The neuropsychology of selective attention difficulties in dyslexi

The study tested four models of maladaptive cerebral lateralization in dyslexia: Poor Structural Lateralization, and three attentional models--Right Hemisphere Excessive Activation, Left Hemisphere Excessive Activation and Bidirectional Excessive Hemisphere Activation. These models were investigated using a directed attention dichotic listening paradigm. One experiment used strings of digit name stimuli and a second experiment used consonant vowel nonsense syllables.;Thirty right handed auditory-linguistic dyslexic boys were chosen using a severe deficiency in pseudoword decoding as the main screening criterion. They were matched by age and Performance IQ with 30 normally achieving right handed boys and by reading comprehension with 30 younger normally achieving right handed boys.;The results of the two experiments taken together did not support the Poor Structural Lateralization Model. Previous findings with digit name stimuli showing poorer results for dyslexics at the left channel reflective of right hemisphere overactivation were confirmed and found to be linked to the main word attack disability. Earlier results with consonant-vowels indicating increased but abnormal ability for dyslexics at the left channel were also supported. Normally developing right hemisphere selective activation ability and inhibition of left hemisphere activation was associated with better word decoding skill. Paradoxical findings with digits and CVs were discussed and a range of theoretical possibilities were outlined through a consideration of the different attentional processing demands of the two types of stimuli.;The strong left channel findings of both experiments were consistent with the Right Hemisphere Excessive Activation Model. However, right channel results were more congruous with the Left Hemisphere Excessive Activation Model. The poor phonemic ability of the dyslexics may possibly be linked to right and left anterior hemispheric overactivation. The lower Verbal IQ ability of the dyslexics and relatively higher Performance IQ for those Dyslexics with large Discrepancies seemed related to left and right posterior hemispheric overactivation respectively. These results taken together reservedly supported the Bidirectional Excessive Hemisphere Activation Model. The findings implicate a deficiency in selective attention combined with a disruption of interhemispheric shifting of attention as a main correlate of dyslexia.