Despite how much noise pop psychology makes about left or right hemispheres, the brain really is a very cohesive unit. The right and left hemispheres have some differences, but they communicate with each other through special neural connections that bridge the two hemispheres.
However, in some people, the two hemispheres of the brain are separated as a treatment for severe epilepsy. What is special is that this has fewer effects than you might think. While there are certainly differences in how a split brain behaves, people who have this surgery usually behave much like we would expect someone else to do, and they are even better at certain types of double-duty.
These split-brain individuals are interesting because they can help us understand how the brain processes information and how it integrates or separates concurrent tasks. For example, we know that in a split brain, the two hemispheres have to process tasks separately (the connection between them is lost, remember), with each hemisphere not knowing what the other is up to.
A group of neuroscientists at the University of Wisconsin-Madison suspected that healthy brains sometimes process tasks separately when multitasking. While the brain wouldn’t literally split, the researchers thought they might be able to detect two separate networks operating independently.
To test this, the researchers devised a functional magnetic resonance imaging (fMRI) experiment that involved multitasking. They chose listening and driving as tasks partly because they’re daily activities, meaning there’s less chance of just finding an artificial effect in a weird lab task. Another reason for the choice is that we already have an understanding of the networks we use for auditory and linguistic processing, as well as the networks used for the visual and motor processes of driving.
What the researchers wanted to see was whether they could find evidence that the two listening and driving networks worked together on one occasion and separately on the other, depending on the task.
The driving simulation that the researchers used was quite simple: the subjects only had to drive on a two-lane road with no intersections or other vehicles. The complication came with the extra tasks they had to perform. In the “integrated” task, the drivers had to listen to GPS-like instructions as they drove, telling them when to change lanes. That’s multitasking, but the two separate tasks have only one goal: to navigate the simulation correctly. In the “split” task, they changed lanes at designated objects (such as traffic signs) and listened to the radio, which obviously had nothing to do with the main task of driving.
There are some obvious potential pitfalls here. For example, GPS voices and radio often sound quite different. To get around this, the researchers used the same voice to read both instructions for the GPS task and articles for the radio. They also asked the participants how difficult the tasks were and how sleepy they felt, eventually testing their driving and listening skills. All checks suggested that the tasks were essentially the same on these points.
When the drivers performed the integrated task, the activation in their brains showed an integrated network: their brains processed both challenges as a single task. But when they ran the split task, the scans showed less connectivity between the two activity networks. “A brain can be functionally split into two separate ‘driving’ and ‘listening’ systems when the listening task is unrelated to simultaneous driving,” the authors write.
The ability to manage these different networks was also related to driving performance, which the researchers defined as driving in a straighter line with less away from the centerline. If drivers could easily switch between high and low information integration, they were better drivers in both integrated and split tasks.
As with many fMRI studies, the sample size in the experiment was small: just 13 male participants. This carries the risk of creating an exciting effect just because there are not enough people to bridge individual differences. If 13 other people were used, the same effect might not occur. “I would certainly like to see it replicated in a larger sample size that includes participants who are not limited to male adults,” said Gagan Wig, who studies brain network organization and was not involved in this study. “But at least this is proof that there are some innovative ways to pursue these ideas.”
The interesting thing about this research, says Wig, is that it shows how flexible and dynamic the brain is. It is capable of running concurrent, split networks, but it can also integrate these networks when needed. This kind of flexibility is suspected, he says, but this research has found new ways to detect the integration and separation.
This research leads to more questions. The brain has many processing capabilities beyond those tested here, and we don’t know which other systems can integrate with each other and which cannot. There’s also a deeper level to explore: Which subnetworks control the brain’s switches between network integration and separation? This is an interesting proof of concept, but there is always more work to be done.
PNAS2016. DOI: 10.1073/pnas.1613200113 (About DOIs).