The how and why of the brain’s division across hemispheres

(Press-News.org) People have a lot of misconceptions about what the brain’s left and right hemispheres do, but one well-known aspect of this division may be even more true than people realize: The brain not only splits up visual spatial perception—processing what’s on our left in the right hemisphere and what’s on our right in the left hemisphere—it takes cognitive advantage of that. A new review by MIT neuroscientists explains what the field has learned about this division of labor, the trade-off it involves and how the brain ultimately bridges the divide.

“People hear all these myths about the left brain being more analytical and the right brain being more artistic, or people being right-brained vs left-brained. Ninety-nine percent of that is nonsense,” said paper co-author Earl K. Miller, Picower Professor in The Picower Institute for Learning and Memory and the Department of Brain and Cognitive Sciences at MIT. “You think with your whole brain.”

But when it comes to visual spatial perception, the brain has evolved separate neural resources for the right vs left sides of gaze even in later stages of cognitive processing, Miller said. Why? To optimize its capacity.

“It’s for good reason,” said Miller, who co-authored the new review in the journal Neuropsychologia with Picower Institute research scientist Scott Brincat. “Perceptual capacity is limited—you can only take in so much at once. If you’re your capacity is fully tied up on the right side of your gaze, you might miss a threat approaching on the left. Splitting resources between both sides helps avoid dangerous perceptual blind spots”.

Separated sight

When Miller was in graduate school, he said, he was taught that the brain neatly divided its visual perception of space between hemispheres until the information reached the prefrontal cortex where it became seamlessly blended. But experiments by Miller and Brincat, as well as many other researchers, have been accumulating evidence to refine that view over the last 20 years. These studies have shown that even in the prefrontal cortex, neural encoding of information about where an object is, is still biased toward the “contralateral” hemisphere, or the hemisphere opposite of where the object appears in the field of view.

“As a result, the two hemispheres appear to function surprisingly independently, even for high-level cognitive functions like attention and working memory,” the authors wrote. The evidence is apparent in measurements of the brain waves produced by coordinated networks of neurons in each hemisphere. Gamma frequency wave power increases in frontal brain hemispheres when they consider the visual stimuli that appear on the contralateral side.

Meanwhile, studies for decades (including one in 1971) have shown that people and animals can remember more things if their presentation is split between hemispheres rather than presented all on one side. Neuroscientists call this the “bilateral advantage,” though it is not perfect. People don’t track multiple things, even if split across both sides, as well as they track just one thing on either side.

People also exhibit individualized differences in perceptual capacity across the visual field. Miller has founded the startup company SplitSage to measure these differences with the goal of helping people with visually oriented complex tasks to improve their performance.

Notably, the studies that Brincat and Miller review in the new paper show that the split bias between hemispheres applies only to spatial information—the where something is. Other features like color or shape are processed by both hemispheres.

Seamless sight

If the brain maintains a separation in its processing of spatial visual perception, even at the stage of allocating attention and juggling objects in working memory, why aren’t we confused or surprised when a bird flying from our left passes into the right side of our field of view? We also easily handle cases where our shifting gaze moves an object from one eye’s field of view to the other’s.

“We experience a seamless world,” Miller said.

It turns out that the brain accomplishes the “handoff” from one hemisphere to the other much like two cellular towers do for your phone as you drive around, according to neural activity measurements in studies such as one in 2014 and one that Brincat and Miller led in 2021.

“As a tracked target approaches the visual midline, the hemisphere about to receive the target shows a ramp-up of activity well before the crossing time, as if it is anticipating the target,” the authors wrote. “Further, activity in the sending hemisphere remains high well after the crossing. Thus, for up to a second or more, neural signals reflecting the target are shared across both hemispheres. It is as if both hemispheres are holding the baton.”

As with the “bilateral advantage,” there is a small performance cost when this transfer happens, studies show.

Deficits in interhemispheric connectivity or synchrony are apparent in neurological and psychiatric diseases including Alzheimer’s, anxiety, depression, schizophrenia, obsessive-compulsive disorder and autism spectrum disorders, the authors note. Brincat and Miller’s review emphasizes that such disruptions could affect cognition.

“A foundational understanding of interhemispheric processing, combined with interventions translatable to human patients offers hope for developing novel network-level treatments,” they wrote.

END