As is well known, our thinking organ has a characteristic dichotomy. But to what extent are both halves necessary for the brain to function? A study shows that living with half a brain is possible. Losing an entire hemisphere can have serious consequences for cognitive abilities. But in some cases, people can even think and speak like healthy people. The thinking organ can apparently compensate for the loss. According to the results of the study, unusually strong connections between different brain networks are formed in the remaining half of the brain. It is possible that this functional adaptation forms the basis for the astonishing ability of the thinking organ to compensate.
No neuron in our brain works on its own. Instead, the thinking organ is organized in networks. Certain regions of the brain become active in the accomplishment of certain tasks and also in the resting state – they are linked to one another. Researchers now agree that these functional connections are critical to our cognitive abilities, our emotions, and our behavior. From the default mode network to the attention network: “In principle, there are a handful of brain networks that underlie all our cognition,” explain Dorit Kliemann from the California Institute of Technology in Pasadena and her colleagues.
Look into half the thinking organ
Many of the well-known networks of our organ of thought extend across both hemispheres of the brain. Nevertheless, there are people who have a surprisingly high level of cognitive and sensorimotor skills, even though they only have one hemisphere. This can be the case, for example, in patients with certain forms of severe epilepsy – they are sometimes given a so-called hemispherectomy as a therapeutic measure. How does this drastic neurosurgical intervention change the brain? The scientists have now investigated this using the example of six adults who had half of their brains removed between the ages of three months and eleven years.
“The people we have dedicated ourselves to are highly functional. For example, you have intact language skills. I was able to hold small talk with them as with any other person, ”says Kliemann. “You almost forget their condition when you meet them for the first time.” In search of an explanation for this phenomenon, the researcher and her team looked into the test subjects’ minds with the help of functional magnetic resonance imaging (fMRI). They were asked to relax but not fall asleep – so the scientists were able to measure the spontaneous brain activity in the resting state. They then compared the results with recordings of six healthy control persons and data from almost 1500 “normal” brains from a database. The focus was primarily on the network structure: How do the brain regions that are jointly responsible for things like vision, movement, emotions and cognition work together?
Increased communication between networks
Surprisingly, there was a striking similarity between the brain activity in subjects with only one hemisphere and those with two hemispheres. The researchers found: Both groups showed a pronounced and comparable connection of areas within the hemispheres that are typically assigned to a functional network. The level of global connectivity was ultimately similar for both groups. The key difference, however: Almost all patients with only one hemisphere were characterized by a significantly stronger link between different networks, as Kliemann and her colleagues report.
According to the researchers, the increased communication between individual brain networks could be an important compensation mechanism. It may be this functional reorganization that forms the basis for maintaining cognitive abilities after the loss of one hemisphere of the brain. “Our insights into the brains of these rare patients suggest that intrinsic mechanisms of brain organization in only one half of the typically available cortex are sufficient to enable extensive cognitive compensation,” the scientists conclude. “This gives rise to exciting new questions about the neural basis of cognition and conscious experience.”
How does the brain compensate?
Kliemann and her colleagues are currently investigating this phenomenon further – they are examining even more people with atypical brain structures. “As remarkable as it is that there are individuals who can live largely normally with only one hemisphere of the brain: Sometimes even a small brain injury from a stroke, an accident or a tumor can have dramatic consequences,” says Kliemann. “We want to understand the conditions under which the brain can reorganize and compensate for the loss of individual structures. This could one day lead to new strategies to help more people with brain injuries. “
Source: Dorit Kliemann (California Institute of Technology, Pasadena) et al., Cell Reports, doi: 10.1016 / j.celrep.2019.10.067