The communication between left and right sides of the brain is key for the development of advanced language skills, according to new research by University of California, San Francisco scientists. In most people, areas related to language processing are located in the brain’s left hemisphere.
Autism-like problems with language comprehension seen in people with defective connections between the two hemispheres may occur, the researchers suggest, because these individuals tend to use the right side of the brain to process language instead of the left.
The findings are the result of the first large-scale study of rare individuals born missing key connections between the brain’s left and right hemispheres. The work suggests that the left-hemisphere’s normal specialization for language processing may be more important for the development language skills than was previously realized.
Led by UCSF neurologist Elliott Sherr, MD, PhD, and UCSF neuroscientist Srikantan Nagarajan, PhD, the study focused on the role of the corpus callosum, a tight bundle of nerves containing as much as 200 million signaling fibers that transmit information between the left and right sides of the brain.
To see whether the loss of the corpus callosum affected which hemisphere was dominant for language, the UCSF team tracked brain activity in people lacking a normal corpus callosum by measuring magnetic fields in while administering linguistic tests that required participants to name verbs and pictures.
The researchers found that in contrast to 21 normal control subjects, who relied on the left hemisphere for key aspects of these tasks, 13 research subjects born completely lacking a corpus callosum were more likely to use the right hemisphere, while 12 subjects born with only a partial corpus callosum fell squarely between the two.
According to Sherr, a professor in the Departments of Neurology and Pediatrics at UCSF,
“This study is the first real evidence that one of the functions of the corpus callosum is to establish laterality between the two halves of the brain, which is most evident in the way that you see language form. This has broad implications for understanding how the brain is organized during development.
Our study also showed that having speech localization on the right side of the brain was associated with lower performance on verbal tasks, as measured by the verbal IQ,” he said.
Autism-like Language Delay
As a pediatric neurologist, Sherr has for more than a decade treated and studied children born without a corpus callosum, a condition that affects between one-in-three-thousand and one-in-four-thousand newborns. The corpus callosum normally develops between the 10th and 14th weeks of gestation, and its failure to develop, a condition called agenesis, can be routinely detected during prenatal ultrasound examination.
“Almost all who are born with this condition, even the highest functioning, have delays in language acquisition, although most of them learn to speak eventually,” Sherr said.
Agenesis of the corpus callosum also is associated with intellectual disability and autism-like social and cognitive deficits. Sherr counsels parents and treats newborns and children with the condition. He also has led research studies that have implicated mutations in single genes as potential causes of agenesis.
Some neurologists are beginning to clinically manipulate brain signaling between the two hemispheres to treat stroke patients, Sherr said, but it is currently unclear how brain stimulation could be used to mimic actions of the corpus callosum to stimulate normal language development.
The researchers also noted an interesting association between language localization in the brain and right- or left-handededness, though the research does not specifically address this connection.
“In almost all right handed individuals language is localized to the left hemisphere, as it is in a smaller majority of left-handed people.” Sherr said.
Research suggests that many left-handed individuals with language localized to the right side of brain are developmentally normal, he said, but some of the right-side language localization in left-handers may be associated developmental difficulties.
L. B. N. Hinkley et al. The Contribution of the Corpus Callosum to Language Lateralization Journal of Neuroscience (2016). DOI: 10.1523/JNEUROSCI.3850-14.2016
K. M. Jasmin et al. Cohesion and Joint Speech: Right Hemisphere Contributions to Synchronized Vocal Production Journal of Neuroscience (2016). DOI: 10.1523/JNEUROSCI.4075-15.2016