Pacemaking neurons emerge in self-organizing neural tissue

Brain organoids are self-organizing neural tissues whose differentiation can be controlled in experiments. Previous studies have observed neural population activity bursts in brain organoids, but we still lack detailed descriptions of the bursts for theoretical models to compare to. We analyze widefield calcium recordings of brain organoids, and using random matrix theory, we identify statistically significant low-dimensional dynamical modes of their population activity. We observe neurons spike in an order that is stable from burst to burst within organoids, where the activity is initiated by putative pacemaking neurons. In many organoids, the spread of activity is not spatially local, and we perform dimensionality reduction to understand how the spread occurs over the population. The presence of pacemaking neurons and the stable firing order are detailed descriptions of population bursts in brain organoids. They potentially require new theoretical models of self-organizing neural tissue to capture.