Our knowledge about the variability of cerebral language lateralization is
derived from studies of patients: with brain lesions and thus possible seco
ndary reorganization of cerebral functions. In healthy right-handed subject
s 'atypical', i.e, right hemisphere language dominance, has generally been
assumed to be exceedingly rare. To test this assumption we measured languag
e lateralization in 188 healthy subjects with moderate and strong right-han
dedness (59% females) by a new non-invasive, quantitative technique previou
sly validated by direct comparison with the intracarotid amobarbital proced
ure, During a word generation task the averaged hemispheric perfusion diffe
rences within the territories of the middle cerebral arteries were determin
ed. (i) The natural distribution of language lateralization was found to oc
cur along a bimodal continuum. (ii) Lateralization was equivalent in men an
d women,. (iii) Right hemisphere dominance was found in 7.5% of subjects, T
hese findings indicate that atypical language dominance in healthy right-ha
nded subjects of either sex is considerably more common than previously sus
pected.