Table 1

 Cognitive components involved in reading and writing

Visual processingThe magnocellular system responds to moving stimuli and stimuli of low spatial frequency and low contrast. Impaired perception of moving stimuli and the neurophysiological correlates of this have been found repeatedly in individuals with dyslexia. The exact nature of this deficiency and its potential relationship to dyslexia is not yet clear
Phonological awarenessThe ability to perceive, segment and manipulate the sounds of spoken words.31 Phonemes are the smallest meaningfully distinct sounds from which an acoustic speech flow can be constructed. The word dog, for example, consists of three phonemes /d/, /o/, /g/. The capacity for phonological awareness is often tested through a phoneme deletion task
Verbal short-term memoryVarious aspects of memory are required for reading. Many known words are no longer dissected into their phonemes, but are recalled directly from memory. Processing of unknown words into their phonemes occurs in short-term memory. Short-term memory is often examined by a digit span task
Phonological codingThe ability to put together the phonemes and then verbally express words which have never been previously read or heard. This ability is tested through reading of pseudowords
Orthographic codingOrthographic coding refers to the assumed process of recognising a word by its holistic form. Orthographic coding is measured by a pseudohomophone task where an orally presented word has to be compared with a visual presentation of two phonologically indistinguishable words, of which one may be orthographically correct
Rapid namingRapid naming is a measure of the speed of processing. The naming of objects, numbers, letters and colours is typically measured