Masters of songs, and inspiring the works of the literati and artists of the past as well as the present, songbirds are an intriguing species. Their mellifluous string of songs is what fascinated renowned poets such as Wordsworth, Keats, and Shelley, and painters such as Imru Al Qays, Ren Yi, and George Baselitz to place the magnificent songbird into their works.
This blog post aims to briefly shed light on the vocal learning behavior in songbirds, with a slight emphasis on the importance of the neurotransmitter dopamine, in vocal processing.
More than four thousand species of songbirds, almost fifty percent of the bird species, entertain human ears around the world with their unique melodious songs (Innovateus, n.d). They include Nightingales, Mockbirds, Musk Duck, Bengalese finches, American robins, Eastern bluebirds, Northern cardinals, to name but a few. Songbirds inhabit a diverse range of areas, encompassing open fields, woodlands, and other terrestrial habitats, as well as vegetated wetlands and shores.
Songs are an important means of communication among songbirds. They are instrumental when demarcating territories, signaling genetic quality to potential mates, and are even a significant driver in speciation. (Elemans, 2014)
The highly intricate songs of songbirds are a fascinating production. They possess specialized vocal organs and brain regions that enable them to form elaborate and various songs in order to communicate. Each individual in each species has its characteristic way of singing, analogous to how each human individual has their own voice and way of talking.
Songbird studies are vital in understanding similar learning patterns in humans. Using auditory feedback to correct mistakes in behavior include speaking, singing and playing an instrument. (Emory Health Sciences, 2016)
Switching first to anatomical structure of the brain, the basal ganglia is an important region in sound control. It is located at the base of the forebrain (shown in pink in the figure above), and is responsible for various functions such as procedural learning and voluntary motor movements, including speech and song behavior (Emory Health Sciences, 2016). Dopaminergic and glutamatergic neurons both exist in the basal ganglia and they are both important in the functioning of the basal ganglia. However, this paper highlights the significance of dopamine.
Certain similarities and differences exist between the basal ganglia of humans and songbirds. A major resemblance of the basal ganglia circuitry between birds and humans is the existence of the closed cortico-basal ganglia-thalamo-cortical loops. Similar to humans, the songbird basal ganglia consists of the striatum (a major portion of the basal ganglia), pallidum, subthalamic nucleus (STN), and the substantia nigra (SN, composed of: substantia nigra pars compacta SNc; and substantia nigra pars reticulate SNr).
The difference, however, between human and bird speech/song production, is where humans utilize the pallidum, SNr and STN for vocal processing, these same regions are not regarded essential components of the bird song system. Instead, songbirds possess a specialized song region in their (dorsal) striatum called Area X. An analog of Area X is present only in song learning bird pedigrees (i.e. songbirds, hummingbirds and parrots), but not in other song non-learning birds studied up to now (Simonyan, Horwitz, & Jarvis, 2012)
During singing, dopamine is released from the songbird VTA and SNc into Area X. Area X is highly active with increased firing of neurons, and context-dependent regulation of plasticity-related genes.
If songbirds hear a tutor sing, they create an auditory memory of the song, which is referred to as an auditory template. When the bird attempts mimicking the song, it refers to its auditory template and practices and monitors its own singing, looking for errors. At this point, it is singing for practice and vocal explorations. This is when the bird is performing undirected singing and at this stage, the neuronal firing rates in its Area X and song pitch vary considerably and the plasticity-induced gene expression is high. During undirected singing, dopamine levels in Area X are low.
When the bird detects an error, it tries correcting the song by referring to its auditory template and keeps on correcting the song until it considers that the song is perfect. It is this perfected version, then, of the song that it presents and sings for attracting a female. On this occasion, the bird is performing directed singing and so its neuronal firing rates and song pitch are more stable and the plasticity-induced gene expression is low. During directed singing, dopamine levels in Area X are quite high. (Simonyan, Horwitz, & Jarvis, 2012)
A recent study at Emory Health Sciences conducted an experiment on song processing using Bengalese finches. Finches have extraordinarily precise singing behavior and possess the ability to polish “their songs in response to auditory feedback”, according to Emory biologist Samuel Sober (Emory Health Sciences, 2016)
The team conducted a simple experiment where they recorded the singing of each finch and interrupted them with an occasional burst of white noise while the bird sang ‘within a particular range of pitches’ (Emory Health Sciences, 2016). This burst of noise is interpreted as an error by the finch. Researchers measured how rapidly each bird changed its pitch to evade the white noise.
They then administered a drug to the finches that lowered the dopamine levels in their basal ganglia by about half. The finches’ singing was recorded again. It was found that their ability to fix their mistakes in response to hearing the white noise also decreased by half. They were still able to learn from the white noise ‘mistake’, but were considerably worse at it. Within the basal ganglia in the fiches’ brain, a reduction in dopamine levels resulted in a reduction in their ability to correct vocal mistakes, while having no effect on their ability to sing. (Emory Health Sciences, 2016)
The researchers concluded that dopamine is crucial in sensory-motor learning and is important in correcting vocal mistakes.
Evidence presented in this blog post along with numerous other neurologic and psychiatric studies establish that dopamine is vital in speech production, motor commands and in processing vocal errors. The subject of future studies on songbirds will be centered on investigating the effect of dopamine levels on activity in the basal ganglia, at the molecular and cellular level. (Emory Health Sciences, 2016). Insights gained from studies of this nature on songbirds, may be extended to speech production and vocal control in humans.
This is where the practical implications of studies on songbirds are related to Parkinson’s disease in humans. This disease is characterized by the loss of (dying off of) these dopaminergic neurons in the SNc over the years, and these are associated with vocal problems in Parkinson’s disease patients (Emory Health Sciences, 2016). This type of research may be instrumental in uncovering more secrets as to the role of dopamine in vocal behavior and in the development of potential and effective therapies for learning deficits like Parkinson’s, Tourette’s Syndrome, stuttering and the likes (Simonyan, Horwitz, & Jarvis, 2012)
Elemans, C. P. (2014). The singer and the song: The neuromechanics of avian sound production.Current Opinion in Neurobiology, 28, 172-178. doi:10.1016/j.conb.2014.07.022
Emory Health Sciences. (2016, March 15). Dopamine key to vocal learning, songbird study finds: Experiments are the first to isolate the role of dopamine in sensory-motor learning. ScienceDaily. Retrieved April 20, 2016 from www.sciencedaily.com/releases/2016/03/160315120951.htm
Innovateus. (n.d.). Facts on Songbirds. Retrieved April 28, 2016, from http://www.innovateus.net/green-home/facts-songbirds
Simonyan, K., Horwitz, B., & Jarvis, E. D. (2012). Dopamine regulation of human speech and bird song: A critical review. Brain and Language, 122(3), 142-150. doi:10.1016/j.bandl.2011.12.009