- Visual experiences are necessary for the the development of visual brain circuitry or areas.
- These visual experiences are needed early in life for proper development of vision.
- Proper function of these brain areas is likely critical for vision restoration.
A newborn sees blurry shape of light and dark initially. But very soon their vision comes online and babies begin recognizing faces.
Then, when the baby looks beyond the face it sees another blurry world of shapes and light. But within a few months they slowly begin to recognize the trees outside.
New study maps the functions of the areas of the brain that play crucial roles in vision.
The study conducted in mice, is part of an ongoing project in the lab of Spencer Smith, PhD, assistant professor of cell biology and physiology.
Proper function of these brain areas is likely critical for vision restoration.
"There's this remarkable biological operation that plays out during development," Smith said. "Early on, there are genetic programs and chemical pathways that position cells in the brain and help wire up a 'rough draft' of the circuitry."
"Later, after birth, this circuitry is actively sculpted by visual experience: simply looking around our world helps developing brains wire up the most sophisticated visual processing circuitry the world has ever known. Even the best supercomputers and our latest algorithms still can't compete with the visual processing abilities of humans and animals. We want to know how neural circuitry does this." Smith added.
Exploring Higher Visual Areas of the Brain
"Most work on restoring vision has focused on the retina and the primary visual cortex," Smith said.
Little in known about the development of higher visual areas of the brain, and their potential for recovery from early deficits.
"I want to understand how these higher visual areas develop. We need to know the critical time windows during which vision should be restored, and what occurs during these windows to ensure proper circuit development." Smith added.
Challenges of Vision Restoration Later in Life
In case of bilateral cataracts, where the lenses of both eyes are cloudy and vision is severely limited, they are surgically removed very early in life in developed countries. If so, vision typically develops appropriately.
"But in less developed, rural parts of the world, people often don't get to a clinic until they are teens or older," Smith said.
When they have the cataracts removed later in life, they recover a large amount of visual function, but it is not complete. They can learn to read and recognize their friends. But they have great difficulty perceiving some types of visual motion.
Visual Deficit in Dorsal Sream
There are two subnetworks of visual brain circuitry- the ventral and dorsal streams. The dorsal stream is important for motion perception.
To understand the importance of visual experience for proper development of the dorsal stream and the changes that occur at the individual neuron level during this early development, researchers are reverse engineering complicated brain circuitry with the help of specialized two-photon imaging systems.
This was designed and built by Smith and his team at the UNC Neuroscience Center.
"If you want to reverse engineer a radio to know how it works, a good way to start would be to watch someone put together a radio," Smith said. "Well, this is kind of what we're doing. We're using our imaging systems to watch how biology builds its visual processing circuitry."
Researchers reared mice in complete darkness for several weeks. Even the daily care of the mice was in darkness with the aid of night-vision goggles.
Using two-photon imaging systems and precision surgical methods, researchers viewed specific areas of the brain with neuron-level resolution.
They showed that in response to visual stimuli, the ventral visual stream in mice did indeed come online immediately, with individual neurons firing. But the dorsal stream did not.
"Keeping the mice in darkness significantly degraded the magnitude of visual responses in the dorsal stream - responses to what they were seeing," Smith said. The neurons in the dorsal area weren't firing as strongly as they did in mice raised with normal visual experience.
Even after a recovery period in a normal light-dark cycle, the visual deficit in the dorsal stream persisted.
This is what happens when bilateral cataracts are not repaired until later in life. There are persistent visual deficits seen in humans.
"Not only did the mice need visual experience to develop their dorsal stream of visual processing, but they needed it in an early developmental time window to refine the brain circuitry," Smith said. "Otherwise, their vision never properly developed."
These findings help explain what happens in the the ventral and dorsal streams of humans as babies, when part of the vision develops.
The study is published in the journal Nature Neuroscience.
- Spencer Smith et al. Stream-dependent development of higher visual cortical areas. Nature Neuroscience ; (2017) doi:10.1038/nn.4469