TRIO (Tracing the Relationship of Inputs and Outputs) experiments systematically inject complementary viruses into different target brain regions to monosynaptically label inputs to a specific projection neuron population. TRIO experiments use 3 different viruses (AAVretro-Ef1a-Cre, AAV8-hSyn-FLEX- TVA-P2A-GFP-2A-oG helper virus, and EnvA pseudotyped G-deleted Rabies mCherry (EnvA RVdG-4mCherry).