HUMANS, SPARROWS MAKE SENSE OF SOUNDS IN SIMILAR WAYS
The song of the swamp
sparrow -- a grey-breasted bird found in wetlands throughout much of North
America -- is a simple melodious trill, repeated over and over again.
It's kind of like a
harmonious police whistle," said biologist Stephen Nowicki.
But according to a new
study by Duke University scientists Nowicki and Robert Lachlan, swamp sparrows
are capable of processing the notes that make up their simple songs in more
sophisticated ways than previously realized -- an ability that may help researchers
better understand the perceptual building blocks that enable language in
humans.
The study appears in
the journal Proceedings of the National Academy of Sciences.
From the finite types
of sounds that make up a stream of speech -- such as the "c" sound in
"cat" or the "b" sound in "boy" -- humans are
able to create and make sense of an almost infinite number of words and sentences
about the present, past and future, unconsciously and automatically.
What's more, how
humans perceive speech sounds is influenced by context, said Lachlan, now of
Queen Mary University of London.
In American English,
for example, listeners recognize that the "t" in "city" and
the "d" in "ready" belong to different categories, even
though they're frequently pronounced the same.
Lachlan and Nowicki
wanted to know if this common aspect of understanding spoken language, called
partial phonemic overlapping, is found in birds, too.
To find out, they
recorded and analyzed the songs of 206 male swamp sparrows near Pymatuning Lake
in Pennsylvania.
Statistical analysis
revealed that the short repeated syllables that make up each song consist of
subsets of roughly 10 types of notes.
In two experiments, the
researchers compared males' territorial responses to songs in which either the
first note or the last note of each syllable was substituted with a note of a
different type -- either short, intermediate or long.
How the birds
perceived a particular note depended on where it fell in a snippet of song.
The birds responded to
the modified songs with an aggressive territorial display when the note
substitution occurred in one position in the snippet, but much more weakly or
not at all when the same note was substituted in another position -- indicating
that the birds are able to assign the same sound to different categories of
notes depending on the context in which it appears.
The study is part of a
larger body of research aimed at understanding how language arose in humans by
studying different forms of communication in animals.
Human language draws
on a complex set of cognitive skills, some of which are also found in
songbirds. That fact alone is not entirely surprising to scientists, especially
in light of recent research led by Duke's Erich Jarvis showing that songbirds
and humans rely on many of the same genes to sing and speak.
But what's exciting
about their results, the researchers say, is it suggests that the ability to
perceive speech sounds differently in different contexts -- a critical skill
necessary for the perception of human speech -- could have arisen before,
rather than after, other aspects of human language such as semantics and syntax
came to be.
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