This Is Your Brain On Dyslexia


Credit: Lou Anna/Pixabay

It’s a safe bet that dyslexics and non-dyslexics have different patterns of brain activity while reading. The real questions are, how do these patterns differ and do the differences shed any light on the possible causes of dyslexia. Researchers from Boston University, the MGH Institute of Health Professions and MIT used fMRI to address these questions and found results that were both unexpected and informative. Their work was reported on Dec. 21, 2016 in the journal Neuron.

Dyslexia is typically viewed as a reading impairment but dyslexics generally evidence reduced levels of performance on other tasks as well. For example, dyslexics do not usually hear speech in noisy environments as well as non-dyslexics. Findings like this suggest that impairments in sensory perception in general, as opposed to the sensory processes that are directly involved in reading, may contribute to dyslexia.

Perceptual adaptation

Have you ever started a conversation with someone who speaks with an unfamiliar accent and found you had difficulty understanding what they were saying? What often happens in situations like this is that the listener quickly adapts to the unfamiliar speech sounds and understanding noticeably improves after just a few sentences. The process is called perceptual adaptation.

Credit: Wikipedia

The researchers used fMRI to examine the neurophysiological correlates of perceptual adaptation in dyslexics and non-dyslexics. The areas of the brain that are active when people listen to someone talking are easily identified using fMRI. Activity in some of these areas is high when the voice is new to the listener but it quickly declines as perceptual adaptation takes place. The activity declines because the brain has become more efficient at processing the sound of the voice. The decline in brain activity that accompanies perceptual adaptation is called neural adaptation.

To examine neural adaptation in dyslexia, the researchers asked dyslexics and non-dyslexics to perform a simple task in which they heard a word which they matched to a picture. The dyslexics and non-dyslexics were each divided into two groups; one group heard each word spoken by the same voice while the other heard each word spoken by a different voice. Neural adaptation should take place when the words were spoken by the same voice. When a different voice said each word, neural adaptation should not occur.

This is just what happened with the non-dyslexics; neural adaptation occurred when all the words were spoken by the same voice. When the dyslexics heard the same voice saying all the words, neural adaptation occurred but it was substantially reduced. The dyslexics did not learn as much from hearing the same voice repeatedly and as a result did not process information about the sound of the voice as efficiently as the non-dyslexics.

Credit: Wikipedia

Credit: Wikipedia

Expanding beyond the sound of a voice

At first glance, research that focuses on the sound of a voice may not seem to have much to do with dyslexia which is supposed to be about reading. However, there is a solid body of prior research that indicates that the reading impairments seen in dyslexia are strongly associated with impairments in the ability to process spoken speech.

Nevertheless, the researchers wanted to know if dyslexic’s reduced level of neural adaptation when words are spoken is also present when words are read. They asked new groups of dyslexics and non-dyslexics to read blocks of words that were either made up of the same word repeated over and over or different words. The results with the written words were the same as with spoken words: neural adaptation occurred when the same word was repeated for the non-dyslexics but was markedly reduced for the dyslexics.

The results thus far indicate that dyslexia involves reduced neural adaptation to both written and spoken forms of language. Is this reduction confined to language, or does dyslexia involve impaired adaptation to other types of perceptual information as well?

The investigators examined this question by repeating the blocks-of-words study with experiments that used pictures of objects or faces. The areas of the brain involved in processing objects, faces and words are different but the pattern of results was the same. Adaptation to repeated presentations of the same object or face was strong among the non-dyslexics and weak among the dyslexics.

The results of all these studies taken together indicate that dyslexia is not just about reading. It involves a reduction in neural adaptation to a variety of perceptual stimuli. People with dyslexia do not learn as much from repeated experience with spoken words, written words, pictures of objects or pictures of faces. This indicates they process these different types of perceptual information less efficiently than non-dyslexics.

Credit: 422737/Pixabay

Credit: 422737/Pixabay

Questions that remain

The results of this research raise several questions. For one, if dyslexia involves a system-wide reduction in neural adaptation to perceptual stimuli, why does it only show up in reading? Why don’t dyslexics have the same difficulties speaking or recognizing faces and objects?

The investigators’ answer to this question is that language and vision are fundamentally important for humans. The human brain has evolved over a very long period of time to overcome deficits in these two areas in multiple ways. The same is not true for reading which is a relatively recent development in human history. Some of the neural substrates that support reading are not as robust as the substrates that underlie vision and language which makes the ability to read more fragile and more susceptible to disruption. This is a plausible hypothesis that awaits empirical support.

A more important question from the viewpoint of those with dyslexia is what causes the widespread reduction in neural adaptation to perceptual stimuli observed in these studies? If the reduction is system-wide, does it have a system-wide cause? If that cause can be identified, can it be counteracted for those who are dyslexic or who are prone to dyslexia? Stay tuned as we await the results of further research.


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