The brain divides vision between its two hemispheres—what's on your left is processed by your right hemisphere and vice versa—but your experience with every bike or bird that you see zipping by is seamless. A new study by neuroscientists at The Picower Institute for Learning and Memory at MIT reveals how the brain handles the transition.
"It's surprising to some people to hear that there's some independence between the hemispheres, because that doesn't really correspond to how we perceive reality," said Earl K. Miller , Picower Professor in The Picower Institute and MIT's Department of Brain and Cognitive Sciences. "In our consciousness, everything seems to be unified."
There are advantages to separately processing vision on either side of the brain, including the ability to keep track of more things at once, Miller and other researchers have found , but neuroscientists have been eager to fully understand how perception ultimately appears so unified in the end.
Led by postdoctoral Picower Fellow Matthew Broschard and Research Scientist Jefferson Roy, the research team measured neural activity in the brains of animals as they tracked objects crossing their field of view. The results reveal that different frequencies of brain waves encoded and then transferred information from one hemisphere to the other in advance of the crossing and then held on to the object representation in both hemispheres until after the crossing was complete. The process is analogous to how relay racers hand off a baton, how a child swings from one monkey bar to the next, and how cell phone towers hand off a call from one to the next as a train passenger travels through their area. In all cases, both towers or hands actively hold what's being transferred until the handoff is confirmed.
Witnessing the handoff
To conduct the study, published in the Journal of Neuroscience, the researchers measured both the electrical spiking of individual neurons and the various frequencies of brain waves that emerge from the coordinated activity of many neurons. They studied the dorsal and ventrolateral prefrontal cortex in both hemispheres, brain areas associated with executive brain functions.
The power fluctuations of the wave frequencies in each hemisphere told the researchers a clear story about how the subject's brains transferred information from the "sending" to the "receiving" hemisphere whenever a target object crossed the middle of their field of view. In the experiments, the target was accompanied by a distractor object on the opposite side of the screen to confirm that the subjects were consciously paying attention to the target object's motion and not just indiscriminately glancing at whatever happened to pop up on to the screen.
The highest frequency "gamma" waves, which encode sensory information, peaked in both hemispheres when the subjects first looked at the screen and again when the two objects appeared. When a color change signaled which object was the target to track, the gamma increase was only evident in the "sending" hemisphere (on the opposite side as the target object), as expected. Meanwhile, the power of somewhat lower frequency "beta" waves, which regulate when gamma waves are active, varied inversely with the gamma waves. These sensory encoding dynamics were stronger in the ventrolateral locations compared to the dorsolateral ones.
Meanwhile, two distinct bands of lower frequency waves showed greater power in the dorsolateral locations at key moments related to achieving the handoff. About a quarter of a second before a target object crossed the middle of the field of view, "alpha" waves ramped up in both hemispheres and then peaked just after the object crossed. Meanwhile, "theta" band waves peaked after the crossing was complete, only in the "receiving" hemisphere (opposite from the target's new position).
Accompanying the pattern of wave peaks, neuron spiking data showed how the brain's representation of the target's location traveled. Using decoder software, which interprets what information the spikes represent, the researchers could see the target representation emerge in the sending hemisphere's ventrolateral location when it was first cued by the color change. Then they could see that as the target neared the middle of the field of view, the receiving hemisphere joined the sending hemisphere in representing the object, so that they both encoded the information during the transfer.
Taken together, the results showed that after the sending hemisphere initially encoded the target with a ventrolateral interplay of beta and gamma waves, a dorsolateral ramp up of alpha waves caused the receiving hemisphere to anticipate the handoff by mirroring the sending hemisphere's encoding of the target information. Alpha peaked just after the target crossed the middle of the field of view, and when the handoff was complete, theta peaked in the receiving hemisphere as if to say, "I got it."
And in trials where the target never crossed the middle of the field of view, these handoff dynamics were not apparent in the masurements.
The study shows that the brain is not simply tracking objects in one hemisphere and then just picking them up anew when they enter the field of view of the other hemisphere.
"These results suggest there are active mechanisms that transfer information between cerebral hemispheres," the authors wrote. "The brain seems to anticipate the transfer and acknowledge its completion."
But they also note based on other studies that the system of interhemispheric coordination can sometimes appear to break down in certain neurological conditions including schizophrenia, autism, depression, dyslexia and multiple sclerosis. The new study may lend insight into the specific dynamics needed for it to succeed.
In addition to Broschard, Roy and Miller, the paper's other authors are Scott Brincat and Meredith Mahnke.
Funding for the study came from the Office of Naval Research, the National Eye Institute of the National Institutes of Health, The Freedom Together Foundation and The Picower Institute for Learning and Memory.
