Until now, researchers thought that the development of the human brain’s visual-processing center stopped in the first few years of life. But a new study challenges this belief, instead suggesting that vision develops until midlife. This may have a significant impact on people with amblyopia, which is an eye disorder that causes what is commonly known as a “lazy eye.”
A new study, published in the Journal of Neuroscience, suggests that human vision might take longer to develop than previously thought.
A team of researchers led by Kathryn Murphy, a professor in the Department of Psychology, Neuroscience and Behaviour at McMaster University in Ontario, Canada, set out to examine the evolution of the primary visual cortex in the human brain by analyzing the postmortem brain tissue of 30 people, ranging in age from 20 days to 80 years.
Until now, the accepted view has been that in humans, the maturation of the primary visual cortex is completed in the first few years of life.
This traditional belief was based on anatomical studies of how the synapses are formed, as well as how connections within the cortex and between the cortex and other brain regions occur.
However, Murphy and colleagues have previously discovered that there are some proteins in the primary visual cortex that continue to develop well beyond the first years of life.
Their new study confirms these preliminary findings.
In their previous research, Murphy and colleagues looked at the so-called GABAergic activity in the brain. GABA stands for gamma-aminobutyric acid, and “GABAergic” refers to the brain’s ability to produce it.
GABA is a neurotransmitter whose main function is to inhibit the action of another neurotransmitter called glutamate.
Their former study looked at the balance between excitation and inhibition in the brain, and how it is linked to the plasticity and aging of the visual cortex. The researchers followed the maturation of this brain region and showed how these GABAergic mechanisms change across the human lifespan.
This previous research showed that the GABA-producing mechanisms continue to mature until quite late in life. However, it still was not clear whether all of the mechanisms that regulate the plasticity of the synapses behaved in the same way. GABA is inhibitory, but what about the excitatory neurotransmitters and synapses?
As Murphy and colleagues explain in their new paper, over 80 percent of synapses in the primary visual cortex are excitatory.
So, to answer this question, the scientists looked at the postmortem tissue of 30 people, of whom 12 were females and 18 were males. They examined their brain tissue using Western blotting – a technique commonly used in molecular biology to separate and identify specific proteins from the mixture of proteins that has been extracted from cells.
The researchers found that some of the glutamatergic proteins – that is, the ones that produce the excitatory glutamate – develop until late childhood, but others develop until around the age of 40.
According to the new research, the visual-processing part of the brain matures until a person reaches 36 years of age, plus or minus around 4.5 years.
This was surprising, as the consensus is that the primary visual cortex stops developing at around the age of 5 or 6 years.
Additionally, the researchers found that the primary visual cortex develops gradually across five stages, which reflect “life-long changes in human visual perception.”
“There’s a big gap in our understanding of how our brains function. Our idea of sensory areas developing in childhood and then being static is part of the challenge. It’s not correct.”
Prof. Kathryn Murphy
These findings may have significant implications for conditions such as amblyopia, which is more popularly known as “lazy eye.” Currently, medical professionals only prescribe corrective therapies to children who have amblyopia. It is currently estimated that “approximately 2 to 3 out of every 100 children” are affected by amblyopia in the United States.
Treating adults is considered futile, as their visual cortex is thought not to be plastic or respond to treatment anymore. However, Murphy says that their research suggests that more brain areas are more flexible and responsive to experience-dependent plasticity than previously thought.