The ability to read requires a series of learned visual and processing skills to be used in tandem. A reader must first and foremost have an understanding of the language of the text (think of how difficult it is to understand what a foreign textbook is trying to convey!). The visual system is then engaged.
A reader must have good visual acuity, which means a clear image falls on the retina of the eyes. First, visual information is passed through the visual pathway to the occipital lobe, where visual processing begins. Both eyes are activated, and the fovea (area of central, clear vision) is directed at the words on a page. The accommodative system of the eye makes an adjustment to help clear the images, and the eyes rotate inward (converge) to point at the same location in space. The images from the right and left eye must be fused together (as double vision when reading would be very debilitating!).
The eyes then begin a series of calculated, short movement across the page–these small jumps are called saccades, and they help the eyes team together to move from one word or group of words across a page and back to the start the next line of text.
The inability to process the words on a page, or if part of the reader's vision is missing, can have a serious impact on his or her ability to comfortably find the next word or process the meaning of the words.
The complex nature of reading means deficits in any of these areas can impact reading ability. It is estimated that upwards of 16% of the population struggle with reading in some form, and 5% of children have significant problems with reading.
Reading can be broken down into a series of stages starting from early awareness of text to expert reading ability.
This often begins prior to schooling or with early schooling. A child starts to develop an awareness of text, some sound or phonetic awareness and may be able to identify common signs or symbols.
This is the early reading of a more formal sense. Letters are now understood, and combinations of letters make words. Sight words are learned, and the ability to "sound out" unknown words is used. This stage is progressed through around early school age (first and into second grade). About 1,000 words can be recognized at this stage.
Decoding continues and phonics improves. The child can read with greater ease and fluency and should be able to recognize over 3,000 written words.
By grade 4, reading can be used to gather information and knowledge. This leads to using reading for the formation of ideas, expanding vocabulary, and enhancing cognition. About this time textbooks are often introduced in schools (sometimes even sooner!) and tasks require an understanding of text, answering questions, and more complex writing organization.
High school years require reading for understanding and study of complex concepts consisting of different narratives and viewpoints. The demand for reading comprehension and thought formation is again elevated.
This is the college-level reader and beyond. Reading is used to extract information, formulate complex thoughts, and to integrate the reader's knowledge into works (or even create new works of text!)
Dyslexia by itself has a definition problem. Some people use the term dyslexia as a "catch-all" label for reading difficulties. In a broad sense, dyslexia refers to significant difficulties with word reading, decoding, and spelling as evidenced by low accuracy and/or fluency on standardized assessments. This broad definition has led to the definition of different subtypes of dyslexia such as:
Accommodation is the focusing system of the eyes. When text is moved closer or farther away, the accommodative system must make an adjustment to keep the words clear. As reading material becomes more sophisticated, the accommodative system must be more precise. Disorders such as accommodative insufficiency, accommodative infacility, and accommodative excess place a strain on the binocular system.
The oculomotor system must provide accurate, fast control for reading to allow the reader to maximally extract visual information from text. The fovea of the retina must fixate on a word that is to be decoded. Small eye movements called saccades are incredibly fast. Most saccades move the eyes along the text in a forward fashion (left-to-right in English text for example), however, about 10-20% of saccades are in the backward direction (regression). The combination of foveation and saccades during reading does not necessarily mean the reader is fixating on each individual word, rather, when used together, the two actions allow the grapheme (a combination of letters) to quick be matched with the phoneme (a phonetic sounds) by visual processing centers of the brain. Poor oculomotor control may lead to a substantially higher amount of regressions and poor foveation. Children with poor oculomotor control may be labeled as inattentive, careless, or distracted readers.
Binocular disorders are often more problematic in the middle and advanced stages of reading. This is a rather broad category (and technically accommodation and oculomotor are at least in part disorders of binocular vision). Any condition that disrupts fusion and inhibits clear, single, comfortable vision has the potential to impact reading ability. Common binocular vision disorders such as exophoria, esophoria, exotropia, esotropia, amblyopia (lazy eye), convergence insufficiency, convergence excess, and multiple other conditions may negatively impact reading ability.
Abnormalities that affect these first three major components (accommodation, oculomotor, and binocular vision) often produce similar symptoms such as:
This is an excellent reason to involve and eye care specialist when reading difficulties are suspected. A thorough binocular vision examination looks for these core visual skills. Some eye providers may even test visual processing.
Visual perceptual skills are developing as the child progresses through reading stages. Word recognition is critical to reading. Early reading begins with simple sight words. Thes are small, familiar words to begin with, and eventually, expand to more complex or "abnormal" words that may not follow the traditional "sound-it-out" rule (think of words like "neighbor" or "weigh"). This also involves directional orientation (think of b d p q - how similar these look!), pattern recognition, and shape and form matching. Taken together, these visual skills include visual discrimination, visual memory and recall, and spatial relationships. To further complicate matters, math adds a whole new compent of visual-spatial thinking to this already complex mix!
Helping children with reading difficulties: some things we have learned so far (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6220295/)
Jeanne S. Chall, Stages of Reading Development. N.Y.:McGraw-Hill Book Company, 1983. (https://lilaacdotcom.files.wordpress.com/2013/05/challs-stages-of-reading-development.pdf)
Understanding Dyslexia in the Context of Developmental Language Disorders (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6430503/)
Attention and Eye Movements in Reading (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2694615/)
Amblyopia and Slow Reading (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6050007/)