Mixed dyslexia, also known as double-deficit dyslexia, is a severe form of dyslexia caused by deficits in phonological processing and visual naming speed.
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Table of contents:
Overview
Dyslexia is a learning disability. People with dyslexia have trouble reading at a good pace and without mistakes. They may also have difficulties with reading comprehension, spelling, and writing.
As with most disorders, dyslexia can vary in severity and there are various types. Children with phonological deficits typically have phonological dyslexia, while children with visual naming speed deficits typically have orthographic dyslexia. Synonyms for phonological dyslexia are auditory dyslexia and dysphonetic dyslexia. Synonyms for orthographic dyslexia are surface dyslexia, visual dyslexia, and dyseidetic dyslexia.
With phonological dyslexia, a person has difficulty associating sounds with letters or groups of letters; with orthographic dyslexia, a person has difficulty recognizing familiar words. Children with mixed dyslexia have both dyslexia types and, therefore, struggle with decoding and fluency, making reading and spelling extremely challenging. Most of them read at the bottom five percentiles on standardized measures.
Indicators of mixed dyslexia
Children with phonological dyslexia have trouble breaking words into syllables and smaller sound units called phonemes. For example, if one says a word out loud to a child with weak phonemic skills, they can hear it just fine and repeat it back. However, they have trouble telling how to split it into the different sounds that make up this word. Difficulties in this area can make it challenging for readers to match phonemes with their written symbols (graphemes), which makes it hard to sound out or “decode” words.
The most typical indicator of a disabled reader’s impaired phonological-based decoding abilities is a severely reduced ability to decode nonsense words. Nonsense words are made-up words one will not find in an English dictionary. Words like fim or stap. The idea is for the child to apply their grapheme-phoneme correspondence rules in a context-free situation. The word ckad, for example, would not be used as a nonsense word, as ‘ck’ is never found at the start of a word in English.
Children with orthographic dyslexia, on the other hand, struggle to recognize words by sight. Sight-reading is an essential skill for a couple of reasons. One is that some words have tricky spellings. For example, words like weight and debt can’t be sounded out — readers need to memorize them. The other reason has to do with reading fluency. To read quickly and accurately, kids need to recognize many common words at a glance — without sounding them out (Newcombe & Marshall, chap. 2, 2017).
A typical indicator of orthographic dyslexia is a deficit in visual naming speed. The best-known measure of visual naming speed is the rapid automatized naming (RAN) test, designed by Denckla (1972) and developed by Denckla and Rudel. This test involves rapidly naming a visual array of 50 stimuli consisting of five symbols in a given category (e.g., letters, numbers, colors, or objects) presented ten times in random order (Wolf & Bowers, 1999).
Rapid letter naming requires attention to the letter stimulus and visual processes such as visual discrimination, pattern identification, integration of pattern identification with stored orthographic and phonological representations, and retrieval from long-term memory (Wolf & Bowers, 1999).
Naming-speed deficits have been demonstrated in impaired readers across several language systems, such as German, Dutch, Finnish, and Spanish. In addition, naming speed plays a vital role in learning to read regardless of whether the sample subjects used an alphabetic or non-alphabetic writing system, such as Chinese or Japanese (Araújo et al., 2015).
Children with mixed dyslexia have both dyslexia types and, therefore, struggle with decoding and fluency, making reading and spelling extremely challenging. Most of them read at the bottom five percentiles on standardized measures.
Causes of mixed dyslexia
Deficits in phonological processing and visual naming speed are not symptoms of dyslexia; instead, they are cognitive deficits that cause dyslexia. In addition to phonological processing and visual naming speed, cognitive psychology has now linked many other cognitive skills to dyslexia, including processing speed, working memory, and visual long-term memory for details.
Brain studies have linked especially two brain areas to dyslexia: the left occipitotemporal cortex (shown in yellow) and the left inferior parietal lobe (shown in red). The left occipitotemporal cortex includes the so-called visual word form area (VWFA), thought critical for reading, and is most likely the brain area that does not work well in pure orthographic dyslexia.
Neuroscientists at Georgetown University Medical Center concluded that skilled readers can recognize words at a lightning-fast speed when they read because the words have been placed in a sort of visual dictionary. This part of the brain, the VWFA, functions separately from an area that processes the sounds of written words (Glezer et al., 2016).
In the most extensive study of its kind up to date, Brem et al. (2020) confirmed that the VWFA is vital to fluent reading and that this brain region should be a prime target for supportive interventions.
The left inferior parietal lobe is said to be involved in word analysis, grapheme-to-phoneme conversion, and general phonological and semantic processing. It is most likely the brain region that does not work well in pure phonological dyslexia.
With mixed dyslexia, both the left occipitotemporal cortex and the left inferior parietal lobe are likely defective.
Imaging also reveals compensatory overactivation in other parts of the reading system (shown in green). The compensatory neural systems allow a dyslexic person to read more accurately. However, the critical visual word-form area remains disrupted, and difficulties with rapid, fluent, automatic reading persist. The person with dyslexia continues to read slowly (Shaywitz, 2005).
Interventions for mixed dyslexia
An important aspect of mixed dyslexia concerns intervention. Since multiple cognitive skills underlie reading, intervention programs that target numerous cognitive skills are a better solution than programs targeting cognitive skills selectively. Edublox’s dyslexia interventions thus aim to develop multiple cognitive skills.
Also, while readers with phonological deficits benefit most from phonological-based interventions, students with naming-speed deficits and mixed dyslexia will not. Edublox’s dyslexia interventions thus target the left parietal lobe (the brain region involved in phonological processing) as well as the VWFA (the brain region involved in fluency).
Below is a case study of a student with mixed dyslexia, whose reading improved by 54 percentiles according to the STAR Reading Test.
A mixed dyslexia case study
Maddie had mixed dyslexia. She showed many signs and symptoms of both phonological and orthographic dyslexia. She scored in the 1st percentile for reading—the lowest percentile—meaning that 99 percent of children her age were reading better than her. She scored in the 6th percentile for phonological awareness, in the 16th for phonological memory, and below the 1st for rapid naming.
People who evaluated Maddie were astounded by the severity of her dyslexia. More than one professional had told Maddie’s parents that she would probably never read. However, her mother, refusing to accept this, embarked on a tireless journey to find help for Maddie.
After many avenues had failed, Maddie started a nine-month journey with Edublox. A reassessment by her psychologist saw improvements in phonological awareness (from the 6th to the 39th percentile), phonological memory (from the 16th to the 30th percentile), and rapid naming (from below the 1st to the 8th percentile). According to the STAR Reading Test, her reading improved by 54 percentiles. Watch the video of their journey below.