21.2 C
Wednesday, October 5, 2022
Dr. Brown: Birdsong, human speech and how practice really can make perfect
Dr. William Brown
Dr. William Brown

No less than Charles Darwin, the father of evolution by natural selection, speculated a century and a half ago that birdsong might be among the many evolutionary precursors to human language.

Turns out he may have been right as new evidence suggests the neural substrate for both share important features.

In the case of the zebra finch and probably most of the other songbirds that make up about half of the estimated 10,000 species of birds worldwide, birdsong is used as a personal calling card for courtship and other social interactions.

For mature birdsong, as David Clayton framed it recently in the Journal Science, “is a precisely structured performance, consisting of a few short sound elements ('syllables') repeated in an exact order, each burst lasting one to two seconds. The syllables can have differing complex structures, with stacks of harmonic frequencies and varied amplitude.”

And each bird’s song differs slightly from the songs of others, making for personalized versions of the birdsong.

Songbirds begin learning birdsong with 20 to 30 days of hatching and continue to learn and perfect it over the next several weeks. Initially, they learn by listening, but it isn’t long before they begin to imitate the birdsong of their elder (usually the father) and by repeated practice, gradually, refine their song until it becomes a close facsimile of their elder’s song.

Here it gets interesting because all that listening and practice creates a remarkably exact physiological template for the song in a special region of the brain, which carries the fancy name of pallial sensorimotor nucleus interfacialis of the nido pallium or mercifully for short, NIf.

To create the song, the latter region projects to a premotor region in the bird’s brain which activates the requisite motor nerve cells and their related muscles in their proper order in the voice tract. To illustrate the importance of the NIf as the region that stores the all-important memory for the birdsong, investigators showed they could impose their own version of birdsong by repeated electrical stimulation of the same area.

Apparently, humans do much the same thing by listening, as early as within the womb, to the voices of their parents and others. Later, within a few months, they begin to copy their elders' sounds, refining them by constant practice and in the process, like the songbirds, create auditory templates in higher level association cortex surrounding the primary auditory cortex in the temporal lobes.

These templates for syllables and other oral speech constructions are connected to premotor cortex which, again as with the songbird, activate appropriate motor systems in the oropharyngeal and laryngeal regions to create oral speech.

So as simple as the system may be in songbirds, Darwin was right – the ancestors of songbirds and other species that communicate by vocalization, acted as evolutionary prototypes for the more complex vocalizations of highly intelligent species to follow, including humans.

Or perhaps birdsong and hominin speech are better thought of as examples of convergent evolution – two very different species arrive at similar solutions for similar challenges – communicating with others of their kind.

It’s worthwhile pointing out that learning other motor skills such as skiing or playing the piano are also acquired by imitation and relentless practice, creating along the way, sensory memories for the tasks – which become more and more refined and fluid with practice and experience – to the point where most haven’t much, if any, conscious awareness of how they do familiar tasks.

At the other end of life, many of those same overlearned sensor-motor skills may be lost, at least partially so, as neurodegenerative and vascular diseases destroy the parts of the brain containing the sensor-motor memories on which those tasks depend.

There’s yet another point here. It turns out that songbirds have an extra disposable chromosome whose genes may play a pivotal role in learning birdsong.

But once mating is over, the chromosome disappears, possibly as a metabolic cost-saving measure, only to reappear again when the next mating season arrives! Nothing like that in humans. Extraordinary.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the InfoHealth series at the Niagara-on-the-Lake Public Library.