New study reveals how brain cells form precise circuits before experience can shape the wiring

In humans, the learning process is driven by different groups of brain cells firing together. For example, when neurons associated with the process of recognizing a dog begin to fire in a coordinated manner in response to cells that encode the characteristics of a dog (four legs, fur, tail, etc.), a young child will eventually be able to identify dogs. But the brain wiring begins before humans are born, before they have experiences or senses like sight to guide this cellular circuitry. How does this happen?

In a new study published in ScienceYale researchers have identified how brain cells begin to cluster together in this hardwired network early in development, before experience has a chance to shape the brain. It turns out that very early development follows the same rules as later development: Cells that fire together wire together. But rather than experience being the driving force, spontaneous cellular activity is the driving force.

“One of the fundamental questions we have is how the brain is wired during development,” said Michael Crair, co-senior author of the study and the William Ziegler III Professor of Neuroscience at Yale School of Medicine. “What are the rules and mechanisms that govern brain wiring? These findings help answer that question.”

For the study, the researchers focused on mouse retinal ganglion cells, which project from the retina to a region of the brain called the superior colliculus where they connect to downstream target neurons.

The researchers simultaneously measured the activity of a single retinal ganglion cell, the anatomical changes that occurred in that cell during development, and the activity of surrounding cells in awake newborn mice whose eyes had not yet opened. This technically complex experiment was made possible by advanced microscopy techniques and fluorescent proteins that indicate cellular activity and anatomical changes.

Previous research has shown that before sensory experience can take place – for example, when humans are in the womb or before young mice open their eyes – spontaneously generated neural activity is correlated and forms waves.

In this new study, the researchers found that when the activity of a single retinal ganglion cell was highly synchronized with the spontaneous activity waves of surrounding cells, the cell’s axon (the part of the cell that connects to other cells) grew new branches. When the activity was poorly synchronized, the axonal branches were eliminated.

“This tells us that when these cells fire together, the associations are strengthened,” said Liang Liang, co-senior author of the study and assistant professor of neuroscience at Yale School of Medicine. “The branching of the axons allows for more connections to be made between the retinal ganglion cell and the neurons sharing the synchronized activity in the superior colliculus circuit.”

The discovery follows what is known as “Hebb’s rule,” an idea put forward by psychologist Donald Hebb in 1949. At the time, Hebb suggested that when one cell repeatedly causes another cell to fire, the connections between the two become stronger.

“Hebb’s rule is very often applied in psychology to explain the brain bases of learning,” says Crair, who is also vice provost for research and professor of ophthalmology and visual sciences. “Here we show that it also applies to early brain development with subcellular precision.”

In the new study, the researchers were also able to determine where in the cell branching was most likely to occur, a pattern that was disrupted when the researchers disrupted the synchronization between the cell and the spontaneous waves.

Spontaneous activity occurs during development in several other neural circuits, including the spinal cord, hippocampus and cochlea. Although the specific pattern of cellular activity is different in each of these areas, similar rules may govern how cellular wiring unfolds in these circuits, Crair said.

In the future, the researchers will study whether these axonal branching patterns persist after a mouse’s eyes open and what happens to the downstream connected neuron when a new axonal branch forms.

“The Crair and Liang labs will continue to combine our expertise in brain development and single-cell imaging to examine how the assembly and refinement of brain circuits are guided by precise patterns of neuronal activity at different stages of development,” Liang said.