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proceeded more than 500 μm before making contact with up to 20 OHCs. Before Spoendlin’s observations, both IHCs and OHCs were assumed to carry out similar roles of mech- anotransduction. The innervation pattern led to the sugges- tion, however, that most, if not all, hearing information was coming from the IHCs. The big question raised by this dis-
a convenient summary of work in the field up to that point. The tuning of the fibers was consistent across the entire sample population. There were no surprising, broadly tuned fibers that may have originated from Spoendlin’s SGC2s. It was also noted that the auditory nerve fiber response was more highly tuned than the traveling wave patterns that von Békésy had described. The neural tuning was closer to what was needed to explain the frequency selectivity of mamma- lian hearing.
Active Hearing:
Early Speculations and Evidence
An active process in the cochlea had been hypothesized by Thomas Gold (1948), a Viennese-born English polymath (Figure 6). He realized that viscous damping in the fluid environment of the cochlea was incompatible with the ex- quisite frequency-resolving powers of human hearing. He postulated that an active, essentially piezoelectric mecha-
Figure 5. Photograph of the Swiss physician-scientist and head of otolaryngology at the University of Innsbruck Heinrich Hans Spo- endlin (1927-1991). Spoendlin provided the first structural evi- dence that most, if not all, neu- ral acoustic information from the ear came via IHCs. His discovery set the stage for accepting OHCs as the cochlear amplifier. Photo- graph provided by Professor Jo- seph B. Nadol.
covery was what were the OHCs doing?
Passive Hearing
Is Not Consistent with Auditory Nerve Fiber Tuning Investigations on cochle- ar mechanics during the late 19th and first half of the 20th century by scien- tists such as Hermann von Helmholtz and Georg von Békésy led to a widely ac- cepted model for how the vibrations of sound were analyzed by the cochlea. von Békésy used a light mi- croscope and stroboscopic illumination to observe acoustically driven pat- terns of movement in the basilar membrane of cadav- ers (von Békésy, 1960). He confirmed the tonotopic or- ganization of high- to low- frequency vibrations going
nism that converted elec- trical to mechanical energy could provide feedback to counteract viscous damp- ing from cochlear fluids. His hypothesis was largely ignored, in part, because of opposition by von Békésy.
Evidence that the cochlea was active began to mount in the 1970s. Brian John- stone in Perth, WA, Aus- tralia, and Bill Rhode in Madison, WI, pioneered the use of the Mössbauer effect to measure the move- ment of a gamma radia- tion source placed on the basilar membrane in living animals (Johnstone and Boyle, 1967; Rhode, 1971). By the mid-1970s, they had established that, in the best preparations, mechanical tuning approached that of single auditory nerve fibers. When the animal died, the motion rapidly deteriorated to the passive tuning that von Békésy had
from the base to the apex of the cochlea. He also described the traveling wave that resulted from hydraulic coupling. von Békésy was awarded the Nobel Prize for his work and helped establish a view of the cochlea as an elegant, but es- sentially passive, device for converting the mechanical en- ergy of sound into electrical signals to the brain.
Neurophysiological investigations of sensory coding by the auditory nerve began with the advent of vacuum tube amplifiers and kymograph recordings. The introduction of oscilloscopes and digital recording techniques facilitated neurophysiological research. A research monograph on the response characteristics of single auditory nerve fibers was published by Nelson Kiang and colleagues in the mid-sixties (Kiang et al., 1965). It was a compendium of sensory coding in the 8th nerve using well-controlled stimuli and provided
Figure 6. Photograph of Thomas Gold (1920-2004) who, over a long active career, studied bio- physics, astronomy, aerospace en- gineering, and geophysics. Gold was born in Austria and moved with his family to England where he studied at Cambridge. He con- tributed to the British war effort by working on improvements to radar. While working with R. J. Pumphrey after the war, Gold concluded that an electrome- chanically active mechanism was necessary to counteract viscous damping by cochlear fluids. Pho- tograph provided by Professor David Kemp.
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