The Developing Concept of Tonotopic Organization of the Inner Ear

This study aims to document the historical conceptualization of the inner ear as the anatomical location for the appreciation of sound at a continuum of frequencies and to examine the evolution of concepts of tonotopic organization to our current understanding. Primary sources used are from the sixth century BCE through the twentieth century CE. Each work/reference was analyzed from two points of view: to understand the conception of hearing and the role of the inner ear and to define the main evidential method. The dependence on theory alone in the ancient world led to inaccurate conceptualization of the mechanism of hearing. In the sixteenth century, Galileo described the physical and mathematical basis of resonance. The first theory of tonotopic organization, advanced in the seventeenth century, was that high-frequency sound is mediated at the apex of the cochlea and low-frequency at the base of the cochlea. In the eighteenth and nineteenth centuries, more accurate anatomical information was developed which led to what we now know is the accurate view of tonotopic organization: the high-frequency sound is mediated at the base and low-frequency sound at the apex. The electrical responses of the ear discovered in 1930 allowed for physiological studies that were consistent with the concept of a high to low tone sensitivity continuum from base to apex. In the mid-twentieth century, physical observations of models and anatomical specimens confirmed the findings of greater sensitivity to high tones at the base and low tones at the apex and, further, demonstrated that for high-intensity sound, there was a spread of effect through the entire cochlea, more so for low-frequency tones than for high tones. Animal and human behavioral studies provided empirical proof that sound is mediated at a continuum of frequencies from high tones at the base through low tones at the apex of the cochlea. Current understanding of the tonotopic organization of the inner ear with regard to pure tones is the result of the acquisition over time of knowledge of acoustics and the anatomy, physical properties, and physiology of the inner ear, with the ultimate verification being behavioral studies. Examination of this complex evolution leads to understanding of the way each approach and evidential method through time draws upon previously developed knowledge, with behavioral studies providing empirical verification.