Hearing and Dyslexia
Reading depends on good Hearing- Warbles
Currently most people who work on dyslexia believe that the main cause of reading difficulties is failure to acquire the skill of separating the sounds of words into their separate phonemes to match with the letters that represent them ie they have a linguistic 'phonological' weakness. It is certainly true that most poor readers have this problem, but this is not really an explanation. The basic requirement of reading is to translate letters into their sounds, so the phonological theory merely restates that these children cannot read. More interesting is to find out why they have this problem. What is the brain difference that underlies it?
Letter sounds are distinguished from each other mainly by changes in speech amplitude and frequency. We can understand speech because the auditory system is adapted to pick up these amplitude and frequency modulations (AM & FM). So we have been measuring how well adult dyslexics and good readers can hear simple changes in amplitude (AM) & frequency (FM); the stimuli sound like warbles.
We found that the dyslexics were indeed significantly less sensitive at hearing these warbles. Whether you are a bad or good reader, your sensitivity to these simple sounds seems to help determine how well you can develop phonological skills. Thus it seems that your basic auditory AM & FM sensitivity plays a very important part in the development of phonological skills.
Magnocells are not confined to vision, but form a whole subsystem throughout the brain dedicated to detecting change. In the auditory system their function is to track these FM and AM variations in speech sounds. Like those in the visual system, auditory magnocells seem to fail to develop quite normally in dyslexics. Hence dyslexics often have difficulty distinguishing similar sounds such as 's', 'sh', 'th', 'f'. If they confuse sounds like this, it is not surprising that they often fail to match the sounds correctly with the letters that stand for them, and this may underlie their phonological deficit.
Again understanding the origin of these problems has led to simple techniques for helping these children, several methods have been developed for training young children's auditory skills to hear these amplitude and frequency changes more easily, such as teaching them rhyme, rhythm and music. It is a great pity that primary schools have cut down on music lessons because these skills turn out to be important for learning to read as well as to appreciate music. Also adaptive computer programmes that emphasise the sound changes have been developed that show promise for helping dyslexics with auditory weaknesses so we are confident that we can help children with these problems to hear phonological distinctions better and thus help them to avoid phonological reading problems later on.