Language dominance and factors that influence language
lateralization were investigated in right-handed, neurologically
normal subjects (n = 100) and right-handed epilepsy patients (n = 50)
using functional MRI. Increases in blood oxygenation-dependent signal
during a semantic language activation task relative to a
non-linguistic, auditory discrimination task provided an index of
language system lateralization. As expected, the majority of both
groups showed left hemisphere dominance, although a continuum of
activation asymmetry was evident, with nearly all subjects showing
some degree of right hemisphere activation. Using a categorical
dominance classification, 94% of the normal subjects were considered
left hemisphere dominant and 6% had bilateral, roughly symmetric
language representation. None of the normal subjects had rightward
dominance. There was greater variability of language dominance in the
epilepsy group, with 78% showing left hemisphere dominance, 16%
showing a symmetric pattern and 6% showing right hemisphere
dominance. Atypical language dominance in the epilepsy group was
associated with an earlier age of brain injury and with weaker right
hand dominance. Language lateralization in the normal group was weakly
related to age, but was not significantly related to sex, education,
task performance or familial left-handedness.