Common Cognitive Deficits in Dyslexic Students

There is now a broad consensus that human thinking, learning, and memory relies on a set of distinct, but interrelated, cognitive abilities. These abilities can be briefly summarized as: auditory processing (correctly processing the sounds of our language, including phonological awareness), visual processing, short-term memory and working memory (including executive attentional skills), long-term memory (placing information in and retrieving it from long-term memory), acculturation knowledge (knowledge of the language, concepts, and information of our culture), fluid reasoning (problem solving and reasoning with unfamiliar information), processing speed (speed of thinking ability on simple visual or auditory tasks), and quantitative knowledge (understanding and applying math skills and concepts). Strengths and weaknesses in these eight cognitive abilities affect the quality and rate of an individual’s learning.
Phonological processing is widely accepted as the core cognitive process underlying most dyslexic students’ reading and writing difficulties. Much research has been published about identifying and remediating phonological processing deficits. Many dyslexic students also present with significant deficits in other basic cognitive processes that are distinct from, but related to, phonological processing. It is important to note that dyslexia is a heterogeneous disorder and numerous studies have been conducted to identify subtype profiles within the heterogeneity of the disorder as a whole. In my practice as an educational diagnostician I conduct evaluations designed to supply information about dyslexic students’ individual profiles of basic cognitive processes, in order to recommend appropriate interventions. I would like to focus this discussion on the inter-relationship between phonological/auditory processing, verbal working memory, processing speed, long-term retrieval (specifically rapid automatic naming or RAN), and executive attentional skills. Most dyslexic students present with deficits in one, many, or all of these areas. Verbal working memory, executive attention, and processing speed are all known to decline with age (beginning approximately in the thirties), making awareness of these possibly comorbid deficits even more germane to the adult literacy community.
Here is one brief explanation of how deficits in those basic cognitive functions inter-relate and contribute to dyslexia. When reading unknown words, slow (non-automatic) retrieval of letter/sound associations from long-term memory negatively affects working memory. Verbal working memory is a limited capacity, time-dependent cognitive process. If information (letters, sounds, and words) is being supplied to working memory too slowly (or in a degraded form) due to phonological processing deficits and/or processing speed deficits, there is some chance that the first letters/sounds or words to arrive in working memory have begun to fade by the time the last letters in that sequence have arrived. Information that has fallen apart (been partially forgotten) in working memory is eventually stored in long-term memory, and information stored in a degraded form is harder to recall. Verbal working memory is also highly dependent upon adequate attentional skills. When a reader is attempting to read, and their attention is inappropriately diverted by irrelevant information (including anxiety), the pertinent information in working memory is forgotten. Working memory contains a limited number of “slots”, and individuals with weak attentional skills fill some of their slots with non-pertinent information. The incorrect or incomplete information encoded in their long-term memory slows down processing and makes long-term memory encoding and retrieval (RAN) more difficult. Slow processing speed can make it more difficult to recall even high quality information from long-term memory.
Marilyn Adams indicates in her seminal work, Beginning to Read, that the development of a functional sight word vocabulary (words recognized instantly on sight without effortful decoding) is dependent upon building mental inter-letter association networks. Letters commonly seen together begin to share neural activation energy and, after sufficient, accurate practice, the sight of the first letter(s) in the common string of letters will automatically activate the other letters. Dyslexic students don’t perceive the adjacent letters quickly enough in sequence to build this shared activation energy (due to phonological processing deficits, processing speed deficits, attentional deficits, RAN deficits, etc.). By the time the second letter has been identified, the activation energy from the first letter has already faded, so no inter-letter association can form. Without the inter-letter associations decoding proceeds letter-by-letter, which is too slow to be maintained in verbal working memory, and greatly slows the growth of a sight vocabulary. Simultaneous processing (figuring out the letter/sound) and storage (remembering the previous letters already identified) significantly taxes the working memories of students with verbal working memory deficits.
This lack of automaticity in word reading then translates up the food chain to comprehension. When decoded words are supplied to verbal working memory too slowly, they begin to be forgotten, and building meaning from incomplete information is difficult. Forgetting in working memory also occurs due to weak attentional skills (inhibiting irrelevant information), and RAN deficits, which cause slow retrieval from long-term memory.
Most dyslexic readers are born with a core deficit in phonological/auditory processing, but some then layer on verbal working memory, attentional, RAN, or processing speed deficits, along with emotional interference as their reading failure experiences accumulate. Appropriate intervention is informed by a well-interpreted profile of strengths and weaknesses in basic cognitive processes. With that information differentiated interventions can be designed, implemented, and monitored.