1. A method of producing an in vitro system in which there are connections between one or more cortical neurons and one or more dopaminergic neurons, comprising: culturing cortical neuron progenitor cells in a first compartment of a culture dish and culturing dopaminergic neuron progenitor cells in a second compartment of the culture dish, wherein the first compartment and the second compartment are separated by a removable barrier that creates a gap; removing the barrier; and culturing the cortical neuron progenitor cells and the dopaminergic neuron progenitor cells under conditions sufficient to produce differentiation of the cortical neuron progenitor cells to cortical neurons and differentiation of the dopaminergic neuron progenitor cells to dopaminergic neurons, wherein the at least one of the cortical neurons forms a connection with at least one of the dopaminergic neurons.