Human vision is very complex. It works so seamlessly that we are fooled into thinking that it is a movie camera capturing the world as it really is, in real time. But vision is not a picture to be looked at. It is an information gathering and processing system concerned with "what is out there" and "how to react" to what is out there.
Actually, from the very first moments of receiving light patterns complexity arises; the image that hits the retina is upside down. Our brain must begin the act of processing by flipping the image over. There are two eyes, of course, so there are two streams of information being sent to the brain; yet we see only a single view of the world. You might think that the information from one eye at least goes to the same area of the brain, but again it is complex. Signals from the same eye go to different hemispheres of the brain and to lower processing centers for instant attention. There are over thirty identified centers for vision processing and these are located throughout the brain; vision is a whole brain process. This suggests that damage anywhere in the brain can have implications for vision processing.
Perhaps the most startling revelation about the human vision system is that it is actually two sophisticated and completely autonomous systems, a central processing network, and a peripheral processing neural network. The two vision systems send their information to two different sets of neural pathways and to two separate sets of brain centers for processing. These parallel processing universes are further complicated by their interaction and coordination with each other and with the human motor systems.
Because the two vision systems are so different in their roles and in their means of processing, it is possible for either of them to be selectively damaged. The peripheral system can be blind or visually impaired, and the central system can be blind or visually impaired. The cross talk between the two systems can (theoretically) be selectively damaged causing a third set of vision anomalies (speculation on my part).
The discussion should start with peripheral processing because this is the first and oldest of the two systems from an evolutionary perspective. All mammals have this kind of vision system, but only the primates have the addition of a central processing system. Because the peripheral network is so old, it is very tough and reliable; it is hard to damage compared to the more delicate central processing system.
Peripheral vision evolved to provide mammals with a survival advantage. It is a very fast responding system that allows the mammal to either get food quickly (wild game) or to get away quickly (from lions, for example). It is very sensitive to motion and is tied into a fast responding part of the motor system. It quickly aligns the animal to objects of regard and positions the body to attack or flee. It is tied to the fight or flight primitive emotional network.
Another way to think about peripheral vision is to understand that it is used for unconscious navigation. It allows us to move freely through openings and around obstacles without having to think about each step we take. We position our bodies correctly to the spaces and the objects and we flow through the environment without hurting ourselves. Because of this navigation feature of peripheral vision, it is often called the "Where" vision system: "Where is it?" "Where am I?" "Where are you?" "Where do I need to go?" The cells in the peripheral retina are linked to the body balancing system in the vestibular network of the inner ear, and to the cerebellum, the master organ for smooth and accurate body movement/alignment.
Because peripheral vision is so old, it is also linked to the oldest of evolution's vision networks, the biological clocks of the body that are triggered by light. Early cells "learned" to either move toward light or move away from light; this was the beginning of the system that eventually developed to set all the body clocks, as the sun came up in the morning and went down at night. There is an entirely different retinal neural system that does not project to the visual cortex; it does not follow the rod system, nor the cone system. It projects directly to the master clock in the hypothalamus.
The sense of self must somehow be a part of the peripheral system, although the central system plays the leading role (because it is conscious- "aware of it's self"). To know where you are in space, and to know that objects and people are stable elements in the environment, must contribute to the defining of "self". This is significant, especially since handicapped individuals sometimes have an impaired sense of self (autism, for example).
The peripheral system has a set of information processing specialties that it performs separate from the central system:
1. It is the primary network for monitoring and responding to motion- it has a high motion acuity; it unconsciously "tracks" moving objects (relative to the background) and it "monitors" the human body in relationship to these objects. It is responsible for the visual control of motor behavior.What happens when there is damage to the peripheral system but not the central?
2. It also makes predictions about the future location of the body and moving objects in the environment, and it positions the body to best respond to future changes.
3. It is colorblind, relying entirely on luminance to make its quick judgements.
4. It monitors the ambient environment and locates salient objects (people, animals, and things), and it passes this information quickly to the central system for analysis.
5. It pulls relevant figures/features out of the background.
6. It is the human night vision system, and as such is very light sensitive (high contrast sensitivity).
7. It has an acuity of about 20/200, which is about three times "worse" than the central systems acuity (it's job is not to see clearly, anyway).
8. It is the system that "understands" space, how we organize the world, what features are where for different environments (forests, hallways, airport terminals, etc.).
9. It sees in three dimensions. It is our depth perception system.
10. It "pays attention"/responds only to change; it is "blind" when there is no movement across the retinal. This is why the eyes constantly oscillate (micro nystagmus); to prevent perceptual blindness. This peripheral system must be suppressed for central fixation to operate. With every glance, the peripheral system completes it's picture of the world differently (it is said to be "spatially imprecise, or to have "illusory conjunction"). It never gives us the same image twice- even of the same scene because it is always in motion. In lay terms, the peripheral system "goes with the flow".
11. It is not so much in need of high level processing and high level memory, since it is a first line, fast acting response system. It picks up information and responses immediately to the input. It has no need to analyze the input. It is a reflex system; it "lives in the moment".
12. It is concerned with the "context" of events, the gestalt.
13. It is unconscious, unaware of itself, non-verbal. It is not about communication.
14. Picking up facial expression/emotion and body language (the gestalt of the face/body) may begin with the peripheral system; the information is then passed on to the central system for identification and interpretation. The image components used to identify individual faces (a role of central vision) may be different from the image components used to identify emotional states- (maybe a role of the peripheral system). Reading emotional states probably was an early survival skill and so evolved with the peripheral system. Mammals with just the peripheral system seem to be able to interpret body stances and posturing. (this whole discussion is speculation on my part)
15. It makes the world appear stable as we move our head and eyes around (the brain "remaps" image inputs to stabilize the images)
The central system is a delicate "new" system that was "added on" to the older peripheral system as mammals evolved. It evolved to freeze motion, lock on to objects of regard, and analyze the image. It therefore required a high level visual cortex for processing "frozen" two-dimensional images (a face, an animal, a word on a page, etc.). It required a high level memory system and the ability to override (suppress) the peripheral system long enough to gather information; what we call "paying attention." It led to language, concepts, learning, consciousness (the sense of self), and intelligence.
Another way to think about central vision is to understand that it's role in survival has to do with identification, of friends, dangerous animals, etc. Because of this role it is also called the "What" system: "What is that?" "Who's face is that?" "What is that word?" "What is that landmark?"
The central system has a set of information processing specialties that it performs separate from the peripheral system:
1. It is "motion blind"; it consciously locks onto and "tracks" moving objects; it "freezes" the object so it can be identified and analyzed.What happens when there is damage to the central system but not the peripheral ?
2. It has two color processing systems, one that sees surface colors, and the other that sees forms- luminance plus color defines shapes. These are two separate brain processing centers. Human color perception is coarse.
3. It is the human day vision system, and as such requires good quality, relatively constant, and bright light to process accurately. It is not as light sensitive as the peripheral system; ie. it has relatively low contrast sensitivity.
4. It has an acuity of about 20/20.
5. It sees in two dimensions. It has "depth perception blindness".
6. It requires high level processing and high level memory. It does not "live in the moment". It tends to "roam around the past" or dream about the future. It is "processing intensive" and so it is slower to respond than the peripheral system..
7. It is concerned with the "subject" of events, the focus, where attention should be placed.
8. It is conscious, aware of itself, and verbal. It is about communication.
9. It is a pattern recognition system; it remembers the spatial arrangement of details (a face has a certain pattern, and a friend's face is distinct). It learns and can retrieve patterns- it can match. It can create whole patterns given pattern fragments. It is an object recogniition system.
10. Identification is the major role of the central system, including identification of faces
11. Repeated looks at a scene always provide the central system with the same pattern information. The central system must have a static/stable spatial layout memory system (in the hippocampus). The central system seems to freeze time.
As I thought about the two vision systems over the years, it gradually dawned on me that we were not talking just about vision. The human body has two separate survival systems of which the peripheral and central vision systems are only a major part. There are analogous sensory and motor components supporting these two major neural networks.
Complex human behaviors like reading require the whole brain and all the senses, including both vision systems. The peripheral system takes in the gestalt of a page, overall word/paragraph shapes. It also darts rapidly and constantly about the page, passing the central system from word groupings to word groupings (or, in slow readers, from word to word or letter to letter). The central system locks on to word groupings and decides the meaning and relevance of what is read- paying attention, storing new data in memory for later retrieval. If either of the systems is not working well, reading will slow down and be difficult.