What Happens if You Lose Half a Brain?

A new study shows the remarkable plasticity of the human brain.

Posted Dec 04, 2019

Sometimes, drastic measures have to be taken to save a human life. One of the most drastic procedures in neurosurgery is a hemispherectomy, the complete removal of one of the halves (or hemispheres) of the brain. This procedure is performed only in very rare cases of intractable epilepsy as a last-ditch effort when all other therapeutic options have failed. The idea behind this is that by removing the affected hemisphere, the frequency of possibly life-threatening epileptic seizures is reduced or completely disappears.

A new study, published in the scientific journal Cell Reports, investigated brain networks in several patients that had undergone removal of one half of the brain in early childhood (Kliemann et al., 2019). The results offer a fascinating insight into the remarkable ability of the human brain to reorganize itself even after severe damage, if that damage happens early in life.

The authors used a neuroscientific technique called resting-state fMRI (functional magnetic resonance imaging). For resting-state fMRI, participants’ brains are scanned using a MRI scanner, a machine found in many hospitals. An MRI scanner is used to make series of pictures of the brain (or other body parts) based on their magnetic properties. Resting-state fMRI is a variation of fMRI, a technique used to make images of the brain while somebody performs a specific task, e.g. speaking or moving their fingers. In order to generate a series of resting state images, the researcher would ask the patients to lay still in the MRI scanner and do not do anything in particular. However, even without being given any specific task the brain shows lots of activity. By determining which brain areas are active at the same time in the idling brain, the researchers can then determine functional networks of brain areas that are usually active together. Using this technique, researchers have identified several different networks in the brain — for example, the so-called default-mode network that has been linked to mind-wandering and daydreaming.

The scientists investigated resting state networks in a group of patients who had undergone removal of one half of the brain in early childhood and compared them to control participants that still had both halves of their brains. The results were quite surprising. While one would expect that the removal of half of the brain would severely impair brain organization in these patients, their networks looked remarkably similar to that of healthy controls. The researchers identified seven different functional networks (e.g. related to attention, visual abilities and motor abilities). Surprisingly, patients who had undergone removal of one half of the brain showed connectivity between brain regions within the same functional networks remarkably similar to those in the controls with intact brains. This shows that within these networks, the patients showed rather normal brain development, despite the absence of one half of the brain.

There was, however, one difference: Connectivity between different networks was markedly increased in the patients. These increased connections seem to reflect cortical reorganization processes after removal of one half of the brain. By getting stronger connections between the remaining parts of the brain, these patients seem to be able to cope for the loss of the other hemisphere. If the surgery was performed early enough in life, many of these patients can retain normal cognitive functions and intelligence and live fairly normal lives. This is even more impressive when you consider that brain damage in later life (e.g. a stroke) can have severe consequences on cognitive abilities, even if only small parts of the brain are damaged.

References

Kliemann D, Adolphs R, Tyszka JM, Fischl B, Yeo BTT, Nair R, Dubois J, Paul LK. (2019). Intrinsic Functional Connectivity of the Brain in Adults with a Single Cerebral Hemisphere. Cell Rep, 29, 2398-2407.e4.