York University researchers have found that the brain is capable of adapting to the loss of an eye within the first four years of life.
York psychology Professor Jennifer Steeves and PhD psychology candidate Krista Kelly (MA ’08) already knew that people who have lost an eye early in life have good vision – rivaling that of people with both eyes intact, except for depth perception – but what they didn’t know was why.
To find out, they and several colleagues looked at magnetic resonance images of the anterior structure of the visual system of 12 adults who had a cancerous eye surgically removed before four years of age. This group was then compared to one person who lost an eye at the age of 59 and to a group of people with intact binocular vision.
“We show reorganization in the brain of individuals with early loss of one eye, indicating that the brain is more plastic and able to adapt in early life,” says Steeves of the Faculty of Health. “The changes that we see are also consistent with the excellent visual ability that people who lose one eye early in life have.”
Unlike most studies to date, which have reported brain changes only in adults who have lost an eye, this study looked at people who had an eye removed in early childhood. The researchers assessed the optic nerves, optic chiasm, optic tracts and the lateral geniculate nucleus (LGN) volumes of each subject. The goal was to find out what affected the maturation of the anterior visual system when it received information from only one eye. These effects were studied years after the eye loss.
The researchers found alterations in the development of the visual system occurred after the eye’s removal. The structure of the visual system – optic tract diameter, and optic chiasm and lateral geniculate volumes – was decreased in all participants who had an eye removed compared to the control subjects.
But what they found surprising, says Steeves, was that the visual structure in those participants who lost an eye early was found to be asymmetrical – larger on the side opposite the intact eye. The structure, however, remained symmetrical in the participant who lost his eye late in life.
Why does this asymmetry occur? It could be that the disconnected cells following surgical removal of the eye are recruited to the intact side or stronger feedback signals from the brain’s opposing hemisphere helped retain those cells in the participants who underwent surgical removal of their eye early, but not the older person.
These alterations in visual structure could also explain why there are no deficits in the spatial vision of those underwent early removal of an eye, says Steeves. It points to a “stronger malleability of the brain following visual deprivation that occurs earlier rather than later in life.”
The study was published in November in NeuroImage: Clinical, a journal of diseases affecting the nervous system. Kelly is lead author and Steeves is senior author of the article, “Altered Anterior Visual System Development Following Early Monocular Enucleation”. Both are from the Centre for Vision Research at York.
The research was funded by the Natural Sciences & Engineering Research Council of Canada, the Canada Foundation for Innovation and the Canadian National Institute for the Blind.