On the relationship of hemispheric specialization and developmental dyslexia

Does dyslexia develop from learning the alphabet in the wrong hemisphere? A cognitive neuroscience analysis

A new perspective is described which views developmental dyslexia as the outcome of learning to write the alphabet in the nondominant (right) hemisphere. The letter-level and whole-word subtypes of dyslexia are seen as differing responses adopted to cope with this predicament. Striking similarities between dyslexics and callosotomy patients in the allocation of covert attention to lateralized stimuli provide direction for integrating a diversity of dyslexic research within this framework. This synthesis, together with information from pure alexia, brain activation, and reading research, lends insight into the neural circuitry of the compensatory strategies adopted by the two dyslexic subtypes.

The relationship of reading disability and the techniques of teaching reading

Lateral Eye Movements were found to be a marker variable for two types of reading disability that can be related to specific stages of the way reading is usually taught. Current techniques used in the teaching of reading rely on the phonological route to meaning: if a child can "auditorize" a word, its meaning can be derived through auditory comprehension. This study has isolated two groups of disabled readers who are deficient in distinct aspects of this phonological route. Approximately 85% of disabled readers have a problem with the first step; they have difficulty converting the written word into its phonological counterpart. The remaining 15% have no difficulty with this first step, but the phonological representation of a word does not lead to its meaning.

Defective lateralized attention for non-verbal sounds in developmental dyslexia

The performance of dyslexic and control children--matched for age, sex, handedness and performance IQ--was compared on a dichotic listening task in which the stimuli were environmental sounds. Two findings emerged. First, the dyslexic group showed a normal, left ear perceptual advantage for these non-verbal materials. Second, the dyslexics were incapable of occluding dichotically presented sounds to the left ear when required to deploy attention only to sounds entering the right ear. This failure of unilateral selective attention was not found in the controls, nor in either group under a condition of left ear only attention. The data were interpreted as undermining the bihemispheric non-verbal processing hypothesis of developmental dyslexia. It was argued that the dyslexics' apparent inability to override voluntarily the reflexive lateral orienting mechanism was most consistent with explanatory models emphasizing defective functioning, or even malformation, of the left cerebral hemisphere alone.

Lateral asymmetry in subgroups of dyslexic children

It has been proposed that in examining hemispheric dominance in dyslexics, investigators must make a distinction between subtypes of the disorder. Using the Denver Reading and Spelling Test, 72 dyslexics were divided into three groups: dysphonetics, dyseidetics, and nonspecifics. Three measures of hemispheric dominance were administered: a dichotic listening test, a hemiretinal test with linguistic stimuli, and a hemiretinal test with spatial stimuli. Results indicated that the three subgroups of dyslexics did not differ from one another on measures of hemispheric dominance. Dyslexics, taken as a group, tended to show an exaggerated right ear advantage on the dichotic listening test, in comparison with that of controls. It was proposed that the dichotic listening test may be confounded by attentional deficits in the dyslexics.

Lateral preference and skill in dyslexics: implications of the right shift theory

The lateral preferences and L-R skill of 109 male and 20 female dyslexics were as expected if the distribution of lateral asymmetry is shifted less far to the right in dyslexics than in controls. Several aspects of the data were consistent with Annett's hypothesis that some dyslexics lack the left hemisphere speech-organising factor postulated by the right shift theory of handedness and that this would be sufficient to account for the proportion of affected relatives. Some dyslexics were strongly dextral and these differed from the less dextral cases in several ways which resembled the distinction between backward and retarded readers.

Patterns of inverted reading and subgroups in dyslexia

Reading performance of 46 poor readers was compared with that of 20 normal control readers. All subjects were second grade children. In Experiment 1 two matching word lists were presented under two conditions: one version of the test was read in the upright position and the other inverted. In Experiment 2 the eye movements of all subjects were recorded during reading of two meaningful sentences in the normal and inverted position. While the controls were negatively influenced by inversion of the text, the poor readers showed a variety of responses. Overall, the poor readers showed a slight tendency to be better at reading in the inverted position when the text must be scanned from right to left. An individual analysis of the data revealed that in 28.3% of the poor readers inverted reading improved performance at least 15 %, a phenomenon found in none of the controls.

A longitudinal study of the development of visual field advantage for letter matching

The visual field advantage for letter matching was investigated longitudinally from 5 to 7 yr. A right visual advantage was found with very little indication of any increase with age. Cross-sectional controls exhibited somewhat less of a right visual field advantage. In a group of 4-yr-olds who could not name letters a left visual field advantage was observed for boys. It is suggested that the right visual field advantage for visual-verbal material occurs from a mapping on to an already existing left hemisphere language system.