Medial visual areas, nearer the midline, receive nerve fibers earlier and refine their connections gradually as the animal grows into adulthood. Lateral regions, farther out, receive connections later. These undergo a rapid surge of axonal growth—the extension of the long fibers neurons use to send signals—and then quickly prune away excess nerve endings, reaching mature levels by 24 days after birth.

The team also found that connections carrying information back to the primary visual cortex form as early as—and in some areas possibly before—those carrying information forward. That challenges the common view that the brain builds its forward pathways first.

Genetic barcodes for single neurons

To build the map, the researchers combined several complementary tracing approaches, using engineered viruses to label neurons according to where they send their connections and where their incoming connections originate.

To examine circuit development at the level of individual brain cells, the team used MAPseq (Multiplexed Analysis of Projections by Sequencing), a technique developed in Anthony Zador's lab at Cold Spring Harbor Laboratory that can map the connections of thousands of neurons at once.

The researchers labeled individual visual cortex neurons with unique RNA barcodes that travel along their axons. Reading these barcodes told the team where each cell sent its long-distance connections. Repeating the procedure at different ages revealed how projection patterns changed during development.

Co-first authors Matthew Jacobs and John Ratliff, both researchers in Kim's lab, led experiments showing that individual visual cortex neurons formed the same types of projection patterns at every developmental stage examined. Neurons functioned either as dedicated channels projecting to a single region or as broadcasting channels reaching specific combinations of targets, with minimal variation in these wiring motifs over time.

"The data showed us that neurons connect to either a single visual area from the get-go, or to some combination of areas, at predictable rates," said Jacobs, a former postdoctoral researcher in Kim's lab. "We are not seeing cells that connect everywhere randomly and then abandon whole areas during refinement."

A baseline for neurodevelopmental research

By clarifying how long-range brain circuits normally form, the study provides a foundation for understanding neurodevelopmental disorders. Disruptions in long-range cortical connectivity during early development have been implicated in conditions such as autism spectrum disorder and schizophrenia.

Kim stressed the value of having a comprehensive, fine-grained map of how these circuits wire up normally. "That matters because atypical wiring during this same developmental window is implicated in conditions like autism," he said. "It sets the baseline that future work on circuit disruption can be measured against, rather than being a finding about autism itself."

In other words, the study itself does not examine disease. It defines what typical wiring looks like, so that future studies can measure deviations against that standard.

The study's co-authors include UC Santa Cruz researchers Alec Soronow, Jordan Nichols, Hylen James, Jorin Eddy, and Adam Murray. As with any single study in mice, the findings describe one species and one sensory system; how far they generalize to other cortical regions or to humans remains an open question for future work.