English: Figure 4. Plasticity in Specific pathways enabling learning. (A) Avian song pathways and the tripartite hypotheses. A: avian brain areas involved in song production and song learning. Premotor pathway (open) includes areas necessary for song production. Anterior forebrain pathway (filled) is required for song learning but not for song production. (B) Tripartite reinforcement learning schema: the actor produces behaviour; the experimenter sends fluctuating input to the actor, producing variability in behaviour that is used for trial-and-error learning; the critic evaluates the behaviour of the actor and sends a reinforcement signal to it. For birdsong, the actor includes premotor song production areas HVC and RA. (C) Plastic and empiric synapses. RA receives synaptic input from both HVC and LMAN. We will call the HVC synapses “plastic,” in keeping with the hypothesis that these synapses are the locus of plasticity for song learning.