Bird Vocalization - Neuroanatomy

Neuroanatomy

The acquisition and learning of bird song involves a group of distinct brain areas that are aligned in two connecting pathways :

• Anterior forebrain pathway (vocal learning): composed of Area X, which is a homologue to mammalian basal ganglia; the lateral part of the magnocellular nucleus of anterior neostriatum (LMAN), also considered a part of the avian basal ganglia; and the dorso-lateral division of the medial thalamus (DLM).
• Posterior descending pathway (vocal production): composed of HVC (proper name, although sometimes referred to as the high vocal center); the robust nucleus of the arcopallium (RA); and the tracheosyringeal part of the hypoglossal nucleus (nXIIts).

The posterior descending pathway (PDP) is required throughout a bird's life for normal song production, while the anterior forebrain pathway (AFP) is necessary for song learning in juvenilles and plasticity/maintenance in adults, but not for adult song production.

Both neural pathways in the song system begin at the level of HVC, which projects information both to the RA (premotor nucleus) and to Area X of the anterior forebrain. Information in the posterior descending pathway (also referred to as the vocal production or motor pathway) descends from HVC to RA, and then from RA to the tracheosyringeal part of the hypoglossal nerve (nXIIts), which then controls muscular contractions of the syrinx.

Information in the anterior forebrain pathway is projected from HVC to Area X (basal ganglia), then from Area X to the DLM (thalamus), and from DLM to LMAN, which then links the vocal learning and vocal production pathways through connections back to the RA. Some investigators have posited a model in which the connection between LMAN and RA carries an instructive signal based on evaluation of auditory feedback (comparing the bird's own song to the memorized song template), which adaptively alters the motor program for song output. The generation of this instructive signal could be facilitated by auditory neurons in Area X and LMAN that show selectivity for the temporal qualities of the bird's own song (BOS) and its tutor song, providing a platform for comparing the BOS and the memorized tutor song.

Models regarding the real-time error-correction interactions between the AFP and PDP will be considered in the future. Other current research has begun to explore the cellular mechanisms underlying HVC control of temporal patterns of song structure and RA control of syllable production. Brain structures involved in both pathways show sexual dimorphism in many bird species, usually causing males and females to sing differently. Some of the known types of dimorphisms in the brain include the size of nuclei, the number of neurons present, and the number of neurons connecting one nucleus to another. In the extremely dimorphic Zebra Finches (Taeniopygia guttata), a species in which only males typically sing, the size of the HVC and RA are approximately three to six times larger in males than in females, and Area X does not appear to be recognizable in females. Research suggests that exposure to sex steroids during early development is partially responsible for these differences in the brain. Female Zebra Finches treated with estradiol after hatching followed by testosterone or dihydrotestosterone (DHT) treatment in adulthood will develop an RA and HVC similar in size to males and will also display male-like singing behavior. Hormone treatment alone does not seem to produce female finches with brain structures or behavior exactly like males. Furthermore, other research has shown results that contradict what would be expected based on our current knowledge of mammalian sexual differentiation. For example, male Zebra Finches castrated or given sex steroid inhibitors as hatchlings still develop normal masculine singing behavior. This suggests that other factors, such as the activation of genes on the z chromosome, might also play a role in normal male song development.

Hormones also have activational effects on singing and the song nuclei in adult birds. In canaries (Serinus canaria), females normally sing less often and with less complexity than males. However, when adult females are given androgen injections, their singing will increase to an almost male-like frequency. Furthermore, adult females injected with androgens also show an increased size in the HVC and RA regions. Melatonin is another hormone that is also believed to influence song behavior in adults, as many songbirds show melatonin receptors in neurons of the song nuclei. Both the European Starling (Sturnus vulgaris) and House Sparrow (Passer domesticus) have demonstrated changes in song nuclei correlated with differing exposures to darkness and secretions of melatonin. This suggests that melatonin might play a role in the seasonal changes of singing behavior in songbirds that live in areas where the amount of daylight varies significantly throughout the year. Several other studies have looked at seasonal changes in the morphology of brain structures within the song system and have found that these changes (adult neurogenesis, gene expression) are dictated by photoperiod, hormonal changes and behavior.

The gene FOXP2, defects of which affect both speech production and comprehension of language in humans, becomes highly expressed in Area X during periods of vocal plasticity in both juvenile Zebra Finches and adult canaries.