We used to think that brain circuits, controlling everything from walking and talking to seeing, hearing and feeling pain, formed during childhood and were, from then on, pretty much set for life.
Neuroplasticity, made famous in Norman Doidge’s bestseller The Brain that Changes Itself, tells us that is not so. After a stroke, for example, restraining the good arm in a sling forces the paralysed side to move, and recruits new brain pathways that can breathe life back into the frozen limb.
The degree to which plasticity can overcome damage to the visual part of the brain has been less certain. A new study, however, led by neuroscientist Marlene Behrmann at Carnegie Mellon University in Pittsburgh, US, has uncovered the brain’s remarkable ability to compensate for even a devastating loss of tissue involved in visual perception.
Writing in the journal Cell Reports, the team recounts the tragic case of a six-year-old boy with a brain tumour that caused seizures that didn’t respond to medication. The only recourse was a drastic one: the tumour and roughly a third of the right side of the boy’s brain were surgically removed, including the entire occipital cortex and most of the temporal lobe.
Those regions are essential for the basic function of sight, and related abilities to recognise faces, objects and words. A diagnosis of face blindness (an inability to recognise previously familiar faces), for example, had been thought to be a specific consequence of damage to right-sided visual pathways.
After the surgery, the researchers undertook detailed study of the boy’s progress over three years, including assessment of his perception and intelligence, and the use of functional brain scans on no fewer than five occasions.
Remarkably, the ability to recall faces, and to identify objects such as an apple, scenes such as a house facade, and words such as “close” were preserved, despite the extensive surgery. Scans showed that most of those functions were taken over by the left side, attesting to the brain’s capacity to repurpose swathes of cortex in order to compensate for lost nerve cells.
“Our results favour dynamic reorganisation and fine-tuning in the functional architecture of cognition … and argue for the critical role of experience in shaping the underlying circuitry,” the researchers write.
One function, however, was not regained. The boy, now 11, is unable to see things in the left half of his visual field, something the researchers say relates to those circuits being “established and fixed at an early age”.
They also note that in a recent review of similar cases, published by two of the authors, recoveries weren’t nearly so promising. They suggest the boy’s case may have been helped by the fact his pre-operative cognitive function was at a high level, and that his slow-growing tumour gave the brain enough time to re-home some of those visual tasks.
The boy remains seizure-free with an IQ of 118, comparable to his pre-surgical score of 116.
“These findings provide a detailed characterisation of the visual system’s plasticity during children’s brain development,” says Behrmann.
“They also shed light on the visual system of the cortex and can potentially help neurologists and neurosurgeons understand the kind of changes that are possible in the brain.”