Dorsomedial Brain Model

The dorsomedial nucleus of the intercollicular complex (DM) in both learners and non-learners is the center nucleus of the midbrain and it is necessary for vocalization and is analogous to the periaqueductal gray area (PAG) in a mammalian brain. Axons spread from here to two nuclei, the tracheosyringeal hypoglossal nucleus (nXllts) and the nucleus retroambigualis (RAm). The syrix is regulated by the nXllts and the RAm regulates the respiratory system. This was the vocal system of all birds for the majority of time until natural selection started acting upon certain anatomical features to improve and diversify until there was another circuit mechanism to which auditory signals could be stored and processed. It should be noted that Nottebohm

1976). The specifics of the neural system of song birds consists of both and anterior and posterior section which are responsible for difference pathways. The anterior circuit is responsible for a process that extends through the high vocal center (HVC) to an Area X, to the medial nucleus of the dorsolateral thalamus (DLM), to the lateral magnocellular nucleus of anterior nidopallium (LMAN) to the robust nucleus of the arcopallium (RA). The posterior pathways consists of the HVC to the RA to the nXllts (Matsunaga, E., Okanoya, K. 2009). Further inquiry suggests a disruption in vocal learning ability of juveniles when manipulating the anterior circuit with no consequence in fully matured individuals. Dislocation of the posterior circuit affects vocal production in both juveniles and adults indicating that the posterior circuit is responsible for the respiratory muscles necessary for vocalization and also the muscles associated with the syrinx (Marler, P., Slabbekoorn, H. 2004; Zeigler, P. H., Marler, P. 2008).

Sexual dimorphism can be seen within the Zebra Finch species as only the male finch sings. Two weeks after hatching all of the neural connections between the anterior and posterior circuits are formed except that off the HVC-RA.Within the next two weeks the HVC sends its axons to the outside of the RA nucleus. Nottebohm (1989) concluded the there is fluctuations in size of the HVC and that it was due to the death of neurons. He was able to prove there was neurogenesis allowing the HVC to rebuild itself giving some context as to the role of annual modification of song (Nottebohm, 1989). Only in male finches does this connection take place after thirty days allowing it to completley finish forming the song system (Holloway & Clayton 2001). Iyengar et al.