Parallels between asymmetries of Planum temporale and of hand skill

Relationships between the skill of each hand and differences between the hands are described for a sample of 14-15-year-olds representative of the general population. The description is modelled on the account of Galaburda et al. (Neuropsychologia 25, 853-868, 1987) of relationships between asymmetries of the Planum Temporale (PT) and measures of PT area on each side, in order to show parallels between findings for the two types of asymmetry. For both PT and hand skill, the extent of asymmetry appears to depend on the greater variability of the weaker side rather than of the larger or "better" side. The findings are also described as a replication of the study of hand skill in children by Annett and Manning (Br. J. Psychol. 80, 213-226, 1989). As no score transformations were needed in the present sample, the grounds for doubt raised by Bishop (Handedness and Developmental Disorder, Blackwell, Oxford, 1990) about the earlier sample are removed. Repetition of the analyses for right-handed and left-handed writers separately demonstrates very close similarities for findings in the two hand preference groups. Theories about the mechanisms responsible for asymmetries appear to require the presence of factors which reduce brain area and hand skill on one side. For most people the reduction is to the right PT and to the left hand.