Bringing order to the jumbled letters

03/Jul/2017

Electric stimulation might help people with dyslexia to strengthen their brain’s interconnection between spoken and written language. By Yvonne Vahlensieck

(From "Horizons" no. 113 June 2017)​​​

​Reading and writing are relatively new achievements in the history of mankind. That means they're not pre-programmed in our brains. As a result, we spend a lot of time in our early school years learning how to translate spoken language into written letters (and vice versa). But some children just can't do this properly. They mix up letters, make innumerable spelling errors and can only read hesitantly. Between 5 and 15 percent of the population suffer from such a reading and writing disability, which is generally known as dyslexia. New findings from brain research might offer them help.

Trouble with syllables

Researchers have long been investigating just what goes wrong in a case of dyslexia. What is certain is that it's got nothing to do with intelligence or a lack of it. Instead, neuroscientists have discovered a deficit in the brain's interface where it processes spoken and written language. "The problem is not primarily to do with hearing or seeing, but lies on a higher level where auditory and visual linguistic processing come together and are concentrated", explains Daniel Brandeis, a professor of neurophysiology at the University of Zurich. "At this interface, spoken language is dismantled into its individual elements and linked to visual patterns of written characters that can be recognised by specialised regions of the brain".

This corresponds to the experience of those who teach children with special needs. Children with dyslexia find it difficult to break down language into its smallest units – phonemes. For example, they find it difficult to clap to syllables and to differentiate between similar sounds such as 't' and 'd'. This is why therapies for dyslexia often concentrate on these deficits. "Purely visual training without language has no positive effect on reading and writing", says Anke Sodogé, a professor at the University of Applied Sciences of Special Needs Education in Zurich. "On the other hand, exercises for differentiating and processing sounds have been proven to be effective". But she also warns about expectations being too high with this therapy because the progress attained is often only minor, regardless of the effort involved.

Correcting brain oscillations

A new neurological approach to treating dyslexia could improve the success of these therapies. It is based on the finding that specific brain waves are out of step in people with dyslexia. With the aid of electrical brain stimulation, the Swiss psychologist Katharina Rufener at the Otto-von-Guericke University of Magdeburg aims to bring these oscillations back into synch. "The idea is to normalise the initial physiological activity of the brain so that the therapy has a better impact".

In the brain, neurons fire synchronously to create different types of oscillations that fulfil different functions, according to their frequency. The gamma oscillations that are disturbed in cases of dyslexia have a frequency of between 25 and 40 hertz and are involved in processing auditory language: "The brain has about 25 milliseconds to recognise individual phonemes. This sampling rate corresponds roughly to the period of a gamma oscillation", says Rufener. "In dyslexia, these oscillations are too slow or too quick, which means that the sampling rate of the acoustic language signal is disturbed. This is why the brain cannot distinguish between different phonemes".

In order to make these altered oscillations synchronous again, Rufener stimulates the brain of her test subjects by means of two electrodes attached to their heads. The desired frequency is transmitted to the brain by a weak alternating electric current. It is known that nerve cells in the regions stimulated will take up oscillations transmitted from outside.

Rufener stimulated her test subjects with these oscillations and at the same time had them carry out tasks to differentiate phonemes. Children and young people with dyslexia were able to carry out these tasks better with this stimulation than without; the frequencies used were in the gamma range. In a second series of tests, Rufener now wants to find out the effect of other oscillations that are also impaired in the majority of dyslexia patients, and which are linked to cognitive ability.

It is still unclear whether or not this method can lead to a long-term improvement, because after a single phase of stimulation, the brain falls back into its incorrect rhythm almost immediately. But there are signs that the effect lasts longer when the brain receives this stimulation several times over a longer period.

Testing the approach

Anne-Lise Giraud, a professor in neuroscience at the University of Geneva, believes that it is too soon for a therapeutic use of this approach. "Actually, we don't really know yet what happens when the brain is stimulated". She is carrying out similar tests in collaboration with the Wyss Center for Bio and Neuro-engineering – though initially only on adults without dyslexia. "When we stimulate them, we see an improvement in their ability to recognise phonemes. But we also get many contradictory results".

This is why Giraud first wants to carry out more basic research. For example, it still has not been definitively explained whether or not gamma oscillations are constantly present. "We believe that the brain itself only gives out a weak oscillation, which is then strengthened through the act of hearing". Her investigations also show that the gamma oscillations are coupled with other waves in the brain that quite possibly control adjustments to different speaking rates. In order to test her hypotheses, Giraud has developed a computer simulation: "The model depicts a network of neurons that produces gamma oscillations, and with this we can run through all these processes".

Even if these connections have not yet been explored in detail, Daniel Brandeis finds the application of electrical brain stimulation a highly promising approach: "I can well imagine that this procedure can decisively increase the efficacy of existing therapies, and thereby achieve quicker, longer-lasting changes". All the same, we should not expect to achieve a complete cure. "For those who are badly affected, reading usually remains difficult despite successful treatment. Only in rare cases will they be able to read with the same ease as people without dyslexia".

Yvonne Vahlensieck is a freelance science journalist who lives near Basel.

 

K. S. Rufener et al.: Transcranial Alternating Current Stimulation (tACS) differentially modulates speech perception in young and older adults. Brain Stimulation (2016)