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Bird vocalization
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==Neuroanatomy== [[Image:Birdbrain.svg|thumb|Song-learning pathway in birds<ref name="Nottebohm 2005"/>]] The acquisition and learning of bird song involves a group of distinct brain areas that are aligned in two connecting pathways:<ref name="Nottebohm 2005" /> * Anterior forebrain pathway ([[vocal learning]]): composed of Area X, which is a homologue to mammalian basal ganglia; the lateral part of the [[magnocellular cell|magnocellular]] nucleus of anterior nidopallium (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).<ref name="Brainard 2000 31-40">{{cite journal|author1=Brainard, M. S.|author2=Doupe, A. J.|name-list-style=amp|year=2000|title=Auditory feedback in learning and maintenance of vocal behavior|journal=Nature Reviews Neuroscience|volume=1|issue=1|pages=31β40|doi=10.1038/35036205|pmid=11252766|s2cid=5133196}}</ref><ref name="carew">{{cite book|title=Behavioral Neurobiology: The Cellular Organization of Natural Behavior|author=Carew, Thomas J.|publisher=Sinauer Associates|year=2000|isbn=978-0-87893-092-0}}</ref> 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, plasticity, and maintenance, but not for adult song production.<ref name="Kao2005">{{cite journal|author1=Kao, M.H.|author2=Doupe, A.J.|author3=Brainard, M.S.|year=2005|title=Contributions of an avian basal ganglia-forebrain circuit to real=time modulation of song|journal=Nature|volume=433|issue=7026|pages=638β642|doi=10.1038/nature03127|pmid=15703748|bibcode=2005Natur.433..638K|s2cid=4352436}}</ref> Both neural pathways in the song system begin at the level of [[high vocal center|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 [[high vocal center|HVC]] to RA, and then from RA to the tracheosyringeal part of the [[hypoglossal nerve]] (nXIIts), which then controls muscular contractions of the syrinx.<ref name="Nottebohm 2005" /><ref>{{cite book|title=Nature's music:The science of birdsong|author=Suthers, R.|publisher=Academic Press|year=2004|isbn=978-0-12-473070-0|editor=Marler, P.|pages=272β295|chapter=How birds sing and why it matters|editor2=Slabbekoorn, H.}}</ref> Information in the anterior forebrain pathway is projected from [[high vocal center|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.<ref name="Kao2005" /><ref name="Brainard_2000b">{{cite journal|author1=Brainard, M. S.|author2=Doupe, A. J.|name-list-style=amp|year=2000|title=Interruption of a basal ganglia-forebrain circuit prevents plasticity of learned vocalizations|journal=Nature|volume=404|issue=6779|pages=762β766|doi=10.1038/35008083|pmid=10783889|bibcode=2000Natur.404..762B|s2cid=4413588}}</ref> 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.<ref name="Brainard_2000b" /><ref name="Kojima2008">{{cite journal|author1=Kojima, S.|author2=Doupe, A.|year=2008|title=Neural encoding of auditory temporal context in a songbird basal ganglia nucleus, and its independence of birds' song experience|journal=European Journal of Neuroscience|volume=27|issue=5|pages=1231β1244|doi=10.1111/j.1460-9568.2008.06083.x|pmc=2408885|pmid=18364039}}</ref> 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 [[high vocal center|HVC]] control of temporal patterns of song structure and RA control of syllable production.<ref>{{cite journal|author1=Long, M.A.|author2=Jin, D.Z.|author3=Fee, M.S.|year=2010|title=Support for a synaptic chain model of neuronal sequence generation|journal=Nature|volume=468|issue=7322|pages=394β399|doi=10.1038/nature09514|pmc=2998755|pmid=20972420|bibcode=2010Natur.468..394L}}</ref> 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.<ref name="BalthazartAdkinsRegan2002">{{Cite journal|title=Sexual differentiation of brain and behavior in birds|last1=Balthazart |first1=Jacques|last2=Adkins-Regan |first2=Elizabeth |journal=Hormones, Brain and Behavior|year=2002|isbn=9780125321044|volume=4|issue=1|pages=223β301|doi=10.1016/b978-012532104-4/50068-8|pmid=18406680}}</ref> 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.<ref>{{cite journal|author1=Nottebohm, F.|author2=Arnold, A.P.|name-list-style=amp|year=1976|title=Sexual dimorphism in vocal control areas of the songbird brain|journal=Science|volume=194|issue=4261|pages=211β213|doi=10.1126/science.959852|pmid=959852|bibcode=1976Sci...194..211N}}</ref> 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.<ref>{{cite journal|author1=Gurney, M.E.|author2=Konishi, M.|name-list-style=amp|year=1980|title=Hormone-induced sexual differentiation of brain and behavior in zebra finches|journal=Science|volume=208|issue=4450|pages=1380β1383|doi=10.1126/science.208.4450.1380|pmid=17775725|bibcode=1980Sci...208.1380G|s2cid=11669349}}</ref> 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.<ref name="BalthazartAdkinsRegan2002"/> 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.<ref>{{cite journal|author1=Tomaszycki, M.L.|author2=Peabody, C.|author3=Replogle, K.|author4=Clayton, D.F|author5=Tempelman, R.J.|author6=Wade, J.|year=2009|title=Sexual differentiation of the zebra finch song system: potential roles for sex chromosome genes|journal=BMC Neuroscience|volume=10|pages=24|doi=10.1186/1471-2202-10-24|pmc=2664819|pmid=19309515 |doi-access=free }}</ref> 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.<ref>{{cite journal|last1=Leonard|first1=S. L.|date=1 May 1939|title=Induction of Singing in Female Canaries by Injections of Male Hormone.|journal=Experimental Biology and Medicine|volume=41|issue=1|pages=229β230|doi=10.3181/00379727-41-10631|s2cid=87078020}}</ref> Furthermore, adult females injected with androgens also show an increased size in the HVC and RA regions.<ref>{{cite journal|author=Nottebohm, F.|year=1980|title=Testosterone triggers growth of brain vocal control nuclei in adult female canaries|journal=Brain Research|volume=189|issue=2|pages=429β36|doi=10.1016/0006-8993(80)90102-X|pmid=7370785|s2cid=25845332}}</ref> [[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.<ref>{{Cite journal|title=Neuroendocrine mechanisms regulating reproductive cycles and reproductive behavior in birds|author1=Ball, G.F.|author2=Balthazart, J.|journal=Hormones, Brain, and Behavior|year=2002|isbn=9780125321044|volume=2|pages=649β798|doi=10.1016/b978-012532104-4/50034-2|name-list-style=amp}}</ref> 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.<ref>{{cite journal|author1=Bentley, G.E.|author2=Van't Hof, T.J.|author3=Ball, G.F.|year=1999|title=Seasonal neuroplasticity in the songbird telencephalon: A role for melatonin|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=96|issue=8|pages=4674β4679|doi=10.1073/pnas.96.8.4674|pmc=16391|pmid=10200321|bibcode=1999PNAS...96.4674B|doi-access=free}}</ref><ref>{{cite journal|author1=Cassone, V.M.|author2=Bartell, P.A.|author3=Earnest D.J.|author4=Kumar, V.|name-list-style=amp|year=2008|title=Duration of melatonin regulates seasonal changes in song control nuclei of the house sparrow, Passer domesticus: Independence from gonads and circadian entrainment|journal=Journal of Biological Rhythms|volume=23|issue=1|pages=49β58|doi=10.1177/0748730407311110|pmid=18258757|s2cid=206544790}}</ref> 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.<ref>{{cite journal|author1=Ball, G.F.|author2=Auger, C.J.|author3=Bernard, D.J.|author4=Charlier, T.D.|author5=Sartor, J.J.|author6=Riters, L.V.|author7=Balthazart, J.|year=2004|title=Seasonal plasticity in the song control system: Multiple brain sites of steroid hormone action and the importance of variation in song behavior|journal=Annals of the New York Academy of Sciences|volume=1016|issue=1|pages=586β610|doi=10.1196/annals.1298.043|pmid=15313796|bibcode=2004NYASA1016..586B|s2cid=42818488}}</ref><ref>{{cite journal|author1=London, S.E.|author2=Replogle, K.|author3=Clayton, D.F.|year=2009|title=Developmental shifts in gene expression in the auditory forebrain during the sensitive period for song learning|journal=Developmental Neurobiology|volume=69|issue=7|pages=436β450|doi=10.1002/dneu.20719|pmc=2765821|pmid=19360720}}</ref> 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.<ref>{{cite journal|last1=Scharff |first1=Constance|last2=Haesler |first2=Sebastian |year=2005|title=An evolutionary perspective on FoxP2: strictly for the birds?|journal=Current Opinion in Neurobiology|volume=15|issue=6|pages=694β703|doi=10.1016/j.conb.2005.10.004|pmid=16266802|s2cid=11350165}}</ref>
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