The right hemisphere in the girl’s brain failed to develop in the womb.
Normally, the left and right fields of vision are processed and mapped by opposite sides of the brain, but scans on the German girl showed that retinal nerve fibres that should go to the right hemisphere of the brain diverted to the left.
Further, the researchers found that within the visual cortex of the left hemisphere, ‘islands’ had been formed within it to specifically deal with, and map out, the left visual field.
This study has revealed the surprising flexibility of the brain. The brain has amazing plasticity but we were quite astonished to see just how well the single hemisphere of the brain in this girl has adapted to compensate for the missing half.
Despite lacking one hemisphere, the girl has normal psychological function and is perfectly capable of living a normal and fulfilling life. She is witty, charming and intelligent.
She only has slight weakness on her left side (hemiparesis) . The girl had a normal developmental and medical history, attending regular school and taking part in activities such as roller-skating.
In other cases, where patients have half of the brain removed (hemispherectomy) , to treat severe epilepsy for example, one field of vision is lost in both eyes - i.e. they see only objects on the left or right side of their vision. In the case of the German girl, her left and right field vision is almost perfect in one eye.
Max Planck Institute for Brain Research
Institute of Medical Psychology at Goethe-University
Proceedings of the National Academy of Science USA