Figure 2.

Motor and vocal learning pathways in songbirds and humans and the role of variability. (A) simplified diagram of the pathways involved in vocal learning and production in songbirds and humans. (i) In songbirds, the vocal motor pathway used for production of established song [shown in red: HVC, RA (robust nucleus of the arcopallium) and brainstem nucleus for vocal output] is used to produce the song. The vocal learning pathway [AFP: anterior forebrain pathway, shown in blue: Area X, dorsolateral nucleus of the medial thalamus (DLM) and LMAN (lateral magnocellular nucleus of the anterior nidopallium)] is used in songbirds during the acquisition of the pattern in song learning. (ii) In humans, the motor pathway (shown in red: laryngeal motor cortex and brainstem nucleus for vocal output), and the vocal learning pathway (shown in blue: anterior striatum, thalamus and premotor cortex). (B) Suggested levels of vocal variability when using the two pathways. I suggest that when using the motor pathway (i), production is stable, with little trial-by-trial variability. When using the vocal learning pathway (ii), trial-by-trial variability gradually increases, reflecting an exploratory phase in which the learners try out different ways of pronouncing the words (‘motor exploration’), before decreasing and stabilizing once the ‘best’ performance has been identified (‘motor exploitation’).