Tuning In
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 communication sounds.   Links related to this story

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 Department.

"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 hears.

"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 humans.

"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 implications?"

by David Munson
March-April, 2007

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