Our brain consists of two halves – but there are people who can think and speak normally even with only one brain hemisphere. (Image: metamorworks/ istock)
Our thinking organ is known to have a characteristic dichotomy. But to what extent are both halves necessary for the brain to function?? Living with half a brain is possible, study clarifies. Although the loss of a complete hemisphere can have severe consequences for cognitive abilities. But in some cases, affected people can even think and speak like healthy people. The thinking organ can apparently compensate for the loss. According to the research, 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 thinking organ’s amazing compensatory ability.
No neuron in our brain works on its own. Instead, the thinking organ is organized in networks. Certain brain regions become active together during the accomplishment of certain tasks and also in the resting state – they are linked with each other. Researchers now agree that these functional connections are crucial to our cognitive abilities, emotions and behavior. From the default mode network to the attention network: "Basically, there are a handful of brain networks that underlie all of our cognition," explain Dorit Kliemann of the California Institute of Technology in Pasadena and her colleagues.
Look into the half thinking organ
Many of the known networks of our thinking organ span both hemispheres of the brain. Nevertheless, there are people who have a surprisingly high level of cognitive and sensory-motor abilities, even though they only have one hemisphere. This can be the case, for example, in patients with certain forms of severe epilepsy – for whom a so-called hemispherectomy is sometimes performed as a therapeutic measure. How does the brain change as a result of this drastic neurosurgical procedure? Scientists have now investigated this using the example of six adults who had one half of their brains removed between the ages of three months and eleven years.
"The individuals we have dedicated ourselves to are highly functional. For example, they have intact language abilities. I could make small talk with them just like with any other person," Kliemann reports. "You almost forget their condition when you first meet them."In search of an explanation for this phenomenon, the researcher and her team used functional magnetic resonance imaging (fMRI) to look inside the subjects’ thinking organs. These were asked to relax but not fall asleep – allowing scientists to measure spontaneous brain activity at rest. They then compared the results with images of six healthy control subjects and data from nearly 1500 "normal" brains from a database. The focus was primarily on network structure: how do the brain regions that are collectively responsible for things like vision, movement, emotion and cognition work together?
Increased communication between networks
Surprisingly, a striking similarity emerged between brain activity in subjects with only one brain hemisphere and those with two hemispheres. Here’s what the researchers found: both groups showed distinct and comparable connectivity within the cerebral hemispheres of areas typically attributed to a functional network. The extent of global connectivity was ultimately similar in both groups. The crucial difference, however, is that almost all patients with only one hemisphere were characterized by significantly stronger connections between different networks, Kliemann and her colleagues report.
According to the researchers, the increased communication between individual brain networks could be an important compensatory mechanism. It is possibly this functional reorganization that forms the basis for preserving cognitive abilities after the loss of one half 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 allow extensive cognitive compensation," the scientists conclude. "This raises 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 studying 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 caused by a stroke, an accident or a tumor, for example, can have dramatic consequences," Kliemann emphasizes. "We want to understand under what conditions 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."