Abstract
Georg von Békésy observed that the onset times of responses to brief-duration stimuli vary as a function of distance from the stapes, with basal regions starting to move earlier than apical ones. He noticed that the speed of signal propagation along the cochlea is slow when compared with the speed of sound in water. Fast traveling waves have been recorded in the cochlea, but their existence is interpreted as the result of an experiment artifact. Accounts of the timing of vibration onsets at the base of the cochlea generally agree with Békésy’s results. Some authors, however, have argued that the measured delays are too short for consistency with Békésy’s theory. To investigate the speed of the traveling wave at the base of the cochlea, we analyzed basilar membrane (BM) responses to clicks recorded at several locations in the base of the chinchilla cochlea. The initial component of the BM response matches remarkably well the initial component of the stapes response, after a 4-μs delay of the latter. A similar conclusion is reached by analyzing onset times of time-domain gain functions, which correspond to BM click responses normalized by middle-ear input. Our results suggest that BM responses to clicks arise from a combination of fast and slow traveling waves.
Highlights
The arrival of sounds at the mammalian ear sets off a chain of signal transformations
Unlike middle ear responses, waveforms of basilar membrane (BM) responses to clicks have a bipolar shape that is characteristic of band-pass systems
To facilitate comparison between the onset times of stapes and cochlear motion, approximations to the first derivatives of the raw response waveforms are presented in Fig 1B. (The derivatives were approximated by finite differences, i.e., the difference between two adjacent samples, divided by 4 μs.) The negatives of the first derivative of BM responses are displayed in that figure; no polarity changes were made for the stapes response
Summary
The arrival of sounds at the mammalian ear sets off a chain of signal transformations. Pressure waves traveling in the air are converted into vibrations of the middle ear bones. Such vibrations, those of the stapes, serve as the mechanical input to the hearing organ, the cochlea. Stapes vibrations induce movements of the cochlear fluids and initiate a displacement wave on the basilar membrane (BM) that travels from the base, near the stapes, to the distal end of the cochlea, or apex. We largely owe this description of cochlear mechanics to Georg von Békésy [1]. Additional confirmation has come from direct measurements of BM vibrations (e.g., [5]), performed mostly at the base of the cochlea
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