Population analysis of dopamine neuron (DAN) activity.

(A) First-order conditioning trials with positive or negative valence US. (B) Responses of model dopamine neurons from a single network. Neurons are sorted according to hierarchical clustering (illustrated with gray dendrogram) of their responses. (C) Principal components analysis of dopamine neuron population activity. Left: Response to CS before conditioning. Middle: Response to a positive (green) or negative (red) valence US. Right: Response to a previously conditioned CS.