Hearing the Sounds Inside Words
Reading also begins with sound
Reading begins with vision, but learning to read also requires the brain to connect what the eyes see with a language system that developed originally through listening and speaking.
A child learning to read has to discover that the apparently continuous stream of spoken language can be divided into words, syllables and individual speech sounds, or phonemes. They then have to learn that written symbols represent those sounds. This ability, broadly described as phonological processing, is one of the most consistently identified areas of difficulty in developmental dyslexia.
But this raises another question: why does phonological processing develop less efficiently in some children? Research by Caroline Witton, Joel Talcott, John Stein and colleagues, including myself, investigated whether part of the answer might lie further down the processing chain - in how accurately the auditory system detects rapidly changing information.
Speech is constantly changing
Although we experience speech as words, the acoustic signal reaching the ears is continuously changing. Frequency, amplitude and timing alter rapidly. The auditory system must extract meaningful patterns from these changes before higher-level language systems can construct stable representations of speech sounds.
Frequency modulation (FM) involves changes in the frequency of a sound. Amplitude modulation (AM) involves changes in its intensity. Caroline Witton's work examined whether sensitivity to these relatively fundamental acoustic changes was associated with children's phonological and reading abilities.
Witton, Stein et al. (2002), for example, examined AM and FM sensitivity in impaired and normally developing readers and investigated their relationships with phonological decoding. Their results supported relationships between auditory sensitivity and phonological reading skills, although the pattern was more complicated than a single general auditory deficit.
Phonological decoding
Phonological decoding is particularly visible when someone encounters an unfamiliar word or nonword. If the reader sees a made-up word such as 'mep', there is no stored pronunciation to retrieve. The reader has to identify the letters, associate them with speech sounds and blend those sounds together.
Compare this with a familiar but irregular word such as 'yacht'. Simple letter-sound rules are insufficient. Skilled readers have developed orthographic knowledge of the written word and connect that representation with pronunciation and meaning.
These processes interact continuously during normal reading, but distinguishing them experimentally allowed the Oxford researchers to ask whether visual and auditory processing relate differently to these components of reading.
The Talcott-Witton dissociation
Talcott, Witton et al. (2000) measured visual motion sensitivity and auditory frequency-modulation sensitivity in children. After controlling for intelligence and overall reading ability, visual motion sensitivity accounted for independent variance in orthographic skill but not phonological ability. In contrast, auditory FM sensitivity covaried with phonological skill but not orthographic skill.
This does not mean that there are two completely independent reading systems, one visual and one auditory. Nor does it demonstrate that sensory differences necessarily cause dyslexia. It suggests something subtler: sensitivity to rapidly changing information in different sensory systems may contribute differently to component skills from which fluent reading develops.
Does dyslexia mean auditory processing disorder?
Not necessarily. Group studies and meta-analyses have reported auditory-processing differences in dyslexia, but there is substantial variation between individuals and experiments.
Witton, Swoboda et al. (2020) reviewed auditory frequency discrimination in developmental dyslexia and found evidence of a group difference alongside considerable heterogeneity. A sensory-processing difference found at group level cannot simply be assumed to exist in every individual with dyslexia.
There is also an unresolved causal question. Does less efficient auditory temporal processing contribute to difficulty learning to read? Does reading development alter auditory processing? Or do both arise from broader developmental differences? Reading-level matched studies have contributed to this debate, and the answer is not settled.
Why this matters clinically
The research does not imply that every person with dyslexia requires an auditory-processing intervention. It tells us that the label dyslexia does not describe one uniform cognitive profile.
One child may have pronounced difficulties manipulating phonemes. Another may struggle particularly with rapid naming or verbal working memory. Another may have difficulty developing stable orthographic representations. Attention, language and broader executive functioning may also influence performance.
At INPSYGHT, the more useful question is therefore not simply whether a reading score crosses a diagnostic threshold, but what is making reading difficult for this particular person.
Phonics remains enormously important. But a phoneme is not physically printed inside a word waiting to be discovered. The developing brain has to construct phonological representations from complex auditory information and then learn how those representations map onto visual symbols.
Reading brings two very different worlds together. One begins with light. The other begins with sound.
References
Talcott, J.B., Witton, C. et al. (2000). Dynamic sensory sensitivity and children's word decoding skills. Proceedings of the National Academy of Sciences, 97, 2952-2957.
Witton, C., Stein, J.F. et al. (2002). Separate influences of acoustic AM and FM sensitivity on the phonological decoding skills of impaired and normal readers. Journal of Cognitive Neuroscience, 14, 866-874.
Stein, J. (2001). The magnocellular theory of developmental dyslexia. Dyslexia, 7, 12-36.
Witton, C., Swoboda, K. et al. (2020). Auditory frequency discrimination in developmental dyslexia: a meta-analysis. Dyslexia, 26, 36-51.
Meilleur, A.A.S. et al. (2020). Unisensory and multisensory temporal processing in autism and dyslexia: a systematic review and meta-analysis. Neuroscience & Biobehavioral Reviews, 116, 44-63.