Theories of dyslexia:

Introduction

It is the five senses, (sight, touch sound, smell and taste) coupled with our genetic inheritance and environmental influence in which all learning takes place.

Thinking in the visual system, uses pictures, colours, abstract plans, diagrams etc.

Thinking in the auditory system, uses sounds, conversations, rhythm, melody etc. whilst the kinaesthetic pathways record information of a feeling nature, balance, weight, temperature, emotional state, ‘gut’ feeling, intuition etc.

At the most basic level, all of these sensory inheritances affect learning outcomes.

Researchers have identified two broad categories of dyslexia, acquired and developmental and these overviews of dyslexia, for EDUVAC/The Education Weekly have been collated from a number of researched sources.

Acquired Dyslexia occurs when someone who is a good reader and speller loses some of that ability due to a stroke, or perhaps a car accident.

Developmental Dyslexia is traditionally defined as a discrepancy between reading ability and intelligence in children receiving adequate reading tuition and since this definition is entirely behavioural, it leaves open the causes for reading failure.

It is now well established that dyslexia is a neurological disorder with a genetic origin, that is, it tends to run in families, which is currently being investigated.

Testing for dyslexia often consists of the student undertaking a full battery of psychometric, phonological, auditory, visual and cerebellar tests and research shows, that the disorder has lifelong persistence, with reading retardation being merely one of its manifestations.

Beyond this consensus, and despite decades of intensive research around the world, the underlying biological and cognitive causes of the reading retardation are still hotly debated.

Three leading theories of dyslexia – Currently there are three leading theories of Developmental Dyslexia, the phonological theory, (auditory), the magnocellular (auditory and visual) theory and the cerebellar theory.

The phonological theory

The phonological theory proposes that dyslexics have a specific impairment in the representation, storage and/or retrieval of speech sounds. Test results suggest that a phonological deficit can appear in the absence of any other sensory or motor disorders and is sufficient to cause literacy impairment.

This theory helps explain the dyslexics’ reading impairment, by appealing to the fact that learning to read an alphabetic system requires learning the grapheme-phoneme correspondence, (the correspondence between letters and constituent sounds of speech).

If these sounds are poorly represented, stored or retrieved, the learning of grapheme-phoneme correspondences, (the foundation of reading for alphabetic systems), will be affected accordingly. While researchers have different views about the nature of the phonological problems, they agree on the central and causal role of phonology in dyslexia.

The phonological theory therefore postulates a straightforward link between a cognitive deficit and the behavioural problem to be explained. At the neurological level, it is usually assumed that the origin of the disorder is a congenital dysfunction of left-hemisphere brain areas underlying phonological representations, or connecting between phonological and orthographic representations.

Auditory disorders, when present, aggravate the phonological deficit, hence the literacy impairment.

Research results also suggest that a phonological deficit can appear in the absence of any other sensory or motor disorder, and is sufficient to cause literacy impairment.


The visual theory

The visual theory is another well studied area of dyslexia research. This theory considers it as a visual impairment giving rise to difficulties with the processing of letters and words on a page of text.

This may take the form of unstable binocular fixations, poor vergence and or increased visual crowding. The visual theory does not exclude a phonological deficit, but emphasises a visual contribution to reading problems in some dyslexic individuals.

At the biological level, the proposed visual dysfunction is based on the division of the visual system into two distinct pathways that have different roles and properties: the magnocellular and parvocellular pathways.

The theory postulates that the magnocellular pathway is selectively disrupted in certain dyslexic individuals, leading to deficiencies in visual processing. Evidence for magnocellular dysfunction comes from anatomical and brain imaging studies.

The magnocellular theory

The magnocellular theory, is a unifying theory that attempts to integrate all the findings mentioned above. A generalisation of the visual theory, the magnocellular theory postulates that the magnocellular dysfunction is not restricted to the visual pathways but is generalised to all modalities (visual and auditory as well as tactile).

Furthermore, as the cerebellum receives massive input from various magnocellular systems in the brain, it is also predicted to be affected by the general magnocellular defect.

Through a single biological cause, this theory therefore manages to account for all known manifestations of dyslexia: visual, auditory, tactile, motor and, consequently, phonological.

The cerebellar theory

Yet another view is represented by the automaticity/cerebellar theory of dyslexia. The theory, that faults lie in the functioning of the postural system and the vestibular-cerebellar loop, was hypothesised as early as 1970.

Here the biological claim is that the dyslexic’s cerebellum is mildly dysfunctional and that a number of cognitive difficulties ensue.

First, the cerebellum plays a role in motor control and therefore in speech articulation. It is proposed that retarded or dysfunctional articulation would lead to deficient phonological representations.

Secondly, the cerebellum plays a role in the automating of over-learned tasks, such as driving, typing and reading. A weak capacity to automate would affect, among other things, the learning of grapheme-phoneme correspondences.

Support for the cerebellar theory comes from evidence of poor performance of dyslexics in a large number of motor tasks, in dual tasks demonstrating impaired automatisation of balance, and in time estimation, a non-motor cerebellar task. Brain imaging studies have also shown anatomical, metabolic and activation differences in the cerebellum of dyslexics.

Some researchers suggest that the cerebellar theory fails to account for sensory disorders, but its proponents entertain the idea of distinct cerebellar and magnocellular dyslexia subtypes.

It also remains uncertain what proportions of dyslexics are affected by motor problems.

A number of studies have failed to find any, whilst other researchers have found motor problems only in a subgroup of dyslexics. It has also been suggested that motor dysfunction is found only in dyslexic children who also have attention-deficit hyperactivity disorder (ADHD).

An overlapping spectrum of neurodevelopmental disorders

Despite their separate diagnostic labels, the clinical overlap between dyslexia, dyspraxia, Attention Deficit Hyperactivity Disorder (ADHD) and Autistic Spectrum Disorder (ASD) is very high, and ‘pure’ cases are the exception, not the rule.

Thus around half of any dyslexic population is likely to be dyspraxic and vice versa, and the mutual overlap between ADHD and dyspraxia is also around 50 per cent.

Dyslexia and ADHD co-occur in 30-50 per cent of cases, although this association is stronger for inattention than for hyperactivity-impulsivity.

All of these conditions also show some overlap with the autistic spectrum, although in severe cases, the autism diagnosis always takes precedence.

Dyslexia, dyspraxia, ADHD and the autistic spectrum

Current practice within our education and health care systems involves separate diagnostic labels for dyslexia, dyspraxia, attention-deficit/hyperactivity disorder (ADHD) and autistic spectrum disorders (ASD).

Each refers to a specific pattern of behavioural and learning difficulties for which the core defining features are quite different.

For dyslexia these involve specific difficulties in learning to read and write; for dyspraxia, specific difficulties in the planning and coordination of movement.

For ADHD, persistent and age-inappropriate difficulties with attention, hyperactivity-impulsivity, or both; and for ASD, marked social and communication deficits and a restrictive, stereotyped range of behaviours.

These developmental conditions are remarkably common, affecting up to 20 per cent of the school age population to some degree, and they account for the vast majority of children with special educational needs. The associated difficulties usually persist into adulthood, with enormous consequences for the individuals affected, their families and society as a whole.

Dyslexics and Learning Styles

Spatial and sequential thinking are two different mental organisations that affect the way people learn and view the world. Sequential thinking is step-by-step linear thinking over time, while spatial thinking is an holistic system where all knowledge is interconnected in space, but not all dyslexics are visual spatial learners.

Auditory thinking is associated with sequential thinking and visual-spatial thinking is associated with spatial thinking.

The sequential system involves analysis, progression from simple to complex, organisation of information and linear deductive reasoning. It is influenced by hearing and language and an awareness of time.

In contrast, spatial thinking involves synthesis, an intuitive grasp of complex systems, (often missing the steps) simultaneous processing of concepts, inductive reasoning (from the whole to the parts).

It also uses imagination and generation of ideas by combining existing facts in new ways (creative thinking). It is influenced by visualisation and images and an awareness of space.

Having grasped the concepts, repetition appears to be completely unnecessary to visual spatial learners and irrelevant to their learning style. Visual thinkers and learners can literally see pictures in their heads while auditory thinkers and learners hear streams of words.

Everyone has a thinking and learning style: intrinsic information-processing patterns that represent a person’s typical mode of perceiving, thinking, remembering, and problem-solving.

The challenge for educators today is to assess the thinking and learning style characteristics of all of their students, not only their gifted students, and to provide teaching that is compatible with those characteristics.

When educators recognise and accommodate the preferred learning style of their students, improved attitudes toward learning and an increase in productivity, academic achievement, and creativity can result. 


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