The visual system — a brief guide to anatomy and physiology in the context of typography
The vast majority of people would classify reading as a visual activity but it can also be a tactile experience such as Braille script or touch-sign language. This article focuses on the visual system and with an understanding of the visual system it may be easier to lower the accessibility barriers some people may experience.
The visual system can broadly be divided into two parts: ophthalmic and neurological. The ophthalmic part affects everything that is in the eyes, including the connection to the optic nerve. Any impairment or defect to the eye will have an effect on the quality of visual information that is transmitted to the brain to be processed accordingly. There are many ophthalmic conditions and many can be overcome with assistive technologies like glasses.
This article about the visual system describes the basic and broad concepts of where things are and how they work in the context of typography. For those interested in academic and medical detail of the visual system a good starting point can be found here.
Ophthalmic
Humans have two eyes set apart horizontally for depth perception which allows us to judge distances and make a 3D map of the world. Eye movement is controlled by a set of six muscles on each eye that allow for rotation of the eyeball, and the muscles are controlled by the brain’s motor systems. Each eye has a left field of vision (LFV) and a right field of vision (RFV), and each is connected to the respective optic nerve strand. The front of the eye, the part that is visible from the outside, is where light comes through. In essence, it functions very much like a photographic camera, with the iris regulating the amount of light that comes through, and the lens focussing the light projection to the back of the eye, the retina.
The back of the eye is covered by light receptors that convert light to electrical charges. There are two types of receptors: rods and cones. Rods are for contrast and black & white vision, while cones regulate colour vision that pick up the red, green and blue frequencies of light waves. The back of the eye has three distinct parts: the retina which is most of the back of the eye, the macula in a central position and within that the fovea at the very center.
The fovea and macula are our central vision; it is where ‘20/20’ vision occurs. The fovea holds the highest density of rods and cones, about 500,000 in a 1.77 mm/sq area, lessening a bit in the macular area. The further out from the retinal center, the less the density of light receptors, in particular cones. At the very peripheries it is mainly rods that make our vision, and although we think we see colour it is simply the brain compensating.
This focus on central vision necessitates saccadic eye movements, the constant micro movements of the eyeballs that capture snapshots of the world around us and are then assembled by the brain into a coherent image. Saccadic movements are also in place when we read text, in particular long form. In the Latin alphabet* the eyes fixate on a specific point in a line of text to capture about seven to nine letters of that line. This number of letters fits more or less into the fovea at a reading distance of approximately 30cm and a font size of about 9 to 11 point, depending on the design of the typeface. Once the letters are captured and sent to the brain via the optic nerve, the next saccade occurs. A change of size of letters in the fovea, either by changing the reading distance or font size, of course changes the number of letters captured in a saccade. The saccadic movements are controlled by the brain’s motor system which if impaired can severely impact the quality of vision. But this article is not the place to discuss the brain’s motor system.
Within the macula sits the ‘blind spot’ which is devoid of any receptors. This is the area that connects all the receptors to the optic nerve which carries the electrical charges to the back of the brain into the visual cortex. It is clear from the above very broad description of the eye that any damage or defect will have an impact on the quality of the signal that travels down the optic nerve.
*Other writing systems may have a different behaviour but I am not aware of any research in this area. Greek and Cyrillic are likely to be the same as Latin as they are both alphabetic and overall have proportional structures similar to Latin.
Neurological
Each eye has one optic nerve per eye, each made up of two strands of nerve fibres that are connected either to the LVF or RVF. The LVF nerve path traverses into the right hemisphere of the brain and the RVF into the left, and eventually end up in the visual cortex at the back located in the occipital lobe. The visual cortex receives all visual stimuli and also acts as a ‘sorting office’ to distribute different types of visual information to other parts of the brain for further processing. This may include facial recognition, object recognition, spatial orientation and written information.
The optic nerve carries sensory input, in this case visual information, of different types: colour, contrast, brightness. Light and accommodation reflexes are conducted by the optic nerve. At some point the optic nerve divides into the magnocellular and parvocellular pathways, each with specific functions. The magnocellular pathway primarily provides static, motion and depth information and is particularly sensitive to contrast. The parvocellular pathway primarily provides higher resolution and fine detail of the visual information.
The visual information arrives into the right part of the visual cortex from the LVF of the left eye via the optic chiasm, and vice versa, in essence leaving a ‘split’ image. For a composite image to appear in other parts of the brain part of the image has to cross the Corpus Callosum, the white matter tissue that connects the two hemispheres. When reading the visual information — the letters — are sent to the Visual Word Form Area (VWFA) that is located at the base of the temporal lobe in the left hemisphere. The left hemisphere primarily deals with linguistic functions in the majority of people. The right hemisphere handles many image areas as well as arithmetic functions.
The VWFA recognises and decodes letter shapes; letters are paired up, syllables are made and word shapes are formed. Some research suggests that the VWFA is also involved in some lexical processes but the visual shape of the word is sent to the lexical (semantic) area of the brain where meaning is attached. Depending on language the phonological area is also involved in detecting the accurate meaning of the written word shape. Although not strictly speaking part of the visual system, recent research indicates that the cerebellum also has an influence on our reading ability.
The entire reading process is hugely complex and involves multiple parts of the brain, many not part of the visual system but working in parallel to successfully make sense of the written content. The quality of the information that is first received in the fovea, then converted to electric charges that travel down the optic nerve and via the visual cortex to the VWFA is what determines how well we can read. It is here where legibility comes to life and readability is determined.
The VWFA does not care whether the letter has serifs or not, whether it is a Latin character or Mongolian; it cares about how clearly identifiable each character is, how much ambiguity there is between letters that are vaguely similar. If additional elements, such as serifs, in a typeface help to distinguish characters they should be used; if they are a hindrance, they should be avoided. Clarity of letter shapes is also determined by the spacing between letters. With tight letter spacing specific letter pair combinations like ‘r n’ can easily be decoded by the VWFA as ‘m’ which leads to the word shape having to be reevaluated as it doesn’t make sense as a word in itself, or within the context of a sentence.
Strictly speaking the VWFA is not part of the visual system as it is not so much acquiring and processing visual stimuli as it is decoding and interpreting the stimuli to facilitate linguistic abilities. But it is clear and inarguable that the design and clarity of a letter shape and its surroundings have a considerable impact on how well the VWFA can interpret that shape. From the point of view of design and accessibility, the VWFA should be considered part of the human visual system.
Vergence and Accommodation Conflict (VAC)
New technologies bring new problems and the emergence of the various new realities (Virtual, Mix, Augmented) has not made life easier for those who care about accessibility. The visual system as described above is obviously engaged in the same way as in other types of visual stimuli with the exception of the conflict mentioned in the title.
In the ‘real’ world your eyeballs rotate in such a way that the line of sight meets at the same spot for each eye: vergence. The reflex mechanism in the optic nerve flexes the lens depending on the distance of the object to the eyes: accommodation. The entire visual system and motor functions in the brain have evolved to be highly successful to see things with depth perception and in clear focus.
Using a new reality device fools your brain into a ‘correct’ vergence — looking at something in the distance or close by. At the same time your focus — accommodation — is exactly at the distance between the lens and the device surface. The brain and visual system therefore have two conflicting tasks to do. This is what causes fatigue, eye strain, headaches, dizziness and in some cases even nausea. Most people can use new reality devices only for a short period of time.
The window for success
This article only looks at the visual system, a tiny fraction of all the parts of the human anatomy and physiology that are involved to enable reading and, by extension, writing. Reading and Writing are some of the most complex and involved activities a human can do, requiring a large amount of resources from the brain, yet we take it all for granted. With a bit of imagination, this brief description of the visual system illustrates the many places things can go wrong and access to written information can be impaired or denied altogether. The window for accessibility success is very narrow, and designers and content creators must do all in their power to at least give the VWFA a small chance of correctly interpreting letter shapes.
