Hyperlexia is a syndrome characterized by a child's precocious ability to read, combined with difficulty in understanding and using verbal language, and problems with social interactions. It was initially identified by Norman E. Silberberg and Margaret C. Silberberg (1967), who defined it as the precocious ability to read words without prior training in learning to read typically before the age of 5. They indicated that children with hyperlexia have a significantly higher word-decoding ability than their reading comprehension levels.
Hyperlexic children are characterized by having average or above-average IQs, and word-reading ability well above what would be expected given their age. First named and scientifically described in 1967 (Silverberg and Silverberg), it can be viewed as a superability in which word recognition ability goes far above expected levels of skill. Some hyperlexics, however, have trouble understanding speech. Some experts believe that most or perhaps all children with hyperlexia lie on the autism spectrum. However, one expert, Darold Treffert, proposes that hyperlexia has subtypes, only some of which overlap with autism. Between 5 and 10 percent of children with autism have been estimated to be hyperlexic.
Hyperlexic children are often fascinated by letters or numbers. They are extremely good at decoding language and thus often become very early readers. Some hyperlexic children learn to spell long words (such as elephant) before they are two years old and learn to read whole sentences before they turn three. An fMRI study of a single child showed that hyperlexia may be the neurological opposite of dyslexia.
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Despite hyperlexic children's precocious reading ability, they may struggle to communicate. Often, hyperlexic children will have a precocious ability to read but will learn to speak only by rote and heavy repetition, and may also have difficulty learning the rules of language from examples or from trial and error, which may result in social problems. Their language may develop using echolalia, often repeating words and sentences. Often, the child has a large vocabulary and can identify many objects and pictures, but cannot put their language skills to good use. Spontaneous language is lacking and their pragmatic speech is delayed. Hyperlexic children often struggle with Who? What? Where? Why? and How? questions. Between the ages of 4 and 5 years old, many children make great strides in communicating.
The social skills of a child with hyperlexia often lag tremendously. Hyperlexic children often have far less interest in playing with other children than do their peers.
Types of hyperlexia
- Type 1: Neurotypical children that are very early readers.
- Type 2: Children on the autism spectrum that demonstrate very early reading as a splinter skill.
- Type 3: Very early readers who are not on the autism spectrum though there are some "autistic-like" traits and behaviours which gradually fade as the child gets older.
- Type 1: Hyperlexia marked by an accompanying language disorder.
- Type 2: Hyperlexia marked by an accompanying visual-spatial learning disorder.
In studies in Cantonese and Korean, subjects were able to read non-words in their native orthography without a delay relative to the speed with which they read real words in their native orthography. There is a delay noted with exception words in English, including the examples: chaos, unique, and enough. These studies also illustrate difficulties in understanding what it is that they are reading. The findings suggest that non-hyperlexic readers rely more heavily on word semantics in order to make inferences about word meaning. 
In the Cantonese study, distinguish homographs and determine the readings for rarely used characters. In this study, the subject also made errors of phonetic analogy and regularization of sound. The authors of the study, Weekes, Wong, Iao, To, and Su, suggest that the two routes model for reading Chinese characters may be in effect for hyperlexics. The two routes model describes understanding of Chinese characters in a purely phonetic sense and the understanding of Chinese characters in a semantic sense.
The semantics deficit is also illustrated in the study of Korean hyperlexics through a priming experiment. Non-hyperlexic children read words primed with a related image faster than non-primed words while hyperlexics read them at the same pace. Lee Sunghee and Hwang Mina, the authors of the Korean study, also found that hyperlexics have fewer errors in non-word reading than non-hyperlexics. They suggest that this may be because of an imbalance in the phonological, orthographical, and semantic understandings of the subjects’ native language and writing system, in this case, Hangul. This combination of the parts of linguistics is known as connectionist theory, in which non-words are distinguished from words by differences in interaction between phonology, orthography, and semantics.
In the Lee and Hwang study, the subjects scored lower on general language test and vocabulary tests than the average for their age groups. Literacy education in South Korea involves teaching students entire words, rather than starting with the relationship between phonemes and letters in Hangul, despite evidence that letter name knowledge is useful for learning to read words that have not been taught. The results suggest that hyperlexics are able to obtain the relations between letters (or the smallest unit of the writing system) and their phonemes without knowing the names.
Comprehension difficulties can also be a result of hyperlexia. Semantics and comprehension both have ties to meaning. Semantics relates to the meaning of a certain word while comprehension is the understanding of a longer text. In both studies, interpretation and meaning based tests proved difficult for the hyperlexic subjects. In the Weeks study, the subject was unable to identify characters based on the logographic aspect of the writing system, and in the Lee and Hwang study, priming was ineffective in decreasing reading times for hyperlexics. 
Although it is generally associated with autism, a 69-year-old woman appears to have been made hyperlexic because of a "cerebral infarction in the left anterior cingulate cortex and corpus callosum".
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