Theories of Hearing
• Helmhotz’s place theory (1883): postulated that the basilar membrane acts as a series of tuned resonators similar to a piano string. Each pitch would cause resonant vibration of the basilar membrane which is particular to its own place. Thus, the frequency was analyzed. High frequency waves excite the basal region and low frequency the apical region.
• Rutherford’s frequency/Telephone theory (1886): Proposed that all frequencies activate the entire length of the basilar membrane along with the hair cells. He postulated that the frequency of the signal is represented by the rate of firing of the auditory nerve fibers. He believed that all vibrations are portrayed by the nerve impulses to the brain without complex vibrations in the cochlea.
• Wever’s volley resonance theory (1949):
Combines both the place and telephone theories postulating that:
• High frequencies (5000 Hz) are perceived in the basal turn;
• Low frequencies (1000 Hz) stimulate nerve action potential equal to frequency stimulation;
• Intermediate frequencies (1000-5000 Hz) are represented in the nerve by asynchronous discharges which then combine actively to represent the frequency of stimulus.
• Von Bekesy’s travelling wave theory (1960) This Wave begins from the base and moves towards the apex. Traveling wave is independent of frequency. The region of maximum displacement varies according to frequency. High pitched sounds causes a short traveling wave not beyond the basal turn. Low frequency stimuli cause maximum displacement near the apex.
Middle frequency changes occur in between these two. It is now known that the basilar membrane is much more sharply tuned for frequency filtering. The basilar membrane becomes less selective in tuning at high stimulating intensities due to non linearity of its response. The sharp tuning and non – linearity is due to an active mechanical amplifier which uses biological energy to boost the membrane vibration.