Abstract
Most people are right-handed and left-cerebrally dominant for speech, leading historically to the general notion of left-hemispheric dominance, and more recently to genetic models proposing a single lateralizing gene. This hypothetical gene can account for higher incidence of right-handers in those with left cerebral dominance for speech. It remains unclear how this dominance relates to the right-cerebral dominance for some nonverbal functions such as spatial or emotional processing. Here we use functional magnetic resonance imaging with a sample of 155 subjects to measure asymmetrical activation induced by speech production in the frontal lobes, by face processing in the temporal lobes, and by spatial processing in the parietal lobes. Left-frontal, right-temporal, and right-parietal dominance were all intercorrelated, suggesting that right-cerebral biases may be at least in part complementary to the left-hemispheric dominance for language. However, handedness and parietal asymmetry for spatial processing were uncorrelated, implying independent lateralizing processes, one producing a leftward bias most closely associated with handedness, and the other a rightward bias most closely associated with spatial attention.
Highlights
Since the 1860s, when Broca discovered left-cerebral control of speech [1], the left hemisphere has been regarded as dominant, explaining the fact that most people are right-handed
It is known that most people are right-cerebrally dominant for some nonverbal functions, such as spatial attention and the processing of faces [4], but less is known about how such asymmetries relate to handedness and lateralization of speech
Indices range from 21 to +1, with extremes representing complete lateralization to the right and left, respectively. This weighted mean index was calculated for three regions of interest (ROIs) that were pre-defined in the LI toolbox [21]; the ROIs were the frontal lobes for word generation, the temporal lobe for the faces task, and the parietal lobes for the landmark task
Summary
Since the 1860s, when Broca discovered left-cerebral control of speech [1], the left hemisphere has been regarded as dominant, explaining the fact that most people are right-handed. It is known that most people are right-cerebrally dominant for some nonverbal functions, such as spatial attention and the processing of faces [4], but less is known about how such asymmetries relate to handedness and lateralization of speech. Face recognition networks are largely bilateral [17], face processing is considered to be more right-hemisphere dominant, based on deficits in face recognition following unilateral right hemisphere lesion [18] and on brain imaging [19]. To our knowledge, this is the first fMRI study to address the question of how left-hemispheric and right-hemispheric asymmetries, and handedness, might be related
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