Use and understanding of spoken language is largely orchestrated by the brain’s left side. However, a team of neuroscientists now suggests that a right-brain area assumes a critical role in deciphering sign language, at least among native signers.
When viewing signed sentences of a particular sign language, such as American sign language, 16 adults who grew up signing in that language displayed unique activation of the right angular gyrus, say Aaron J. Newman of the University of Oregon in Eugene and his colleagues. This brain region has already been implicated in the perception of bodily motions and spatial information.
The conveyance of meaning in various sign languages depends heavily on hand, arm, and facial movements. “We suspect that the right angular gyrus is recruited into a neural system for grammar comprehension in native signers,” Newman says.
This right-brain connection to sign language appears to be cinched during childhood, the scientists argue in an upcoming Nature Neuroscience. The right angular gyrus showed no language-related activity in 11 individuals who had attained sign language proficiency as young adults, the researchers report.
Newman’s group used functional magnetic resonance imaging to compare blood flow in volunteers’ brains as they read English sentences and random strings of consonants, and as they watched the presentation of sentences and nonsense signs in American Sign Language. Only those who learned sign language as children showed increased blood flow in the right angular gyrus.
The new data build on preliminary evidence of a right-brain contribution to sign language (SN: 11/23/96, p. 326). Still, the theory that the right angular gyrus influences grammar comprehension in native signers remains controversial.
“It’s hard to know what this brain area is really doing, based on the imaging data,” remarks Gregory Hickok of the University of California, Irvine. In his view, surges of activity in the right angular gyrus of native signers may occur because this area integrates various lines of linguistic information, including grammar, word meanings, and emotional tone. Further research needs to explore whether similar right-brain activity accompanies speech use and comprehension, Hickok says.
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Brain-damaged patients provide contrasting clues to the biology of language, Hickok notes. For instance, right-brain injuries usually spare language skills in fluent signers. But some studies suggest that damage to the right angular gyrus and nearby tissue weakens the grasp of complex sentences presented either in writing to hearing patients or as signs to deaf ones.