How do we perceive the pitch or frequency of a sound?

We hear sounds because they cause vibrations in parts of the ear and information about these vibrations is transmitted by the auditory nerves to the temporal lobe of the brain. But how do we perceive the pitch or frequency of a sound?

Some psychologists believe that sounds are perceived as being louder when more number of receptor neurons on the organ of corti gets fired. But they are not so sure about the perception of pitch. We have three theories to explain pitch discrimination: the place theory and the frequency theory, and the volley theory.

Place Theory:

The place theory was proposed by Helmholtz and was modified by the Noble Prize winner, Bekesy (1960). The theory holds that the pitch of a sound is determined depending upon which section of the basilar membrane vibrates in response to the sound. Bekesy observed that receptor neurons at different sites along the basilar membrane are excited in response to different frequencies of tones. The higher the pitch of a sound, the closer the responsive neurons lie to the oval window. However, tones that are of low frequency excite the entire basilar membrane.

Frequency Theory:

The place theory explains high frequency sounds, but fares poorly with low-frequency sounds. A frequency below 400 Hz does not stimulate a particular spot on the basilar membrane; it is virtually registered to all parts of the basilar membrane on an equal basis. Thus, frequency theory suggests that the frequency of auditory nerve’s firing is involved in our perception of sound. This theory argues that the basilar membrane vibrates as a whole at lower frequencies.

Volley Theory:

This theory proposed by Weven (1949) is a modification and expansion of the frequency theory to account for high-frequency sounds. A neuron has a maximum firing capacity of about 1000 times per second. Hence frequency theory cannot be applied accurately to sounds over 1000Hz. This theory argues that sounds above 1000Hz can be accounted for by team of neurons with each neuron of the team firing at a different time. The volley theory derives its name from the fact that neurons fire in a sequence of rhythmic volleys at higher frequencies.

For example, for a tone of 3000 Hz, there would occur in the auditory nerve a spurt of neural activity, called volley every 0.003 second, with different groups of fibers responding each time, and some fibers because of their greater excitability contributing more of the spurt than others. Pitch, depends upon the frequency of volleys rather than the frequency carried by the individual fibers.