There is a remarkable genetic and
neural parallel between birdsong and human voice production, and the study
suggested that this may have important consequences for understanding the
evolution of auditory-vocal learning and its neural mechanisms.
Production of learned vocal
expressions is confined only to humans, bats, dolphins, elephants, and sea
lions among mammals, and in zebra finches, canaries, crows, parrots and
hummingbirds among birds.
Human brains are asymmetric and
language tends to be organized in the left hemisphere as opposed to the right.
Birds are also often assumed to show similar hemispheric specialization for
song. And the brain chemical responsible for this is dopamine.
Dopamine is a neurotransmitter
produced in several areas of the brain and plays an important role in brain
processes that control movement, emotional response, and ability to experience
pleasure and pain.
Cases have been reported where
dopamine receptor antagonists (drugs blocking dopamine receptors) temporarily
disrupted normal vocal motor control, especially seen in neurological and
psychiatric patients with voice and speech problems, such as Parkinson's
disease (PD), stuttering, Tourette's syndrome, and schizophrenia. The evidence
thus indicates the major role played by dopamine in proper execution of motor
commands and cognitive processing associated with speech production.
Similarly, in birds, dopamine
release from the front part of the brain and into the nerves in the brainstem
that control muscle movements triggers bird song.
The behavioral mechanisms of
vocal learning to produce human speech and birds song are thought to be similar
because 'both have the critical periods for vocal learning in early life before
adulthood and require auditory experience and feedback to develop and maintain
their learned vocal sequences'.
However, there are a couple of
differences, according to the researchers. First, while humans are able to
learn new vocal expressions throughout the lifespan, some birds such as zebra
finches are not able to imitate new songs well, if at all, in adulthood, and
others such as African Grey parrots, have only limited ability to imitate new
sounds as adults. Secondly, birds use most of their songs for affective
communication while human speech use semantic communication.
The researchers concluded 'Due to
similarities with humans in the ability to produce learned vocalizations,
songbirds present a potentially useful model to study some shared mechanisms of
dopaminergic transmission underlying speech and song production. However, as
humans possess several unique features of speech control, human dopaminergic
regulation may need to be studied in humans only'.
Simonyan, K., et al. Dopamine regulation of human
speech and bird song: A critical review. Brain & Language (2012),