Dyslexia is not only about sight, but also about sound

Gemma, flickr.com, CC BY 2.0
Gemma, flickr.com, CC BY 2.0

Teaching the brain to distinguish between elemental speech sounds helps dyslexic patients improve their reading and writing skills. This insight might lead to an effective long-term treatment for dyslexia.

Think about a typical day in your life. How many emails, WhatsApp messages and text messages are you likely to write? Do you read the newspapers or check for the latest news online? How much time do you spend reading and writing at work?

Now imagine what your day would be like if reading took three times as long because you had to put together one syllable after another in a painstaking process, hoping that it would result in a semantically meaningful word. Imagine how it would feel to struggle with orthography to such an extent that you would rather simply meet people in person or call them instead of writing an email. Does this sound exhausting? Well, this is life for people suffering from a reading and writing disorder known as dyslexia.

“Dyslexia does not occur randomly; it is common for parents or siblings of dyslexic children to show impaired reading and writing skills as well.”

Dyslexia is a condition characterized by profoundly diminished reading and/or writing skills that cannot be explained by a lack of education, intelligence or motivation. People suffering from dyslexia are not lazy, nor do their parents fail to support them appropriately. Dyslexia is a developmental disorder that results when certain structures in the brain fail to develop as they do in non-dyslexic children. In Western countries, about 1 person in 10 suffers from this disorder. However, there is considerable variability in how profoundly an individual’s skills are affected.

The number of people who suffer from dyslexia differs from one language to another. In languages with a high level of congruence between written letters and pronunciation, fewer people are affected. The prevalence of dyslexia is much higher among speakers of English and German, languages in which there are many exceptions and in which letters and sounds are not always congruent.

Why is it important to look more closely at dyslexia? We need only remember how essential written language is in our culture. Reading and writing skills are important predictors of school grades and, perhaps even more important, of success in later professional life. Moreover, dyslexics are more likely than non-dyslexics to report lower self-esteem and emotional disorders such as depression or anxiety. Those who suffer from dyslexia usually develop strategies to deal with their disorder. However, so far there is no effective treatment or therapy. Consequently, symptoms often persist into adulthood.

Certain brain regions are structurally different in people with dyslexia

One robust finding is that dyslexia does not occur randomly, but that in many cases parents or siblings of dyslexic children also show impaired reading and or writing skills. This suggests that physiological factors are at the root of this disorder. Over the past decades, numerous studies have sought to identify the physiological underpinnings of dyslexia. There is a broad consensus that specific brain regions are structurally different in dyslexics than in the rest of the population.

These differences seem to be more prominent in the left hemisphere of the brain ­- the hemisphere that is typically described as the “language-relevant” side. Yet just how these structural differences lead to impaired reading and writing performance is still a matter of debate. There are a number of different theoretical models that seek to answer that question.

Spelling when “ba” and “pa” sound the same

Most researchers and speech therapists agree that success in learning to read and write requires an awareness of the hierarchical structure of language and a recognition that phonemes, the smallest meaningful units of speech, can be put together to form syllables that, in turn, form words. This is known as “phonological awareness.”

To acquire writing and reading skills, it is also essential to understand that written symbols, i.e. letters, correspond to sounds, i.e. phonemes, and vice versa. To fully understand this so-called phoneme-grapheme correspondence, however, and to recognize that each written letter represents a sound, the brain’s auditory system needs to be fully functional. In the majority of people with dyslexia, this is not the case. Not only are their reading and writing skills impaired; certain aspects of basic auditory processing are affected as well. Dyslexics find it very difficult, for example, to distinguish between the similar phonemes “da” and “ta,” or “ba” and “pa.”

“The auditory system of dyslexic patients has to be repaired if they are to be successful in acquiring reading and writing skills.”

Most children with dyslexia also find it very difficult to memorize a series of phonemes and to create rhymes. Interestingly, this difficulty is not limited to speech, but also applies to non-speech sounds. The majority of these children have a hard time when they are asked to tap out a rhythm or to identify small gaps (milliseconds) between two tones. This is not because of impaired hearing or damage to the outer ear. Instead, the neurophysiological cause of this difficulty can be found in certain areas of the human brain that are responsible for the processing and integration of incoming auditory signals.

Many researchers therefore believe that it is necessary to repair the auditory system of dyslexic patients if they are to be successful in acquiring reading and writing skills.

Put the relevant brain functions on the right track

This is exactly what our research group – a cooperation between the section of Neuropsychology and the Clinic for Child and Youth Psychiatry at the Otto-von-Guericke University Magdeburg (Germany) – is currently investigating. The goal is to bring the functioning of the auditory system to more normal levels and thus allow for a more accurate processing of incoming acoustic information. To that end, weak electrical currents are applied to the brain regions that are relevant for the processing of auditory signals as well as speech. These currents mimic communication between neurons in the brain.

In the brain of a non-dyslexic person, the regions relevant for processing speech information parse incoming acoustic signals in a specific and functional manner. In dyslexic people, this process is imprecise, leading to inaccurate encoding of the incoming information. By means of electrical impulses, the functional parsing rate is imposed in order to “persuade” the auditory system to adjust its activity according to the externally applied pattern.

This method is known as transcranial electrical stimulation (tES) – a technique that is attracting more and more interest in the field of neuropsychological and psychiatric impairments, but also in basic science. Since the applied current is below the sensory threshold, the subjects do not notice it at all.

We have demonstrated the beneficial effect of tES on phoneme processing in two previous studies. First, we assessed 20 non-dyslexic adults between 20-35 years. In the follow-up study, we looked at age-specific differences of our intervention on phoneme processing. In both studies we registered a positive effect.

While the results of our ongoing research are promising, tES is yet not able to “cure” dyslexia. So far, we have only been able to achieve short-term and transient effects. Our next step will be to focus on more persistent effects. If repeated application of tES is indeed able to make the auditory system function more precisely, this would set the stage for the successful acquisition of reading and writing skills among the dyslexic population.