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.