Answer: The dual stream hypothesis suggests that visual perception takes place with two different neural processes, a dorsal stream and a ventral stream.
Our eyes are the primary sense organ for collecting visual information about the world around us. Photons of light bounce off objects, and those reflected photons enter into the eyeballs. From there, photoreceptors in the retina at the back of the eyeball transduce these photons into electrical and chemical signals which the brain is now able to interpret into images.
After the information signal arrives at the primary visual cortex (V1) located in the posterior end (back) of the brain, higher order visual processing is believed to take place along the route to the frontal cortex in the anterior end (front) of the brain. Information is sent from V1 using two different neural pathways. According to the dual stream hypothesis of visual perception, information regarding the location or motion of an object is conducted through the dorsal stream, which is sometimes called the “where” or “how” pathway. On the other hand, information regarding the recognition of an object is passed through the ventral stream, or the “what” pathway. As expected based on the naming convention, the dorsal stream sends information along the dorsal side (top) of the brain, while the ventral stream sends projections along the ventral side (bottom) of the brain.
Injuries to these pathways of visual processing creates deficits in perception. Damage to dorsal stream brain structures can cause poor spatial awareness and a difficulty in perception of the location or movement of objects. Damage to ventral stream structures causes a difficulty in object recognition.
Dorsal stream injury
Injury to the dorsal stream may occur after a cardiovascular incident such as a stroke, resulting in a loss of blood flow to parts of the brain that may kill a population of brain cells.
Akinetopsia may result from dorsal stream damage. A person with akinetopsia may not perceive that objects are moving through space (a- none, kine- motion, op- sight or vision). For someone with akinetopsia, the world may appear to them only in short clips of still images. This may make everyday tasks such as pouring a drink into a glass without overfilling difficult. Driving would be nearly impossible.
Ventral stream injury
There are specific regions of the ventral stream that produce unique deficits when injured. As with the dorsal stream, these structures may be damaged after a stroke. Injury to the ventral stream results in a deficit to the person's ability to recognize objects.
Injury to the fusiform face area may cause a visual disorder called visual agnosia. People with visual agnosia have difficulty in recognizing or identifying objects. They may be unable to draw objects, even though they know what those objects are.
One form of visual agnosia is prosopagnosia. In a patient with prosopagnosia, or face blindness, they have difficulty with the identification of faces. They may look at a person, and could identify the parts on their face, but wouldn't be able to identify who the person is by looking only at their face. It is believed that the memories for individual faces is stored throughout the fusiform face area.
Some interesting clinical cases of visual agnosia are described by Dr. Oliver Sacks in his book “The Man who Mistook his Wife for a Hat.”