half -- is involved in some way with vision. The parts of the brain that help us see objects and tell what they are, are referred to collectively as the visual brain.
Most vision takes place in the rear portion of the part of the brain called the cerebrum. This area is what is called the visual brain, and it consists of two equally important halves: the dorsal stream, and the ventral stream. The ventral stream is the lower part of the cerebral cortex between the cerebellum and the brain stem, and in technical terms is known as the inferotemporal cortex. In the ventral stream, visual data from the optic nerves is processed in a way that helps us determine the identity of what it is were are looking at. When we recognize the faces of family and friends, for example, or distinguish between a cat and a dog, this perception takes place in the ventral stream.
Damage to the ventral stream because of injury or disease results in the inability of that person to identify what an object is, although they are able to see it clearly. This condition is called visual agnosia, and may present in the elderly as part of a degenerative disease such as Alzheimer's. In some rare cases, damage occurs to the ventral stream at very young age, so the person develops without this area of the visual brain.
The dorsal stream is the part of the visual brain which perceives the location of an object. Known also as the parietal cortex, it is located near the top of the cerebral cortex, above the cerebellum, and is interconnected with the ventral stream at the very back of the brain. When we reach for an object or judge its distance from us, we are using the dorsal stream.
It also gives us the ability to perceive our visual field as a whole, the way we look at a map. When any part of this visual map moves or changes, the dorsal stream processes what this movement means. Damage to this sector of the visual brain can present as a variety of disorders, all of which are characterized by some type of inability to perceive or interact with objects.