According to the British Dyslexia Association (BDA), the term dyslexia refers to a disorder that impedes the learning process in reading, spelling and/or writing, independent of socio-economic factors and intelligence. This definition includes accompanying weaknesses, such as impairments in sequencing and organisation, auditory and/or visual perception, spoken language and motor skills. This is an intriguing theory, because it provides a simple explanation for a broad range of impairments associated with dyslexia.

Although the BDA's definition does not explicitly state the cause of dyslexia, it reflects an important assumption about its cause. It suggests that a general mechanism that affects language, perception and motor skills causes dyslexia. This assumption is consistent with the 'temporal processing deficit hypothesis', or 'timing hypothesis', proposed by a number of researchers, such as Paula Tallal. Although there are different versions of the timing hypothesis, they all agree that difficulties in processing rapid
information and the temporal characteristics of stimuli interfere with one's ability to process language and print. For example, timing deficits would cause difficulties in processing oral language because the individual phonemes (or sounds) are so fast. They could also impair fine motor coordination, balance, visual perception and speech. Furthermore, timing deficits would be present for tasks with rapid demands, but not slower tasks.

We gave dyslexic and non-dyslexic adults a set of reading measures, oral language tasks, and tasks that were thought to tap temporal processing. To increase our chances of finding timing deficits, we used a
wide range of timing tasks. Some were visual timing tasks (did you see one flash or two?), auditory (did you hear the high tone first?), speech production (name these digits as rapidly as you can), and fine
motor coordination (tap your fingers in time to the metronome). Support for the timing hypothesis would be if dyslexics performed more poorly when the timing tasks were presented quickly, but not when
they were slower.

However, we did not find support for the timing hypothesis. We found that in all but one of the timing tasks, dyslexic adults performed just as well as non-dyslexics. That is, dyslexics tended to be as accurate as non-dyslexics whether the tasks were presented quickly or slowly. The only task in which dyslexics showed deficits was the rapid naming task - when adults were asked to name the digits on a chart as quickly as possible. Dyslexic adults named the digits more slowly and less accurately than non-
dyslexics. Furthermore, performance on the digit-naming task was more closely related to reading and language tasks than the other timing tasks. Therefore, rapid naming may reflect language processes instead of a general timing mechanism.

In short, we found that timing deficits are unlikely to cause dyslexia in adults. Instead, it is more likely a language-based disorder. Indeed, we found that adults with dyslexia showed impairments in all the reading and language tasks, as well as rapid digit naming, but not on the timing tasks. Therefore, our
research suggests that the cause of dyslexia is more likely specific to language, rather than a more general timing mechanism.

Understanding the mechanisms that underlie dyslexia can have important implications for the intervention provided to individuals who suffer from it. Unsupported theories may lead to inappropriate
interventions that may be ineffective or harmful. Because we found that adult dyslexics' difficulties were restricted to literacy and language tasks, interventions that specifically address these skills would be appropriate. More specifically, programmes that do not explicitly address literacy and language skills should be avoided. For example, sensory-motor training has been found to enhance sensory-motor skills, but has no effect on literacy skills. More success is likely with traditional programmes that focus on word recognition, spelling and comprehension strategies.

If an adult's decoding skills are below a third grade level, phonological awareness training coupled with systematic, explicit phonics instruction would be appropriate. Structural analysis can also help: breaking
large words into meaningful, smaller units to make them less intimidating and using word families (e.g. light, might, sight), prefixes, suffixes and roots as clues to pronunciation and meaning. English spelling-sound relationships are more predictable when you use reliable chunks rather than letter-by-letter.

Finally, lessons should be reinforced with authentic reading materials. Many dyslexics experience difficulties in transferring skills learned in isolation to real-life situations; reinforcing lessons with real reading materials will help dyslexics to see how strategies translate to the real world.

The full details of the research are reported in Reading and Writing: An Interdisciplinary Journal, vol. 15, 2002.

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