Extracellular Recordings from Human Brain Organoids Using High-density CMOS Arrays

Human brain organoids replicate much of the cellular diversity and
developmental anatomy of the human brain. However, the physiological
behavior of neuronal circuits within organoids remains relatively
under-explored. With high-density CMOS microelectrode arrays (26,400
electrodes) and shank electrodes (960 electrodes), we probed broadband and
three-dimensional extracellular field recordings generated by spontaneous
activity of human brain organoids. These recordings simultaneously
captured local field potentials (LFPs) and single-unit activity extracted
through spike sorting. From spiking activity, we estimated a directed
functional connectivity graph of synchronous neural network activity,
which showed a large number of weak functional connections enmeshed within
a network skeleton of significantly fewer strong connections. Treatment of
the organoid with a benzodiazepine induced a reproducible signature
response that shortened the inter-burst intervals, increased the
uniformity of the firing pattern within each burst and decreased the
population of weakly connected edges. Simultaneously examining the
spontaneous LFPs and their phase alignment to spiking showed that spike
bursts were coherent with theta oscillations in the LFPs. Our results
demonstrate that human brain organoids have self-organized neuronal
assemblies of sufficient size, cellular orientation, and functional
connectivity to co-activate and generate field potentials from their
collective transmembrane currents that phase-lock to spiking activity.
These results point to the potential of brain organoids for the study of
neuropsychiatric diseases, drug mechanisms, and the effects of external
stimuli upon neuronal networks.