A major goal of translational neuroscience is to identify neural circuit abnormalities in neuropsychiatric disorders that can be studied in animal models to facilitate the development of new treatments.¹ Oscillations in the gamma band (30-100 Hz) of the electroencephalogram (EEG) have received considerable interest in this effort because the basic mechanisms underlying these oscillations are understood² and are believed to be conserved across species. Schizophrenia (SZ) is characterized by abnormalities in gamma oscillations elicited by a variety of stimuli and tasks,^3,4 particularly deficits in the auditory steady-state response (ASSR) to gamma frequency stimulation.⁵ Dysfunctional gamma oscillations have been proposed to be caused by abnormalities in parvalbumin (PV)-expressing fast-spiking basket cells (PVBCs).⁶ The PVBCs are a critical element in neural circuits that generate gamma oscillaitons,⁷ and neuropathological studies have demonstrated abnormalities in PVBCs in SZ.⁶

An inter-regional cortical tract is one of the most fundamental architectural motifs that integrates neural circuits to orchestrate and generate complex functions of the human brain. To understand the mechanistic significance of inter-regional projections on development of neural circuits, we investigated an in vitro neural tissue model for inter-regional connections, in which two cerebral organoids are connected with a bundle of reciprocally extended axons. The connected organoids produced more complex and intense oscillatory activity than conventional or directly fused cerebral organoids, suggesting the inter-organoid axonal connections enhance and support the complex network activity. In addition, optogenetic stimulation of the inter-organoid axon bundles could entrain the activity of the organoids and induce robust short-term plasticity of the macroscopic circuit. These results demonstrated that the projection axons could serve as a structural hub that boosts functionality of the organoid-circuits. This model could contribute to further investigation on development and functions of macroscopic neuronal circuits in vitro.