At eye opening, neurons in a young brain’s visual cortex begin adding connections at a striking pace: within the first few days of seeing, their number of tiny signal-receiving sites grows nearly fivefold. The change is not simply a matter of keeping some links and cutting others. A study from the Fitzpatrick lab at the Max Planck Florida Institute for Neuroscience in Jupiter, Florida, finds that new connections help tune the brain’s responses to what the eyes see.
Before an animal’s eyes open, genes help lay down an initial framework for visual circuits. Once vision begins, information from the outside world helps refine that framework. In the primary visual cortex, a brain area important for sight, neurons first respond broadly to different visual features. Signals from the two eyes also do not yet line up. Over the days that follow, responses become more selective and the two eyes’ signals become better matched—changes that help build mature vision.
Scientists have often explained this process mainly through pruning: useful connections remain while less useful ones are removed. The findings, published in Current Biology, point to another important part of the story. In the early days of visual experience, neurons add many connections, and the kinds they add are closely linked to the visual information they are learning to process.
Lead authors Dr. Juliane Jaepel and Dr. Clara Tepohl mapped inputs to individual neurons at eye opening and after several days of visual experience. They studied dendritic spines, tiny bumps along a neuron’s branches where connections from other neurons form. At hundreds of spines on a single neuron, the team recorded what information arrived. By presenting visual patterns to each eye separately, they could tell whether each spine responded to the left eye, the right eye or both. They also tested which edge orientations each spine responded to, and whether similarly tuned spines sat near one another along the neuron.
The team found that spine numbers expanded nearly fivefold in the first few days of seeing. But the growth was selective. Connections that responded strongly to edges at a particular angle increased more than connections that responded equally to every angle. Connections that favored the same edge orientation when it was shown to both eyes also rose sharply. After visual experience, connections tuned to similar edge angles tended to cluster beside one another, helping their signals reinforce each other.
That pattern suggests neurons sharpen their responses early on largely by building up well-matched connections, rather than only by removing poorly matched ones. “Visual experience really kicks off a dramatic remodeling of neural circuitry that leads to mature vision,” Jaepel said. Senior author Dr. David Fitzpatrick said the work shows how neurons expand and reorganize their connections while building on an existing scaffold.
The study offers a fuller picture of how experience shapes a brain circuit: development can involve both removing links and adding carefully organized ones. The researchers’ maps capture changes at the very start of seeing, when visual circuits are especially responsive to experience. Understanding how new connections help refine those circuits may help scientists explain how early sensory experience guides the development of the brain.