Dirk J. Bakker

Developmental parameters of the ear asymmetry: a multivariate approach.

The present paper has two objectives: (1) a critical review of dichotic listening studies in children, and (2) an investigation of developmental parameters of the ear asymmetry, based on factors revealed in the review. A dichotic listening task was administered to approximately 20 boys and 20 girls at each of five ages (5,6,7,9 and 11). The results were as follows: (1) significant ear asymmetry was not found in children younger than nine years of age; (2) the magnitude of the differences between ears, while not significant until age nine, continued to increase with age until eleven at which time the slope functions for each ear plateaued; and (3) ear asymmetry was independent of sex. An attempt is made to explain the present results, and those of previous research, within a developmental framework.

Hemispheric differences and reading strategies: Two dyslexias?

Evidence for Physiological Differences For a time, only functional right-left differences could be shown. With the introduction of modern EEG apparatus, however, the hemispheres were shown to differ physiologically; the electric potentials evoked by speech stimuli, for instance, may be different depending on the side of diversion (Molfese, Freeman, and Palermo 1975). In addition, it recendy has been shown that the two brains are anatomically different. An area called planum temporale mostly shows larger extension in the left than in the right temporal lobe, a difference which possibly underlies hemispheric asymmetry in speech reception and production (Geschwind 1972,1978).*

Ear-asymmetry with monaural stimulation: Relations to lateral dominance and lateral awareness

Abstract The relations between list length, lateral dominance and lateral awareness on the one hand and ear-asymmetry on the other hand were investigated by means of the results obtained with 175 normal and learning-disturbed children in the age groups 7–13. Digit-series and sound-patterns of various lengths were presented to each ear on separate moments. Ordered recall was required. The results showed that ear-asymmetry subsequent to monaural stimulation depends on the length of the series and that lateral dominance as well as lateral awareness are related to the phenomenon.

Effects of hemisphere-specific stimulation on brain activity and reading in dyslexics.

Dyslexic readers, classified as L-types or P-types, received direct or indirect stimulation of the right (L-types) or left (P-types) hemisphere. Direct stimulation was produced by presenting words in the left (L-types) or right (P-types) visual field. Indirect stimulation took place through the presentation of visual-perceptual (L-types) or phonetic (P-types) demanding texts. Analyses of event-related potentials (ERP), elicited by centrally presented words, revealed the component reflecting P250 activity to be asymmetrically affected by experimental vs. control treatments. Treatment effects on scholastic achievement were shown in L-type dyslexics who had received direct stimulation of their right hemisphere and in P-type dyslexics whose left hemisphere had been indirectly stimulated. Training-induced electrical changes in brain asymmetry correlated with changes in measures of reading accuracy and speed. The set of findings replicated most of the results of a previous study (Bakker, Moerland, & Goekoop-Hoefkens, 1981).

The development of lateral event-related potentials (ERPs) related to word naming: A four year longitudinal study.

Event-related potentials (ERPs) were recorded from the left and right temporal and parietal sites during a word naming task. Subjects were a group of children that were followed over four consecutive years starting at Kindergarten. ERP waveforms contained a sequence of positive and negative components (N150, P240, N360, N530 and SW). All components, except N150, showed changes in amplitude as a function of age, whereas SW, N360 and N150 also changed in hemispheric distribution. In addition, a relationship was found between reading performance and ERP amplitudes over the right parietal hemisphere in young children, and over the left temporal hemisphere in older children. Proficient readers showed larger (more negative) parietal N530 amplitudes than less proficient readers, especially when stimuli were degraded words. The results are discussed in terms of age-related changes in right and left hemisphere functions involved in early and advanced stages of reading, that might possibly be related to visual word recognition.

Neuropsychological Classification and Treatment of Dyslexia

Evidence shows that initial and advanced learning to read are predominantly mediated by the right and left cerebral hemispheres, respectively. Premature reliance on left-hemispheric reading strategies may result in L-type dyslexia, characterized by hurried and inaccurate reading. P-type dyslexia, characterized by a slow and fragmented style of reading, may result from the inability to shift from the predominant generation of right-hemispheric reading strategies to the predominant generation of left-hemispheric reading strategies. Results of investigations concerning the validity of the L/P-typology are discussed, as are the reading effects of hemisphere-specific and hemisphere-alluding stimulation in children with L- and P-type dyslexia.

Birth Stress, Autoimmune Diseases, and Handedness

A large nonclinical sample of left-handed and right-handed subjects completed laterality and medical health questionnaires. The group of left-handed subjects showed a raised incidence of birth complications and learning disorders whereas no differences were found for birth order and maternal age. Left-handed and right-handed subjects showed similar incidences of autoimmune diseases, allergies, migraine, and stuttering. These results suggest that an association between early pathology and left-handedness may be found in a subset of the nonclinical left-handed population. The underlying pathological influence seems to manifest itself in pregnancy and birth complications rather than in immune disorders.

The brain as a dependent variable

The mainstream of neuropsychological research and practice has been devoted to the impact of the brain as an independent variable on behavior as a dependent variable. Evidence is currently available to make clear that the order of causation may be reversed: Behavioral changes can have a durable impact on the brain. The results of extensive research indicate that a large number of neuroanatomical, neurophysiological, neurochemical, and neuropsychological parameters of the animal brain can be modified through environmental manipulation, sensory experience, and systematic training. Some evidence is available to show that psychological stimulation has certain effects on the physiology of the human brain. For instance, hemisphere-specific stimulation through the presentation of words flashed in a visual hemifield appears to modify the electrophysiological activity of the contralateral hemisphere in dyslexic children and to affect their subsequent reading performance. Neuropsychology may profit from paying more attention to the ecology of the human brain.

Cortical responses to word reading by right- and left-eared normal and reading disturbed children.

Abstract There is behavioral evidence that children showing left ear dominance (LED) generate hemispheric reading strategies which are different from those who show right ear dominance (RED) subsequent to dichotic verbal stimulation. Electrophysiological evidence of this was sought in the present investigation. Normal and reading-disturbed children were required to read monosyllabic words that were flashed at the point of fixation. Event-related potentials (ERP) were analysed in relation to reading level (normal vs disturbed readers), ear dominance (LED vs RED), and side of diversion (left temporal vs right temporal and left parietal vs right parietal). Significant interactions were found for P310 and N440 amplitudes and latencies, indicating that normal and reading-disturbed LED and RED children generate different reading strategies.

The reliability of dichotic ear asymmetry in normal children

Abstract Two, three and four pair dichotic digit tests were presented twice to 250 normal kindergarten and primary school children. Test-retest as well as inter-test reliability and consistency coefficients were calculated. Their values were shown to match similar indices found in normal adults.