Marcelo Magnasco, who recently joined ICTP's condensed matter physics group, has spent the past decade exploring biological and physical phenomena behind our sense of hearing.

Hear Here

When we hear a songbird sing, the melodious sounds often generate warm gentle emotions. Sweet-sounding sounds have the ability to do that to us.
However, when a songbird hears another songbird sing, the emotions and reactions are often much more significant.
"Song for a songbird is a matter of life and death. The harmonious sounds that they make are used, for example, to signal territorial rights and sexual intentions," says Marcelo Magnasco, who joined ICTP's condensed matter physics group last August after spending the previous decade with Rockefeller University in New York City. "Male songbirds often sing continuously for two or three hours before mating to show their potential partner their fitness for fathering birdlings."
Despite vast differences in purpose and reaction, hearing among songbirds and humans have this much in common: Both must somehow transform sound waves into nerve signals that can be received and interpreted by the brain.

Marcelo Magnasco

"Hearing," explains Magnasco, "is in many ways the least well understood of our senses. We can produce technological analogues of our sense of sight in cameras, and of our sense of smell in detectors. But, except for implants that operate only at marginal efficiency, we have yet to produce a technological analogue for our hearing organ--the cochlea."
Magnasco's research, which he has conducted in partnership with biologists and physicists (both theorists and experimentalists), has sought to expand our understanding of how organisms hear what they hear and respond to those sounds in particular and predictable ways.
"Only a very few organisms in addition to humans have the ability to learn and replicate 'meaningful' vocalised sounds," notes Magnasco. "Whales, dolphins, and songbirds such as canaries, hummingbirds and zebra finches are among those in this select group."
He and his colleagues chose to study songbirds for obvious reasons. "Transporting whales into our laboratory to examine their brain's neuron-firing patterns and the biological and physical dynamics that drive their sense of hearing," Magnasco wryly notes, "would be a difficult task."
One research project conducted by Magnasco and his colleagues has shown that the wide range of tonal sounds generated by songbirds is due to a complex series of behavioural and physiological responses that begin with a songbird's ability to alter the pressure and velocity of the air passing through its vocal organ, the syrinx. The syrinx, in turn, channels the air to the songbird's two bronchial passages just where these passages meet the windpipe. The labia or tissue flaps that lie between the bronchial passages and windpipe vibrate in response to the air current. A diverse set of melodious tunes is created by the joint action of the pressure created in the syrinx and the stiffness of the labia. Think of the syrinx as the body of a clarinet and the labia as the reed. By manipulating the two, a songbird can croon a wide variety of songs.
In another research project, Magnasco and a group of researchers investigated how songbirds can distinguish between sounds that have a similar pitch and tone but originate from different sources--for example, the same tune made by a bird and imitated electronically. "When songbirds hear another songbird sing, our laboratory research reveals intense neural firings and brainwave activity in specific locations. When the sound is reproduced electronically, no neural firing takes place in the same locations."
Clearly, songbirds know the difference. Or, more accurately, songbirds recognise and respond to a 'real' song sung by a 'real' songbird but do not respond to the same song when it is 'artificially' replicated. As science writer Henry Gee commented in Nature, these research findings raise intriguing questions about whether what we hear is actually taking place or is solely a reflection of what we are genetically predisposed and trained to hear.
But the most noteworthy research Magnasco has been involved in--research that has attracted widespread attention in both the scientific and popular press--has challenged some basic assumptions concerning the dynamics behind our sense of hearing.
This research has drawn not only on laboratory studies examining brain wave behaviour in songbirds, but also on a deep understanding of theoretical physics applied to studies examining how a mammal's spiral-like hearing organ--the cochlea--may function.
Historically, scientists believed that the cochlea passively absorbed sound waves, transformed the waves into nerve impulses, and then electronically transmitted the impulses to the brain in a step-by-step linear process.
Magnasco, however, has been involved in a series of research initiatives showing that the cochlea doesn't just passively accept and then transmit sound waves to the brain, but actually modulates the waves much like a public address system mixes together the original sound and its feedback. Or, to change the metaphor, much like a turbo engine uses the pressure created by the flow of its exhaust gases to compress the gasoline/oxygen mix.
"The result," Magnasco explains, "is that faint sounds detected by the cochlea are amplified, which may explain why mammals, among all organisms, have the capability to hear faint tones. Conversely, when the sound waves reach a high screeching pitch, the cochlea has the ability to narrow the sound waves into a reasonable range that can be tolerated and interpreted by the brain."
Magnasco's research has continually blurred the boundaries between disciplines, particularly between physics and biology. Trained as a theoretical condensed matter physicist at the University of Chicago, USA, his knowledge and understanding of biology and neuroscience has been acquired largely through intricate laboratory experiments and studies involving songbird brain tissue to determine how a songbird's brain reacts to acoustic stimuli. It has been a form of on-the-job training that has allowed Magnasco to cross disciplines without being labelled a renegade by physicists or an intruder by biologists.
Magnasco realises that the vastly different mindsets that have shaped the study of physics and biology have kept the two disciplines far apart. But several recent developments may now be driving the two closer together, particularly in the study of such phenomena as the dynamics of hearing, which involves a deep understanding of many different fields of inquiry.
"First," Magnasco notes, "the sheer volume of data and information related to biological phenomena, made possible by electronic data collection, renders it useful--and perhaps imperative--to develop overarching principles and perhaps models that can help provide a framework for deciphering information that may otherwise become overwhelming. Physicists, who are trained to develop and use theories and models, could assist biologists in making more effective use of the information they acquire and in uncovering connections that may be overlooked."
"Second," Magnasco observes, "physicists have shown an increasing interest in applying their skills in areas beyond the conventional boundaries of their discipline. And third, both biologists and physicists (and, for that matter, chemists and neuroscientists) increasingly recognise that the most advanced research in their fields is taking place not within but across disciplines."
In fact, teams of researchers trained in a wide range of fields are opening new scientific frontiers and pathways of understanding. That is exactly the strategy that has been applied by the diverse teams of researchers that Magnasco has worked with in studying the complex dynamics associated with the sense of hearing. If the scientific community's enthusiastic response to this research is any indication, then people are certainly listening.

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