The First Sparks Of Thought Appear Long Before Experience

The idea that the brain begins its work long before the world touches it feels almost mystical, yet the evidence keeps arriving. Now a team led by UC Santa Cruz shows that human brain organoids produce structured electrical sequences with no sensory input at all, a finding that shifts how we think about the earliest steps toward thought.

The work, published in Nature Neuroscience, probes a period of development normally sealed away inside the womb. Researchers grew tiny three dimensional models of human brain tissue and tracked the electrical chatter of thousands of neurons as they organized themselves from stem cells into early cortical circuits. What emerged was not noise. It was order.

A primordial operating system

The organoids were grown by prompting stem cells to form forebrain tissue, then placed on dense microelectrode arrays capable of capturing the spiking of individual neurons. Over weeks and months, the researchers watched as bursts of activity began to appear, each lasting on the order of one hundred milliseconds. Inside those bursts, a striking pattern surfaced: a minority of neurons fired in the same sequence every time, while a larger group behaved more flexibly around them.

“These cells are clearly interacting with each other and forming circuits that self assemble before we can experience anything from the outside world,” said Tal Sharf, the study’s senior author. “There’s an operating system that exists, that emerges in a primordial state.”

The team refers to this rigid minority as backbone neurons. They sit in the high firing tail of the network and anchor the timing of each burst. No matter how many times the organoid fired, these neurons kept the same order, like a scaffold that the rest of the circuit worked around. When the team disrupted inhibition with a drug, the sequences tightened. When they blocked excitatory signaling, they vanished entirely.

This consistency also appeared in neonatal mouse cortical slices, taken before the animals had opened their eyes. But it was absent in two dimensional primary cultures, where the cells lack the layered architecture of a developing brain. In other words, the phenomenon seems to depend on three dimensional cytoarchitecture, not experience.

A default mode before the senses arrive

Decades of neuroscience suggest that the mature brain contains a background repertoire of firing motifs that set the range of possible sensory responses. The new study shows that pieces of that repertoire appear even when sensory information has never been available. The organoids produced what the authors call temporally rigid and flexible firing motifs, much like activity recorded from human cortex during memory retrieval or from animal cortex during navigation.

“An organoid system that’s intrinsically decoupled from any sensory input gives you a window into what’s happening with this self assembly process,” Sharf said.

Hidden Markov models applied to the data revealed that the organoids cycle through discrete electrical states in a reproducible order. Population level analyses showed that the backbone neurons define a low dimensional trajectory that the rest of the network echoes. Randomizing the spike trains destroyed this structure, reinforcing that it was carried by true temporal relationships rather than average firing rates.

The results echo a growing view that the brain begins life with an intrinsic set of rules for organizing time and information. Later sensory experience tunes these templates, but does not create them from scratch. Some of the same dynamics appear in hippocampal development, where preconfigured sequences form before animals begin exploring their environment.

Why it matters

Because organoids assemble themselves without input from the body, their activity offers a rare look at early circuit logic. This could help researchers pinpoint how toxins, genetic variants or developmental disorders alter the brain’s foundational timing patterns. The work also supports the expanding use of organoids as testbeds for circuit level questions once considered unreachable in human tissue.

By showing that structured neuronal sequences emerge spontaneously, the study suggests that the capacity for ordered computation is wired into the architecture of the developing brain. It is a reminder that long before we learn anything, the brain is already preparing its internal map of the world, sketching the temporal backbone that later becomes perception, memory and meaning.

Journal: Nature Neuroscience
DOI: 10.1038/s41593-025-02111-0