“It was quite a surprise to see that something like this is possible,” says Lars Muckli, a neuroscientist at the University of Glasgow, UK, who was part of the team that imaged the girl’s brain.
Doctors discovered that she was missing the right half at the age of three, after she began suffering from seizures.
However, the seizures proved treatable and the girl – known as AH – lives an otherwise normal life. The left side of her body is slightly weaker than the right, but this hasn’t stopped her from bicycling or roller-skating.
But what’s most amazing, Muckli says, is her ability to see out of the left and right visual fields. Patients who have half of their cortex removed to treat epilepsy invariably lose half of their visual field. “They would only see half of the world; this is what’s expected,” he says.
That’s because, each eye sends visual signals to two different halves of the brain via two distinct bundles of nerves. The nerves on the side of the eye nearest the nose are routed to the opposite side of the brain. The nerves nearest the temple, however, send information to the same side of the brain as the eye.
For example, the nose side of the left eye sends left visual field data to the right side of the brain; while the temple side of the left eye sends right visual field data to the left side of the brain.
For this reason, the right side of the brain processes our left visual field, and vice versa.
AH, on the other hand, has no right hemisphere to receive any signal from her left visual field. What’s more, her right eye never developed, so she should get visual information only from one half of her left eye – that is, from just one nerve bundle.
Brain scans performed by Muckli’s team explain why that’s not the case. Her retinal nerves that should normally connect to the right half of her brain instead set up shop in two parts of the left brain: the thalamus and the visual cortex.
In some cases, the diverted nerves seemed to have followed the molecular cues that would have guided nerves from the right eye, were they not missing. But for the most part, the left visual field neurons carved out their own islands in the right brain, Muckli says.
This kind of organisation allows AH’s brain to process the left and right fields of vision distinctly from one another, ensuring that she sees both halves of her world.
“It’s fascinating to have someone who is absolutely, completely behaving normally and then knowing she only has half of a brain,” Muckli says.