Assembloids help close that gap. Unlike neurons grown flat in a lab dish, which are among the models that have previously been used in these types of studies, the assembloids contain interconnected circuits that produce coordinated electrical activity. And unlike animal brains, they let researchers isolate and manipulate those circuits in a controlled human system.

Slow waves, quiet neurons

Created from induced pluripotent stem cells — adult cells that have been reprogrammed into a stem cell-like state — the assembloids combine three types of brain cells: excitatory neurons, which transmit signals; inhibitory neurons, which restrain activity; and glial cells, which play multiple roles in brain function. Together, these cells form circuits capable of generating coordinated rhythms similar to those in the human brain.