Dyslexia linked to shorter memory trace of previous stimuli

Researchers have provided new insight into the brain mechanisms underlying a condition that causes reading and writing difficulties.

Humans have a type of long-term memory (called ‘implicit memory’) that means we respond less to stimuli as they are repeated over time, in a process called neural adaptation. But the new research suggests that dyslexics recover faster than non-dyslexics from their responses to stimuli such as sounds and written words, leading to their perceptual and reading difficulties. The discovery could pave the way for earlier diagnosis and intervention of the condition.

Dyslexia is a common learning difficulty that affects one in every 10 to 20 people in the UK alone, impacting their ability to read and spell words but not affecting their general intelligence. Researchers from the Hebrew University of Jerusalem, led by Professor Merav Ahissar of the Psychology Department and The Edmond & Lily Safra Center for Brain Sciences, decided to carry out a number of experiments with dyslexics and non-dyslexics to shine new light on the mechanisms behind this condition.

“While dyslexics are mainly diagnosed according to their reading difficulty, they also differ from non-dyslexics in performing simple perceptual tasks, such as tone-frequency discrimination,” says first author Sagi Jaffe-Dax.

“Our lab previously found that this is due to ‘poor anchoring’, where dyslexics have an inefficient integration of information from recent stimuli, collected as implicit memory. This memory typically forms ‘anchors’ that provide specific predictions that clarify noisy stimuli, and we wanted to see why this is not the case in dyslexics,” says Ahissar.

In the current study, the team gave 60 native Hebrew speakers, including 30 dyslexics and 30 non-dyslexics, frequency discrimination and oral reading tasks. During the frequency-discrimination task, participants were asked to compare two tones in each trial. All participants’ responses were affected, or biased, by implicit memory of previous stimuli. Both groups were affected in similar ways by very recent stimuli, but dyslexics were less affected by earlier stimuli.

“This suggests that implicit memory decays faster among dyslexics,” says Jaffe-Dax. “We decided to test this hypothesis by increasing the length of time between consecutive stimuli and measuring how it affects behavioral biases and neural responses from the auditory cortex, a section of the brain that processes sound.

“Participants with dyslexia showed a faster decay of implicit memory on both measures. This also affected their oral reading rate, which decreased faster as a result of the time interval between reading the same nonword — a group of letters that looks or sounds like a word — numerous times.”

The team concludes that dyslexics’ faster recovery from stimuli can account for their longer reading times, as it causes less reliable predictions for both simple and complex stimuli.

Co-author Orr Frenkel explains: “The formation of adequate predictions is crucial for becoming an expert in general, and an expert reader in particular. Achieving this depends on matching printed words with predictions based on previous encounters with related words, but such predictions are less accurate in dyslexics.

“However, while shorter implicit memory means they are unable to yield efficient predictions, it may be advantageous with unexpected stimuli, such as novel events in a sequence of predictable, familiar events. Further studies will be needed if we are to establish whether this is indeed the case.”

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Dyslexics show a difference in sensory processing

Neuroscientists at MIT and Boston University have discovered that a basic mechanism underlying sensory perception is deficient in individuals with dyslexia, according to study published December 21 in Neuron. The brain typically adapts rapidly to sensory input, such as the sound of a person’s voice or images of faces and objects, as a way to make processing more efficient. But for individuals with dyslexia, the researchers found that adaptation was on average about half that of those without the disorder.

The difference may explain some of the challenges people with dyslexia experience, such as discerning speech in a noisy environment and learning to read. “Adaptation is something the brain does to help make hard tasks easier,” says first author Tyler Perrachione, assistant professor of Speech, Language and Hearing Sciences at Boston University, who completed this research as a graduate student and post-doctoral fellow at MIT. “Dyslexics are not getting this advantage.”

Perrachione, who has a background in linguistics, wanted to investigate the theory that reading difficulties in dyslexia come from difficulties in associating sounds with written words. Working in the lab of lead investigator John Gabrieli, professor of Brain and Cognitive Science at MIT, he decided to investigate early, fundamental processes in the brain that could make this association difficult. “Part of the mystery of dyslexia is that the brain doesn’t have an area that evolved for reading,” says Gabrieli.

They zoomed in on the process of rapid neural adaptation. The researchers used functional magnetic resonance imaging (fMRI) to examine the brains of adults with and without dyslexia as they listened to voices. In some cases, the same voice spoke a series of words; in others, different voices spoke each word.

Brains typically adapt to a single, consistent voice within a second or two, but they don’t adapt to many different voices. As brains adapt, the fMRI measures of brain activity in relevant brain areas drop.

Individuals without dyslexia adapted to a consistent voice and not to multiple voices. But for dyslexics, brain activity remained high in both cases, suggesting that they did not adapt as much. Dyslexics with better reading skills showed greater adaptation levels. “Brains typically tune in and figure out what is consistent about a voice,” says Perrachione. “We saw much less adaptation in those with dyslexia group compared to typical readers.”

These results raised questions, since difficulty understanding speech is not seen in dyslexia. “If you were to talk to someone on the street, you’d have no idea if they were dyslexic or not,” says Perrachione.

So Perrachione and Gabrieli decided to look at adaption to visual stimuli, too. They recruited another group of individuals with and without dyslexia and examined adaptation to images of written words, faces, and objects, either in a series of different images or repeated images. Again they saw much less adaptation in participants with dyslexia.

The reduced adaptation was observed in the regions of the brain responsible for processing the stimuli in question. “This suggests that adaptation deficits in dyslexia are general, across the whole brain,” says Perrachione.

They repeated this experiment with yet another group of individuals, this time focusing on children aged 6 to 9 with and without dyslexia. The results were the same. Overall, the study involved over 150 individuals, and dyslexics on average had adaptation levels about half those of typical readers. “I am surprised by the magnitude of the difference,” says Perrachione. “In people without dyslexia, we always see adaptation, but in the dyslexics, the lack of adaptation was often really pronounced.”

Perrachione and Gabrieli speculate that dyslexics don’t struggle with processing of heard speech or seen objects and faces because human brains evolved to process these inputs. The systems that perform this processing are likely very robust. “The brain devotes a lot of infrastructure to solving these problems, and has multiple routes,” says Perrachione. “Adaptation is just one of the things that helps take the load off.”

But reading is a different story. It is a learned skill that requires multiple regions of the brain to work together, potentially with the harmony and complexity of a Rube Goldberg machine, says Perrachione. As rapid neural adaptation deficits simultaneously affect auditory and visual processing during reading, they may compound to make reading very difficult. “We have to see letters, map them onto words, map those to sounds, and connect them to semantics,” says Perrachione. “There are lots of places for things to go wrong.”

It isn’t known yet exactly where things do go wrong as a result of deficits in rapid neural adaptation. “This study presents strong evidence for a foundational brain difference in dyslexia, but it isn’t clear how to bridge that to the specific properties of reading,” says Gabrieli. “It opens up as many questions as it answers.”

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