Traveling Waves, Second Filters, and Physiological Vulnerability: A Short History of the Discovery of Active Processes in Hearing

Abstract

Descartes, in the Treatise of Man (1662), suggested how nerve stimulation could evoke reflex responses. Descartes was aware of the principle of place coding for touch and for vision, and suggested how the stimulation of different nerves in the sensory input could lead to stimulation of different nerves in the motor output and so to different reflex responses. However, he did not report any idea of frequency analysis (later to be Fourier analysis), and therefore was not able to suggest the place coding of frequency by the ear. Instead, Descartes suggested that neural responses to sound were entirely driven by the amplitude of the individual sound impulses, and that frequency coding in hearing was accomplished by the time pattern of the neural stimulation from the ear to the brain. By the time of Helmholtz (1863, 1885), Fourier analysis was well established. Also well established was the use of hollow vessels, constructed to resonate in response to sound waves of different frequencies, which could be used as a way of detecting the component frequencies of a complex sound. Helmholtz suggested that the cochlea worked in a similar way; the rods of the arches of Corti would resonate in response to different frequencies of stimulation, and this would be detected by the neural innervation. The perception of sound was therefore entirely dependent on detecting the Fourier components of the stimulation. Temporal information was preserved in the transients of the envelope of the stimulus, but time coding as we now know it, in the sense that frequency information was also conveyed in the time pattern of the neural stimulation, was absent. One objection to Helmholtz's theory was that the fluid in the ear would damp the vibrations of the fine rods, and do this to such an extent that the degree of frequency resolution of hearing that was observed behaviorally would not be possible. In addition, indirect evidence from patterns of noise damage had suggested that tones produced vibrations along a considerable length of the cochlea, rather than only discrete regions, with high-frequency tones producing 39