Real -time image reveals how visual experience worship the brain again

Scientists have long known that the visual system of the brain is not completely wired from the beginning, it is refined for what babies see, but the authors of a new MIT study were not yet prepared for the degree of wiring they observed when they analyzed the process in the mice as happened in real time.

As researchers from the Picower Institute for Learning and Memory tracked hundreds of “spine” structures that house individual network connections, or “synapse”, in the branches of dendrite neurons in the visual cortex for 10 days, they saw that only 40 percent of those that began the process survived. Refine the binocular vision (integration of the entry of both eyes) required numerous additions and eliminations of thorns along the dendrites to establish an eventual set of connections.

The former graduated student Katya Tsimring directed the study in the communications of nature, which the team said that it is the first in which scientists tracked the same connections to the “critical period”, when the binocular vision is refined.

What Katya could do is imagine the same dendrites in the same neurons repeatedly for 10 days in the same living mouse during a critical development period, to ask, what happens to synapses or spines? We were surprised how much there is. “

Mriganka Sur, Principal Author, Professor of Paul and Lilah Newton at the Picower Institute and the Department of Cognitive and Cognitive Sciences of MIT

Extensive rotation

In the experiments, young mice observed how black and white grilles with specific guidance lines and movement addresses deviated through their field of vision. At the same time, scientists observed both the structure and activity of the main body of neurons (or “soma”) and thorns throughout their dendrites. When tracking the structure of 793 dendritic spines in 14 neurons on approximately day 1, day 5 and day 10 of the critical period, they could quantify the addition and loss of the thorns and, therefore, the synaptic connections they housed. And when tracking their activity at the same time, they could quantify the visual information that neurons received in each synaptic connection. For example, a column can respond to a specific guidance or direction of grid, several orientations or not respond at all. Finally, by relating the structural changes of a spine in the critical period with their activity, they sought to discover the process by which the synaptic replacement refined the binocular vision.

Structurally, the researchers saw that 32 percent of the evils evident on day 1 disappeared for day 5, and that 24 percent of the apparent spines on day 5 had been added since day 1. The period between day 5 and day 10 showed a similar turnover: 27 percent were eliminated, but 24 percent were added. In general, only 40 percent of the thorns observed on day 1 were still there on the 10th.

Meanwhile, only 4 of the 13 neurons that were tracking that they responded to visual stimuli still responded on the 10th. Scientists do not know certainly why the other 9 stopped responding, at least to the stimuli to which they once responded, but it is likely that they now fulfill a different function.

What are the rules?

Having seen this extensive wiring and wiring, the scientists then asked what he was entitled to some thorns to survive during the critical period of 10 days.

Previous studies have shown that the first entries to reach the neurons of the binocular visual cortex are from the “contralateral” eye on the opposite side of the head (so in the left hemisphere, the entrances of the right eye arrive first), Sur said. These entries drive the soma of a neuron to respond to specific visual properties, such as the orientation of a line line, a diagonal of 45 degrees. By the time the critical period begins, the “ipsilateral” eye tickets on the same side of the head begin to join the race to the neurons of the visual cortex, which allows some to become binocular.

It is not an accident that many visual cortex neurons conform to lines from different directions in the field of vision, South said.

“The world is made up of oriented line segments,” South said. “They can be long -line segments; they can be short -line segments. But the world is not just amorphous balloons with nebulous limits. Objects in world trees, soil, horizons, grass leaves, tables, chairs are limited by small line segments.”

Because the researchers were tracking the activity into the thorns, they could see how often they were active and what orientation that activity triggered. As the data accumulated, they saw that the thorns were more likely to support if a) were more active, yb) responded to the same orientation as the one preferred by the soma. In particular, the thorns that responded to both eyes were more active than the thorns that responded to only one, which means that binocular thorns were more likely to survive than non -binocular ones.

“This observation provides convincing evidence of the ‘use it or lose it,” said Tsiming. “The more active it was a backbone, the more likely it was to be retained during development.”

The researchers also noticed another trend. Throughout 10 days, the groups emerged throughout the dendrites in which the neighboring thorns had more and more that they were active at the same time. Other studies have shown that when grouping together, thorns can combine their activity to be greater than they would be isolated.

According to these rules, during the critical period, neurons apparently refined their role in binocular vision by selectively retaining the entries that reinforced their preferences of budding guidance both through their volume of activity (a synaptic property called “Hebbian plasticity”) as its correlation with its neighbors (a property called “Heterosinaptic plasticity”). To confirm that these rules were enough to produce the results they were seeing under the microscope, they built a computer model of a neuron and, in fact, the model recapitulated the same trends as what they saw in the mice.

“Both mechanisms are necessary during the critical period to boost the rotation of the thorns that are misaligned to the soma and the neighboring column pairs,” the researchers wrote, “which finally leads to a refinement of responses (binoculars) as the guidance coincidence between the two eyes.”

In addition to Tsimring and South, the other authors of the newspaper are Kyle Jenks, Claudia Cusseddu, Gregory Heller, Jacque Pak Kan IP and Julijana Gjorgjieva. The sources of financing for research came from the National Health Institutes, the Picower Institute for Learning and Memory, and the Freedom Together Foundation.