Sunday, March 16, 2008

Birdsong gives voice and motion to the mysteries of speech


A new study of vocalization in birds sheds possible light on the relationship between voice and movement in humans and other mammals:

Movement-Associated Areas in the Avian BrainGesa Feenders et al., Molecular Mapping of Movement-Associated Areas in the Avian Brain: A Motor Theory for Vocal Learning Origin, PLoS ONE [Public Library of Science] 3(3): e1768. doi:10.1371/journal.pone.0001768 (March 12, 2008):

Bird brainsVocal learning is a critical behavioral substrate for spoken human language. It is a rare trait found in three distantly related groups of birds — songbirds, hummingbirds, and parrots. These avian groups have remarkably similar systems of cerebral vocal nuclei for the control of learned vocalizations that are not found in their more closely related vocal non-learning relatives. These findings led to the hypothesis that brain pathways for vocal learning in different groups evolved independently from a common ancestor but under pre-existing constraints. Here, we suggest one constraint, a pre-existing system for movement control. Using behavioral molecular mapping, we discovered that in songbirds, parrots, and hummingbirds, all cerebral vocal learning nuclei are adjacent to discrete brain areas active during limb and body movements. Similar to the relationships between vocal nuclei activation and singing, activation in the adjacent areas correlated with the amount of movement performed and was independent of auditory and visual input. These same movement-associated brain areas were also present in female songbirds that do not learn vocalizations and have atrophied cerebral vocal nuclei, and in ring doves that are vocal non-learners and do not have cerebral vocal nuclei. A compilation of previous neural tracing experiments in songbirds suggests that the movement-associated areas are connected in a network that is in parallel with the adjacent vocal learning system. This study is the first global mapping that we are aware for movement-associated areas of the avian cerebrum and it indicates that brain systems that control vocal learning in distantly related birds are directly adjacent to brain systems involved in movement control. Based upon these findings, we propose a motor theory for the origin of vocal learning, this being that the brain areas specialized for vocal learning in vocal learners evolved as a specialization of a pre-existing motor pathway that controls movement.

The upshot of this study, as described in Science Daily, is that neural pathways affecting basic motor control hold the key to the neurology of birdsong — and quite possibly of speech in humans as well. If areas in charge of movement in three distantly related avian taxa share many functional similarities with the brain areas for singing, then brain pathways used for vocal learning in humans may have evolved out of the brain pathways used for motor control.

Tree of vertebrate lifeThis study may also explain why humans talk by gesture and by voice, but chimps talk only with their hands. "In its most specialized way, spoken language is the ability to control the learned movements of our larynx," senior author Erich Jarvis told Science Daily. "It's possible that human language pathways have also evolved in ways similar to these birds. Perhaps the evolution of vocal learning brain areas for birds and humans exploited a universal motor system that predates the split from the common ancestor of birds and mammals."

According to National Institutes of Health Director Elias A. Zerhouni, "The discovery that vocal learning brain pathways are embedded in the parts of the brain that control body movement offers unexpected insights on the origins of spoken language and could open up new approaches to understanding vocalization disorders in humans."

While all birds vocalize, most avian sounds are genetically dictated. Only songbirds, parrots, and hummingbirds can learn new songs. That skill resembles human speech, and the resemblance quite possibly has a shared genetic basis. Human speech may trace its origins 300 million years back to a type of vocal learning enabled by the neural pathways of stem amniotes, the common ancestor of reptiles, birds, and mammals.


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