Reshaping Auditory Neuroscience: Dr. Christopher A. Shera Receives the Hartmann Prize

This past spring, Dr. Christopher A. Shera (PhD) of the USC Caruso Department of Otolaryngology-Head and Neck Surgery was awarded the prestigious Hartmann Prize in Auditory Neuroscience from the Acoustical Society of America. The Acoustical Society of America (ASA) is the world’s foremost scientific society dedicated to the field of acoustics and its applications, encompassing a wide range of disciplines, including physics, engineering, physiology, psychology, oceanography, music, and architecture. The Acoustical Society of America was conceived at a Santa Monica beach club on July 30, 1928, and the details were fleshed out later that year at an organizational meeting held at the Bell Telephone Laboratories in New York City, where Harvey Fletcher was Director of the Acoustics Research Department. The ASA was officially born at its first formal meeting, held on May 10, 1929, with the purpose of “increasing and diffusing the knowledge of acoustics and promoting its practical applications.” The William and Christine Hartmann Prize, established by the ASA in 2011, recognizes research that “links auditory physiology with auditory perception or behavior in humans or other animals.” Dr. Shera was honored for the work that he and his collaborators have conducted over the years on estimating cochlear frequency selectivity noninvasively and for their findings (initially controversial) that human frequency selectivity is substantially better than that of the common laboratory animals usually taken as models for human hearing.

Like many other animals, humans can hear a wide range of sounds, and there are several ways to measure how “good” our hearing is compared to other species. One metric is to look at the range of sounds, from low to high, that we can perceive. For instance, as Dr. Shera detailed in his article “What Makes Human Hearing Special?” in Frontiers for Young Minds, a young human ear can hear from about 20 to 20,000 Hz, which, if one is familiar with music, would be the equivalent of about 10 octaves. In comparison, dogs and cats can hear up to 40,000 Hz and mice up to 80,000 Hz (though the latter cannot hear frequencies below about 1,000 Hz, frequencies important for human speech and music). Another way to measure hearing is to look at sensitivity, or the ability to detect quiet sounds. As a species, humans are fairly average in our hearing sensitivity compared to other animals—cats, for example, can hear quieter sounds than humans, while elephants are less sensitive.

Given the above, many assumed that when it came to the third primary metric used to determine how well we hear—the ability to separate and distinguish different pitches, or tones of different frequency—humans would prove similar to other species. Dr. Shera explained that frequency selectivity involves “the mammalian cochlea [inner ear] acting as an acoustic prism, mechanically separating the frequency components of sound so that they stimulate different populations of sensory cells within the inner ear. As a consequence of this frequency separation, or filtering, each sensory cell responds preferentially to sound energy within a limited frequency band. The resulting frequency selectivity greatly facilitates the detection and perceptual segregation of different sounds.” While it is relatively easy to measure the range of sounds that humans can hear, as well as our sensitivity to quiet sounds, for a long time, it was more difficult to effectively measure frequency selectivity in humans. In anesthetized, non-human mammals, the frequency selectivity of the sensory cells can be measured directly by making recordings from the auditory-nerve fibers that contact them. However, since this method necessitates highly invasive procedures (i.e. traumatic brain surgery), it would be unethical to conduct similar research and experimentation on humans. Dr. Shera and his team knew that it would be important to figure out a noninvasive way to measure human frequency selectivity that would produce reliable and valid results.

In 1978, Dr. David Kemp (PhD, FRS) discovered that sounds do not just go into the ear—they also come out as a kind of echo called an otoacoustic emission. Dr. Shera realized that perhaps one could use otoacoustic emissions to estimate cochlear frequency selectivity noninvasively. Dr. Shera discussed how the method he developed “​​…exploits the fact that the healthy cochlea makes sound while listening to sound, generating quiet echoes in response to acoustic stimulation. These otoacoustic emissions can be recorded by listening to the ear using sensitive, low-noise microphones placed in the ear canal. We showed that the delays of these echoes correlate well with the sharpness of frequency tuning, enabling us to obtain reliable estimates of cochlear frequency selectivity in humans.”

Dr. Shera and his collaborators’ findings were unexpected—according to their measurements, human frequency sensitivity is substantially better than that of the other laboratory animals typically compared to humans. As a result, these findings were often considered contentious, and thus, after many years of controversy and debate, Dr. Shera was surprised and thrilled when he learned that the research had received the Hartmann Prize. He was deeply honored to accept it on behalf of all those who worked so hard to help to develop this innovative, noninvasive method of measuring human frequency selectivity, noting that: “The collaborative aspects of this decades-long project were essential at every stage. The work required expertise in a wide range of experimental and theoretical approaches, including otoacoustic measurements, animal psychophysics and human behavioral testing, neurophysiology, and computational modeling. Altogether, the project involved important contributions from more than a dozen collaborators. In retrospect, it’s amazing that everything somehow came together.” Dr. Shera hopes that any publicity surrounding the award helps attract graduate students and postdocs to the outstanding research opportunities available in hearing and communication at USC, especially in the laboratories of the Department of Otolaryngology’s amazing Assistant Professors, including Drs. Karolina Charaziak (PhD), James Dewey (PhD), Ksenia Gnedeva (PhD), and Seiji Shibata (MD, PhD).