References

Biscaldi, M., Fischer, B., & Aiple, F. (1994). Saccadic eye movements of dyslexic and normal reading children. Perception, 23, 45-64. Boder, E. (1971). Developmental dyslexia Prevailing diagnostic concepts and a new diagnostic approach. In H. R. Myklebust (Ed.), Progress in Learning Disabilities (pp. 293-321). New York Grune and Stratton. Critchley, M. (1964). Developmental dyslexia. London Whitefriars. Eden, G. F., Stein, J. F., Wood, H. M., & Wood, F. B. (1994). Differences in eye...

The Neurobiology of Dyslexia

Visual Receptor Brain

It is now almost universally accepted (but see Ehri, 1992) that dyslexia is a constitutional condition, almost certainly genetic, rooted in the biology of the central nervous system. The question is, how do the relevant genes have the effect that they do To show that a particular gene locus is implicated in, for example, phonological awareness does not explain how the gene (or more probably genes) code for biological mechanisms which underlie this ability. One step towards solving this problem...

Dyslexia And The Thalamus

In an experiment concerned with speed discrimination, Demb, Boynton, and Heeger (1998b) found a significant positive correlation between reading rate and level of brain response recorded by fMRI in the middle temporal region of the brain and between reading comprehension and activity at a number of other sites, including the primary visual cortex (V1), the cortical area at which most visual information is first received. The finding of a group difference in brain activation as early in the...

Laterality And Dyslexia

It used to be thought that all right-handers speak with the left hemisphere, while left-handers speak with the right hemisphere. This is often referred to in the literature as Broca's rule though not in fact formulated by Broca himself (Harris, 1991). In any event, we now know that this is not true most left-handers speak with the left hemisphere. However, it is the case that relatively few right-handers have language lateralized to the right hemisphere, while a greater proportion (though still...

Heterophoria

Most people do not have a strabismus, but instead, the eyes are aligned during everyday vision. But this alignment is not a fixed feature the visual system is not like the two monocular tubes of binoculars, which are permanently aligned. Even when the eyes are normally aligned in everyday vision, their resting position is usually out of alignment and this is called a heterophoria. This is best demonstrated by a variation of the cover test described above. With a heterophoria, when one eye is...

Visual Acuity and Refractive Error Visual Acuity

Visual acuity refers to the angular subtense of the smallest size of detail that the eye can resolve. It is usually expressed as a fraction where the numerator refers to the testing distance in meters (feet in the United States). Six meters is the reference distance (20 feet in the United States), and the denominator changes proportionally according to the change in size of the lettering, while reading from the same reference distance. The easiest way to interpret a visual acuity is to divide...

Why Study Twins

Identical and fraternal twins are uniquely informative about the relative contributions from genes and environment to individual differences in reading across the normal range, and to the severe reading deficits found in dyslexia. Identical twin pairs are derived from the same sperm and egg (the egg is called a zygote, so identical twins are Monozygotic or MZ). Therefore, MZ twins share all their genes. Fraternal twins develop from two different sperm-egg fertilizations (i.e., they are...

Phonological Deficits In Dyslexia

Since the 1980s, the most widely accepted view of dyslexia has been that it can be considered part of the continuum of language disorders. However, there has been a gradual shift from the verbal deficit hypothesis to a more specific theory, that dyslexia is characterized by phonological processing difficulties. What is the difference between these views The language processing system that the child brings to the task of learning to read can be thought of as comprising different subsystems....

Issues For Phonological Representations Theory Of Dyslexia

The concept of dyslexia as a deficiency in the way in which the brain codes phonology is appealing but the idea of fuzzy phonological representations is vague. What exactly is meant by phonological representation How are phonological representations created Does each word have one (a localist account) or are these really distributed patterns of activation across a set of yet to be specified processing units We do not have answers to these questions. However, one influential theory proposes that...

The Temporal Deficit Hypothesis

If skilled motor actions and language share neural mechanisms (Tzeng & Wang, 1984) associated with the control of serial order and or precision of timing mechanisms, this might help to explain the frequent co-occurrence of phonological and motor difficulties in dyslexia. Certainly, many of the deficits in motor function seen in dyslexia, such as in maintaining asynchronous tapping rates with the two hands, might be regarded as impairments in implementing motor control commands at the right...

What Is The Cognitive Deficit In Dyslexia

At about the same time as the first epidemiological studies were being conducted, cognitive psychologists began comparing groups of normal and dyslexic readers in a range of experimental paradigms. These researchers pursued the then popular idea that dyslexia was a perceptual deficit and studies investigated visual perception, visual memory, cross-modal transfer between visual and verbal codes and perceptual learning, and other skills in relation to reading ability. These early studies were...

Morphology Of The Corpus Callosum In Dyslexia

As a result of the behavioural findings, some investigators have compared structural aspects of the corpus callosum between dyslexics and controls. In particular, the relative size of the corpus callosum or of its sub-divisions has been compared. These studies have all been carried out in the intact brain I know of no postmortem investigations of the morphology (size and shape) of the callosum in dyslexia. The usual approach is to compare the size of the callosum between groups (controlling for...

Phonological Awareness Its Nature And Measurement

It is possible to measure phonological awareness, that is, the ability to identify, segment, and manipulate speech segments of words in a variety of ways. Adams (1990) divides phonological awareness tasks that successfully predict reading skills into four main types. First are syllable and phoneme segmentation tasks in which the child taps, counts out, or identifies the constituent syllables and or phonemes within words, for example, for the word cat, the child taps three times to indicate the...

The M And P Visual Systems

Almost all we know about the neurophysiology of vision comes from animal research but converging evidence from different sources, principally psychophysical experiments, strongly implies that the human visual system is organised in a similar manner to that of the rhesus macaque monkey. The visual system of this species is divided into two main streams the magnocellular or M pathway and the parvocellular or P pathway. Cells of the M sub-division are larger than those of the P sub-division and...

Dyslexia And Interhemispheric Transfer The Role Of The Corpus Callosum

The adult corpus callosum is a band of approximately 175 million fibres connecting the two sides of the brain. Its role as demonstrated by the classic split-brain studies is predominantly to convey information from one cerebral hemisphere to the other. If the callosum is surgically sectioned for the relief of otherwise intractable epilepsy, then information directed to only one side of the brain, which is possible by virtue of the anatomic arrangement of the major sensory pathways, remains...

Ocular Dominance

The concept of a dominant eye is simplistic and its relevance to dyslexia is probably overstated. One cause of confusion is that there are at least three different types of ocular dominance sighting, motor, and sensory ocular dominance (Evans, 2001). In a given individual, the right eye might be the dominant eye for a sighting task and the left for a motor task and it may even be the case that the right eye is the dominant eye for one motor task and the left for another motor task. It is not...

Screening And Assessment Of Phonological Skills

The foregoing discussion has highlighted the importance of prediction studies that tell us a great deal about the knowledge and skills young children bring to bear on the task of learning to read. Research-generated phonological tasks might, if given to sufficiently large samples of children, provide norms for the purposes of screening young children, or against which individual slow starter youngsters might be compared. However, whether such studies can suggest a strategy for reliably...

Higher Visual Processing

There have been many attempts to classify dyslexia, although it should be noted that one large study failed to support the concept of clearly defined subgroups (Naidoo, 1972). Nevertheless, many authors have classified dyslexia into three subgroups and a good example is the classification of Boder (1971). Her system is based upon three reading-spelling patterns. Dysphonetic dyslexia is characterized by people who read words globally, as instantaneous visual wholes from the limited sight...

Saccadic Eye Movements in Reading

During reading, the eyes constantly make a series of flick (saccadic) eye movements. The eyes typically fix on a group of letters for about 1 4 of a second while the information from these is assimilated and they then make a saccadic eye movement to the next group of letters, and so on. At the end of the line, the eyes make a large backward saccade to the beginning of the next line. Occasional smaller backward saccades, regressions, occur to aid comprehension. The control of this pattern or eye...

Genetics And The Neurobiological Basis Of Phonological Awareness And Dyslexia

There is accumulating evidence that dyslexia is an inherited condition not just the reading difficulty per se, but in particular, the underlying phonological deficit (for a fuller discussion see Chapter 2 by Olson, this volume). Results of large-scale twin studies have shown that there is greater heritability of phonologically based reading skills (e.g., as measured by a nonword reading test) than of orthographic skills (e.g., selecting the correct spelling from phonologically viable pairs,...

The Hormonal Hypothesis

In a series of influential papers Geschwind, Galaburda, and their colleagues proposed that excessively high levels of pre-natal foetal testosterone, a male sex hormone circulating during early maturation of the brain but present also in female foetuses in much smaller amounts, are associated with increased rates of left-handedness, and dyslexia. This was intended to explain not only an association between dyslexia and non-right handedness, but also the higher incidence of dyslexia and other...

Short History Of The Concept Of Dyslexia

Medical approaches to dyslexia, such as that exemplified by the work of pioneers such as Orton and Macdonald Critchley (Critchley, 1970) fell from favor mostly *A more detailed version of the argument in this chapter can be found in Snowling (2000). Margaret J. Snowling, Department of Psychology, University of York, Heslington, YorkY01 5DD, UK. The Study of Dyslexia, edited by Turner and Rack. Kluwer Academic Publishers, New York, 2004. 77 because they were difficult to put into practice. The...

The Consequences Of A Magno System Deficit

While correlations between reading and performance on movement perception tasks are interesting, they do not tell us how a magnocellular deficit might lead to difficulties in learning to read and to spell. Breitmeyer and Ganz (1976) suggested that in the brief interval between successive fixations of a visual scene, when the eyes move and vision is impaired (although we are not usually aware of this), the M pathway suppresses or masks activity in the P pathway. This would ensure that the images...

Myopia Shortsightedness

Shortsightedness (myopia) blurs distance vision (e.g., the board in class), and the prescribing of spectacles for myopia by eye care practitioners is usually a fairly straightforward decision. An optometrist might consider prescribing a refractive correction if the myopia reduced the binocular visual acuity to 6 9 and would almost certainly prescribe if it was reduced to 6 12. Low to moderate myopia will not impair near vision and some studies have even found a correlation between myopia and...

Comorbidity for Dyslexia and Attention Deficit Hyperactivity Disorder ADHD

The above bivariate genetic analyses have also been performed for reading deficits and ADHD. ADHD has commonly been diagnosed in the United States by asking teachers and or parents to rate their children on a list of symptoms for attention deficits and hyperactivity. If a sufficiently high number of symptoms are checked, the children are categorized as ADHD. Researchers in the United States have noted a high rate of ADHD among children with dyslexia, ranging from about 20 to 40 across studies....