Dual-route hypothesis to reading aloud

The dual-route theory of reading aloud was first described in the early 1970s. This theory suggests that two separate mental mechanisms, or cognitive routes, are involved in reading aloud, with output of both mechanisms contributing to the pronunciation of a written stimulus.

One mechanism, termed the lexical route, is the process whereby skilled readers can recognize known words by sight alone, through a "dictionary" lookup procedure. According to this model, every word a reader has learned is represented in a mental database of words and their pronunciations that resembles a dictionary, or internal lexicon. When a skilled reader sees and visually recognizes a written word, he or she is then able to access the dictionary entry for the word and retrieve the information about its pronunciation. The internal lexicon encompasses every learned word, even exception words like 'colonel' or 'pint' that don't follow letter-to-sound rules. This route doesn't enable reading of nonwords (example 'zuce'). There is still no conclusive evidence whether the lexical route functions as a direct pathway going from visual word recognition straight to speech production, or a less direct pathway going from visual word recognition to semantic processing and finally to speech production.

The nonlexical or sublexical route, on the other hand, is the process whereby the reader can "sound out" a written word. This is done by identifying the word's constituent parts (letters, phonemes, graphemes) and, applying knowledge of how these parts are associated with each other, for example how a string of neighboring letters sound together. This mechanism can be thought of as a letter-sound rule system that allows the reader to actively build a phonological representation and read the word aloud. The nonlexical route allows the correct reading of nonwords as well as regular words that follow spelling-sound rules, but not exception words. The dual-route hypothesis of reading has helped researchers explain and understand various facts about normal and abnormal reading.

According to research, the amount of time required to master reading depends on the language's adherence to phonological rules. A written language is described as transparent when it strongly adheres to spelling-sound rules and contains few exception words. Because of this, the English language (low transparency) is considered less transparent that French (medium transparency) and Spanish (high transparency) which contain more consistent grapheme-phoneme mappings. This difference explains why it takes more time for children to learn to read English, due to its frequent irregular orthography, compared to French and Spanish. The Spanish language's reliance on phonological rules can account for the fact that Spanish-speaking children exhibit a higher level of performance in nonword reading, compared to English and French-speaking children. Similarly, Spanish surface dyslexics exhibit less impairment in reading overall, because they can rely upon consistent pronunciation rules instead of processing many exception words as a whole in the internal lexicon. The dual-route system thus provides an explanation for the differences in reading acquisition rates as well as dyslexia rates between different languages.

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