Imagine you are an all-star high school athlete, standing at the plate on a softball diamond. The ball comes to you. You quickly calculate when you must swing to make contact. Then, crack! – you hit the ball and bolt.

To a spectator, the instant between the pitch and your swing passes in a blink of an eye. But within this short amount of time, your brain has made a massive number of rapid-fire connections across both hemispheres to execute the task.

In an average human brain, these connections are possible because of an important region between the left and right brain hemispheres called the corpus callosum. This large bundle of more than 200 million myelinated nerve fibers acts as a signal highway between the two sides of the brain and is also crucial for people recovering from brain damage. By sending signals, the corpus callosum can help other parts of the brain learn and take on the functions of a damaged part over time. This helps some patients regain their full motor capacity following strokes or brain traumas even if they were completely paralyzed initially.

But what happens when you don’t have a corpus callosum?

In May 2022, researchers at Yale School of Medicine published the first recorded case of a patient without a corpus callosum recovering after the removal of the supplementary motor area (SMA) in the journal Neurology.

The patient, a 17-year-old high school softball player, was admitted to the Yale School of Medicine pediatric neurosurgery team in 2021. She had complained of headaches for years and was recently diagnosed with a low-grade glioma in the right supplementary motor area, a delicate region of the brain that controls complex movements and speech. She was also born without a corpus callosum.

Treating the tumor would require removing the right SMA, which would temporarily impact the patient’s speech and motor functions. In an average patient, this would not be a problem since the corpus callosum often facilitates a re-wiring of those functions in the other side of the brain. However, there had never been any documented cases of a patient without a corpus callosum recovering from SMA removal.

To better assess the risks of surgery, Eyiyemisi Damisah, MD, assistant professor of neurosurgery at Yale School of Medicine, and corresponding author on the study, performed an fMRI to better understand how her patient’s brain performed functions without a corpus callosum. What Dr. Damisah found was very unusual. In most human brains, only one side of the brain lights up when the person talks, in this patient, both sides of the brain lit up.

This posed many questions for the surgery team. Did this mean that the speech and motor functions were split among both hemispheres, or were both sides self-sufficient? Would removing the SMA on one side of the brain permanently impact the patient’s ability to function? Would she ever be able to play softball again?

Based on experience, the surgery team knew that while operation posed unknown risks, leaving a brain tumor in a child would lead to known risks of impaired function and even possible death. Following lengthy discussions, the patient’s family decided to proceed with the surgery.

The team removed the tumor entirely with minimal impact to adjacent brain structures. As expected, the patient experienced speech difficulties and weakness of her left arm and leg immediately after the surgery. Yet within a few weeks, despite the absence of corpus callosum, her speech and movement functions came back fully, and she was back on the softball field two months after the surgery.

"The impressive recovery seen in our case is not only striking and surprising, but also contributes to our understanding of how the brain rewires following a brain injury," says Sami Obaid, MD, epilepsy fellow at the Yale School of Medicine and first author in the study.

How was the patient able to recover without a corpus callosum?

A post-surgery fMRI scan showed that her left brain had taken over some motor functions of the injured right brain. Dr. Damisah hypothesized in the Neurology paper that in patients recovering from SMA resection without a corpus callosum, the same function is likely represented on both sides of the brain but silenced on one side in everyday functioning. However, if one of those areas is removed, the silenced function on the other side is unmasked, allowing the patient to regain their motor and speech functions.

This insight could also help patients born with a corpus callosum. Research into technologies such as transmagnetic stimulation to trigger reorganization could be promising for patients recovering from stroke and head traumas. Such a treatment option could hasten recovery and improve the quality of life for many people.

For Dr. Damisah, who has made a career of pushing the boundaries of what is possible in neurosurgery, this unique case has exciting implications for understanding brain function. “This case challenges our dogmatic understanding of what we think is happening. This case was groundbreaking because most people…have a corpus callosum, it’s hard to challenge the idea that those theories as not as powerful as we think they are.”