The neurophysiology of birds' auditory sorting process
offers clues to how humans discriminate between sounds
A spectrogram of a zebra finch song shows how the frequency or pitch
changes over time. Dark blue indicates no sound; red, high
intensity. The song is composed of syllables (short .1- to .2-second
sounds separated by silence), which create repeating motifs
(syllables from 0 to .8 seconds are repeated again from .9 to 1.6
seconds). Like humans, songbirds learn to produce their
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Many colorful descriptors could be used to characterize the
life of the average suburban songbird, but quiet certainly isn't one
of them. Scratching and pecking their way through the hustle and
bustle of the urban landscape, house finches, English sparrows and a
myriad of other urban species pursue life amid a cacophony of sound.
Speeding cars, rattling trains and whining sirens all contribute to
the auditory landscape within which the birds must function.
According to University of Maine researcher Thane Fremouw, a bird's
nervous system responds to all that noise much like ours does, tuning
out the superfluous to avoid auditory overload. Still, selecting the
truly important sounds remains critical, even for those birds that
have abandoned field and forest for the bounty of suburban feeders and
downtown dumpsters. From finding food and avoiding predators to
locating mates and identifying members of the flock, being in tune to
the sounds of the environment is key to birds' survival.
What Fremouw discovered is that songbirds are able to sort out certain
sounds based on the temporal and spectral modulations likely to be
most important to them, effectively homing in on sounds that are
different than the often repetitive background chatter of everyday
life. By allocating more of their nervous system resources to sound
patterns that differ from the norm, they are making the most of their
listening abilities — and their brains.
"We typically think of the brain as having some limited attentional
capacity. From a neuroscience perspective, we wanted to look at how
the auditory system can optimize its abilities through specialized
processes," says Fremouw, a recent addition to the UMaine Psychology
"Traditional auditory neurophysiology focused on playing pure tones
and simple sounds to measure neuronal response. There is evidence that
such reductionistic approaches might be a little misleading. There
might be a benefit to looking at how birds process very complex
sounds, including the whole song. By playing complex sounds and using
normalized reverse correlations that get at the specific frequency and
timing of the auditory processes, we were able to create maps of the
neuronal response that show how the birds respond differently to one
part of the song than they do to another."
Fremouw was then able to apply the birds' neural response maps to bird
song data from the field to show that they were allocating more
processing capacity to specific parts of the song.
"They were concentrating their resources on the outliers to
discriminate between songs, homing in on certain sounds," says Fremouw.
"They were finding more efficient ways to process sound."
The neurophysiology of this type of auditory sorting process in birds
not only offers clues to how they respond to their environment, it
also helps to provide the basis for understanding how humans
discriminate between sounds and how the human brain processes what it
"How we code information in the brain depends on the nature of the
stimulus. It's important to understand how the system optimizes itself
so that we can discover how best to treat individuals with hearing
problems," says Fremouw.
In a related line of research, Fremouw is working to bridge the gap
between the physiology-based research being done on birds and other
animals, and the study of human language and consciousness. By looking
at the neurological basis for processing spatial relationships,
categorization techniques and memory, and comparing that data to what
is known about those same factors in humans, he hopes to provide
scientists with better tools for understanding the similarities and
differences between humans and other animals.
"In many areas, there seems to be a rift between the research work on
humans and the studies being done on animals related to consciousness
and language," says Fremouw. "Studies on humans rely heavily on
language and consciousness to measure memory and categorization, while
the work on the neuronal level is happening in animal research. Quite
frequently, we tend to view having consciousness as being the same as
having language, and there are already some examples where this simply
isn't the case."
Fremouw is working to build connections between the separate worlds of
human and animal research by studying how pigeons behave in
categorization experiments. By comparing the pigeons' performance with
that of humans, Fremouw is able to map the relationships and fine-tune
the experiments to create a bird model that applies to human research
in a meaningful way. By identifying a clear connection between humans
and birds in behavioral and cognition studies, Fremouw hopes
researchers will be better able to apply what is known about the
physiology and chemistry of the avian nervous system to research for
"This research could not only help us to answer specific questions,
like determining which neurotransmitters and neuronal circuits
affected by Parkinson's disease play a major role in cognition, it
also gets at what it means to have language," he says. "Songbirds can
create novel arrangements and develop new strings that are
grammatical. They have some of the same processes that are involved in
language. This is an opportunity to find out what brain functions
allow that type of processing. It says a lot about what the animal
mind is like. Are they like us? And, if so, what are the
by David Munson
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