What goes on inside skulls can be distributed outside of them.
Today, the cerebellum is understood to be the brain’s quality-assessment centre, ensuring that actions the higher brain has decided on go according to plan. The electrical signals that travel from the upper brain out to, say, the arm, to tell it to type, pass through the cerebellum, which takes down a copy of the plan. When information describing the movements in progress comes back from the arm muscles, the cerebellum’s tightly meshed network of billions of nerve cells compares the situation described to the original plan, detects any deviations, and sends a better-honed signal back out to the arm. At the same time, the network of cells adjusts its internal wiring so as to make sure any error is less likely to happen again. As well as supervising the smooth running of the body through endless cycles of error correction, the cerebellum helps the brain and the body learn from their mistakes and miscommunications.
Until recently, this quality-assurance infrastructure was thought to be limited to the oversight of physical movements. But brain scans now suggest that the cerebellum’s remit is much broader. “The cerebellum does the same thing to emotion and cognition that it does for motor control,” says Jeremy Schmahmann, a neurologist at Harvard Medical School. Just as physical movements are monitored, controlled and regulated by the cerebellum, so too, it turns out, are other sorts of thought, including speech and emotional work. The set of mental actions required to remain calm under pressure, for instance, runs through the cerebellum.
All this takes place far below the threshold of consciousness. It is a sort of internalised automation, allowing processes to go on in the background without any attention. You don’t know that you are doing it. But you would notice very quickly if you stopped.
Ethan had his first brain scan on October 15th 2010, just before his 4th birthday, at Le Bonheur Children’s Hospital in Memphis. By this stage, his doctors had begun to suspect a damaged cerebellum. But that was not what the scan (reproduced here, with a scan of a normal brain below for comparison) showed. His cerebellum was not damaged. It was absent; there was a featureless dark hole where the cerebellum would be. His brain — or his body, depending which way round you look at it — lacked an autopilot.
Heather says the scan “opened up a whole new world”. The family had felt alone, isolated by a condition that no doctor quite understood. Now Heather was finding Facebook groups devoted to cerebellar issues and learning the right keywords to put into search engines. The term “cerebellar hypoplasia”, meaning underdevelopment of the cerebellum, led her and Ethan to Dr Schmahmann’s office in Boston the following year.
Learning that her son’s brain was not so much damaged as simply missing a part also allowed her to see her and her family’s supporting role more clearly. “If you were born without a leg, you would put on a prosthetic and that would be it,” she says. Being born without a cerebellum was more complicated. But prosthesis was still possible. It just had to be provided by people.
The person doing the most work is Ethan. Unfortunately, he cannot tell you much about how he does it. When you ask Ethan how it feels to do things that the cerebellum is supposed to govern, such as walking, he has trouble answering, and that is fair enough. To him, walking — something he does with a loose, slightly swaying gait — just feels like walking. But he is doing it in a very different way to most people.
Mind the gap
“We think the neural circuits are learning this somehow,” says Dr Schmahmann, “but without the smoothness and automaticity of the cerebellum.” One possibility is that the basal ganglia, parts of the brain responsible for the formation of habits, are taking up at least some of the slack. This would mean that Ethan remembers how to walk in the same way that you remember morning rituals, or your preferred route to work. You might think of those as being things you do “on autopilot”, but the process is not quite the same. A habit does not provide the same sort of self-regulating feedback that an aircraft or cerebellum’s autopilot provides. It is more a stereotypical sequence of actions. To make good the absence of a cerebellum with circuits designed to encode habits is to bridge a gap that is not meant to be bridged. If that is what Ethan’s brain is doing, it is something that takes a lot of support.
Of his handful of patients without cerebellums, Dr Schmahmann says: “What we’re finding is that with intensive rehab, bringing things to conscious awareness and making [patients like Ethan] conscious about what they need to focus on, you allow them to improve.” Those without such intense rehab learn to walk and talk much later than Ethan.
The Devineys have, without really knowing it, built some of the functions of Ethan’s cerebellum into his life, and their lives, through other means.