Dyslexia refers to an unexpected reading failure. A child with poor reading skills in a negative environment is to be expected but a child with poor reading skills in a favourable environment is unexpected. This could be indicative of dyslexia.

There are common behaviours of someone with dyslexia:

1. Phonology
   The impaired performance on phonemic tasks includes deletion (1), matching (2), and blending (3).
2. Reading aloud
   The performance is often slow (1), dysfluent (2) and error-prone (3). There is particular difficulty with irregularly pronounced words.
3. Processing speed
   The performance is often slow when it comes to naming familiar digits (1), colours (2) and objects (3).
4. Orthography
   The limited knowledge of orthographic structure expresses itself in difficulty distinguishing valid from invalid letter strings (1), weak knowledge of word spellings (2), misspellings (3), misidentification (4) and dysfluency in generating spellings (5).
5. Working memory
   There is a deficit in working memory tasks.
6. Language
   There is a limited vocabulary size and lexical quality (1), familiarity with a narrower range of sentence structures and expressions (2) and difficulty reading texts aloud with appropriate intonation (3).

Reading is a highly complex skill and a skilled reader can read an isolated word in less than a second. It is a relatively newly acquired skill on the evolutionary timeline. Most people acquire speech naturally but they require explicit instruction for reading. Reading is how writing is turned into speech and speech is primary in reading.

During reading, people make quick saccades and the fixation point is very small. This means that a person processes a small area during reading. Every content word in reading is just quickly fixated on (i.e. 250ms). People use the context but cannot know the sentence by only using the context. People seldom go back during reading.

Beginning readers fixate on each content word briefly. However, compared to skilled readers, they fixate longer on each word and have more difficulty decoding the words. They also show more backtracking.

While learning to read, children have to go from the concept to the spoken sound to the written word and translate it back to the sound and then to the concept. Children have to learn how to go from orthography (i.e. spelling) to phonology (i.e. sound) to semantics (i.e. meaning) and vice versa. Learning to read is thus the integration between orthography and phonology.

Children have letter by letter decoding at first (e.g. b-a-t). Adults, on the other hand, have parallel activation of letters and word superiority effect. The words are read faster but all individual letters are still being read (just faster). People with dyslexia decode letter by letter longer and have problems in integrating which is central for reading. All children face the same difficulties in learning to read but dyslexics have more difficulty with the essential components. The accumulation of these deficits and their multiplicative effect on each other affect performance to the point where a child can be described as dyslexic.

A phoneme refers to the smallest unit of sound within a spoken word (e.g. p). A grapheme refers to a letter or letters that represent a speech sound (e.g. ea). A morpheme refers to the smallest unit of meaning (e.g. ed).

The writing system influences learning to read and the process of reading. In the alphabetic language system, graphemes represent phonemes. There is not a 1-1 correspondence of letters and sounds. In the syllabic language system (e.g. Japanese), the graphemes represent syllables in spoken language. This is harder to learn but reading is easier in the end. In the morpho-syllabic language system (e.g. Chinese),  the characters represent semantics and phonetics. Many homophones have the same sound but different spelling and meaning so characters have to be learned individually. This is very difficult to learn but easier to read.

The orthographic depth determines how difficult it is to learn to read a language. A shallow orthographic depth includes languages where a specific letter is always pronounced the same. A deep orthographic depth includes languages where the relations between letters and sounds are more arbitrary. In shallow orthographies, dyslexia is manifested in slower but more accurate responses than in deeper orthographies.

Dyslexia has social and emotional effects and there are behavioural deficits which are a solid basis for identification. The basic characteristics and causes of dyslexia are understood well enough for identification and treatment of dyslexia.

According to the DSM-5, dyslexia refers to persistent difficulties in reading during formal years of schooling. The symptoms may include inaccurate or slow and effortful reading. The learning difficulties begin during the school-age years and the individual’s difficulties must not be better explained by another deficit (e.g. neurological disorder). However, this does not include anything about the nature and the causes of the disorder.

The Dutch definition of dyslexia refers to a specific reading and spelling disability which is below age-expected levels. There is a neurological basis and it is caused by cognitive processing problems in the connection of orthography and phonology that are also below age-expected levels. The other language processing skills are less impaired, thus the difficulties are in the basic language skills (e.g. spoken language comprehension is age-appropriate). The specific reading and spelling problems must interfere with academic achievement and must not be explained by poor education (i.e. no response to typical education).

There are several things about IQ and reading difficulties:

1. Wide IQ range
   Dyslexia occurs across a wide IQ range as there are the same behavioural characteristics of poor readers.
2. IQ as a predictor
   IQ is not a strong predictor of intervention responses.
3. Arbitrary boundaries
   Reading ability and IQ are continuous meaning that the discrepancy between boundaries are arbitrary.
4. Normal distribution
   The low end of the normal reading distribution includes severe reading difficulties and requires treatment.

Dyslexia is not diagnosed if the reading problems are secondary to other problems (e.g. very low IQ). However, the genetic component of dyslexia is greater for higher IQ children. Dyslexia is comorbid with other developmental disorders (e.g. ADHD). This may be because of deficits in the shared capacities of these disorders. In dyslexia, other behaviours may be affected because reading impairments may result from anomalies in capacities that are not just for reading (e.g. visual problems in recognizing letters).

Dyslexia does not have a single cause and underlying deficits vary in severity (1), the behaviours they affect (2) and their persistence (3). Impaired reading could result from the co-occurrence of several relatively mild deficits that would not be debilitating in isolation. The effects of these deficits can be modulated by protective factors.

It is possible that the dyslexic brain develops in a way that interferes with discovering commonalities among words involving orthography and phonology. Non-dyslexic readers show an adaptation effect when listening to people’s speech. Their brain activity in speech-related areas was reduced when words were spoken by the same person rather than multiple. This is because an individual picks up on a speaker’s characteristics. This effect is weaker in dyslexics.

Dyslexics may have difficulty with generalizing patterns of words. This leads to dyslexics needing to learn the same thing multiple times. They fail to benefit from shared structure including exact repetitions. There may be oversensitivity to irrelevant information (e.g. font).

Many beginning readers falling behind the age group catch up with ordinary support and effort. Dyslexics, on the other hand, do not grow out of the conditions and the problems multiply. Dyslexia falls on a continuum and identification and targeted treatment are important for well-being, health and success. This means that there is purposeful medicalization and it is not just a characteristic of falling on the low end of the normal distribution when it comes to reading.

Phonological awareness and rapid naming are predictors of reading development. The phonological awareness refers to the awareness of the sound structure of words. Sounds are central and not the letters. Rapid naming refers to the ease of access to representations stored in memory. Deficits in both phonological awareness and rapid naming predict dyslexia. However, deficits in phonological awareness are more predictive in opaque orthographies (e.g. English). The deficits in rapid naming are more predictive in more transparent orthographies (e.g. Dutch).

The best treatment to dyslexia is prevention and this requires to identify at-risk children. Definitive identification is not possible until a later age when interventions may be less effective.

The response to intervention (RTI) approach consists of several steps:

1. Screening for risk (i.e. simple tests of basic pre-reading skills)
2. Family history reading and language difficulties (i.e. check family history)
3. Tier 1 intervention (i.e. trained classroom teachers)
4. Tier 2 intervention (i.e. 1-1 instruction or small groups)
5. Tier 3 intervention (i.e. special education classroom)

A new tier intervention is introduced if the progress in the previous tier was inadequate. The tier 1 and tier 2 intervention have to be of high quality for RTI to be an effective approach.

Training has to focus on phonological awareness while reading (1), on letter-sound connections (2), on short visual language stimuli (3) and the training should teach rule-based strategies for reading (4). The training should be repeated (1), intensive (2) and very structured (3). After the intervention, the reading accuracy can be age-appropriate but reading fluency can lag behind.

Dyslexia is a behavioural continuum. Insights are needed into individual differences and individual development. Besides that, observable behaviour and underlying causes need to be studied. A better understanding of condition and aetiology leads to more effective interventions and better outcomes.