Instability of hemispheric asymmetry in dyslexic children.

The study tested the hypothesis of abnormal brain asymmetry in dyslexic children. Two dyslexic groups classified as "phonetic disorder" and "language disorder" and a control group participated in two experiments. In both experiments was employed a dichotic listening procedure consisting in recalling pairs of words presented simultaneously to two ears. In Experiment I the children were to recognize four words presented at a level of loudness typical for natural speech. In Experiment II only two words were used in each trial but they were presented at a low intensity level. The recognition scores for stimuli presented to the left and right ears were compared. In Experiment I all groups of children showed a typical right ear/left hemisphere superiority, i.e. their recognition scores were higher for the right than for the left ear. Dyslexics, however, performed significantly less well. In Experiment II the control children and those from "phonetic disorder" group again performed better when words were presented to their right ears. Unlike those two groups, the children from "language disorder group" showed the right hemisphere superiority, i.e. they performed better in the left ear presentation condition. The results do not support the hypothesis that dyslexic children have abnormal lateralization of verbal functions. They suggest that the pattern of hemispheric asymmetry in dyslexics is less stable and depends both on the kind of dyslexia and on task variation.