The brain is divided into two halves that are famously specialized in different ways — for instance, the right side of the brain controls most of the left side of the body and vice versa. Now findings detailed this week in the Proceedings of the National Academy of Sciences reveal another fundamental difference between the brain’s halves — they interact with each other differently, with left-side regions biased to interact more strongly with the same hemisphere, while right-side areas interact more strongly with both hemispheres.
The specialization the different halves of the brain practices, known as lateralization, goes beyond which hand a person favors using. For instance, the left hemisphere is often biased toward language while the right hemisphere often favors processing of visual stimuli.
Scientists have long suggested lateralization results in enhanced mental performance by efficiently dividing up labor. However, many details on the degree of lateralization each region of the brain experiences and the effects this might have on function were murky.
To learn more, neuroscientists Stephen Gotts and Hang Joon Jo at the National Institute of Mental Health in Bethesda and their colleagues used functional magnetic resonance imaging (fMRI) to analyze 62 right-handed male volunteers. Instead of using a specific task during fMRI that would engage only a few brain regions, they measured slow fluctuations in activity that rippled spontaneously throughout the brain while the participants were at rest. The aim was to more comprehensively evaluate lateralization over the entire cerebral cortex.
“Previous functional imaging studies have been hampered by two basic limitations,” Gotts says. “First, the measurement of lateralization was wholly dependent on task performance. As a result, assessment was constrained to the activated regions, and since the basic measure is the magnitude of lateralized brain activity, examining the underlying dynamical interactions is problematic.” Second, previous studies had no straightforward way of identifying corresponding points in the two hemispheres due to the unique ways the cerebral cortex might fold in one hemisphere versus the other across individuals.
“Our method overcame these limitations by utilizing ongoing fluctuations in neural activity throughout the entire brain at rest, and by finding corresponding points by their locations relative to cortical landmarks within each flattened hemisphere,” Gotts says. “Combining these two advances has now positioned us to address fundamental questions about large-scale human brain circuitry.”
The scientists found the left hemisphere showed a preference for interacting more exclusively with itself, especially when it comes to brain regions past research found were involved in language and fine motor control. On the other hand, right hemisphere’s brain regions linked to attention and visuospatial processing interacted in a more integrative fashion with both hemispheres.
The researchers next tested the verbal and visuospatial ability of volunteers afterward outside the fMRI scanner and compared these results with the level of lateralization they detected. They found the greater the degree of lateralization, the better volunteers performed on verbal and visuospatial tasks, suggesting the lateralization did have measurable functional benefits.
These findings raise a number of potential avenues of research, says researcher Alex Martin, the senior author on the paper and a neuroscientist at the National Institute of Mental Health.
“Is the left hemisphere organization we observed established prior to language acquisition? Or does it develop as a consequence of language acquisition?” Martin asks. “Does the current organization found for right-handers apply equally to left-handers? And similarly, are there systematic gender differences? How do other species differ — for example, monkeys and chimpanzees — from the patterns we’ve observed in the human brain? Finally, are there aberrant patterns of lateralization associated with particular neuropsychiatric disorders?”