Both genes and the environment play a role in spatial vision. How exactly these factors interact has now been investigated using mice. Even before the newborn mice open their eyes, numerous nerve cells are eliminated in the region of the brain that is responsible for spatial perception. If the process is disturbed by genetic manipulation, the animals later have problems with the 3D view. Complete darkness, on the other hand, has no detrimental effect on the development of visual skills.
So that we can see in three dimensions, our brain combines the information from both eyes into a three-dimensional overall picture. The calculation takes place in the visual cortex, a brain region with a complex network of inhibiting and exciting nerve cells. The structure of this network is fundamentally laid down in our genes. However, the fine-tuning only takes place through mutual influences between the different cells: Certain connections are strengthened, others weakened or broken down.
Interaction between genes and the environment
A team led by Bor-Shuen Wang from the Cold Spring Harbor Laboratory has now investigated how exactly genes and experience interact in the development of spatial vision. “The question is not whether it is genetic or environmental influences – it is clearly both,” says Wang’s colleague Josh Huang. “The question is how they influence each other.” In order to answer this question, the researchers made so-called chandelier cells visible in the brain of mice with the help of genetic modifications. These are nerve cells that are widely branched like a chandelier and inhibit the signals from surrounding nerve cells. While they are evenly distributed across most parts of the brain, they have been known to be remarkably rare in the visual cortex.
But that obviously does not apply to newborn mice: As Wang’s team found out, there are significantly more chandelier cells in the visual cortex of the animals immediately after birth. Signals from the retina decimate these cells within a few days. “Even before the mice open their eyes for the first time, that is, before the first visual experience, a dramatic process takes place that eliminates these cells. More than half of them are killed, ”says Huang. The cells of the retina send signals that ultimately lead to the programmed cell death of numerous chandelier cells.
Requirement for spatial vision
According to the researchers, the extinction of many inhibitory nerve cells ensures that the remaining cells in the visual cortex can effectively process the images of both eyes. “Spatial vision requires rapid communication between the two visual hemispheres,” says Huang. “We suspect that this requires reducing the inhibition in the brain region responsible for this.” To test this thesis, the researchers genetically modified some mice so that more chandelier cells were retained. And indeed: these animals later had problems seeing three-dimensionally, while their other visual abilities were not restricted.
In another experiment, the researchers let newborn mice grow up in complete darkness so that they did not receive any visual stimuli. Nevertheless, within the first two weeks of life, her retina sent the necessary signals to eliminate the excess chandelier cells in the visual cortex. According to the researchers, the killing of chandelier cells does not happen randomly, but depends on the incoming signals: the signals from the retina cause the chandelier cells to form connections. Those who make the wrong connections succumb to programmed cell death.
“The entire process of brain development is an ongoing process in which genetic information plays an important role in fundamentally building the network,” says Huang. “But later on, processes that depend on learning and experience ensure fine-tuning. The phenomenon we are talking about lies precisely at the interface between genetically predetermined and use-dependent mechanisms. “
Source: Bor-Shuen Wang (Cold Spring Harbor Laboratory) et al., Neuron, doi: 10.1016 / j.neuron.2020.11.004