2nd Essay question:
‘It might be tempting to assume that the pitch of a complex harmonic tone is simply determined by this lowest frequency component. However, the phenomenon of the ‘missing fundamental’ indicates that this is not the case.’ (STAINSBY) Give an account of the processes involved in the perception of pitch, with specific reference to the missing fundamental.
In order to perceive the pitch of a sound, the mechanical energy of the sound waves must be transformed into the electrical energy of nerve impulses. The auditory system is able to process the sound waves travelling through the air and carry out this transformation of energy which leads to the perception of a pitch. When a pitch is heard, it is the frequency of the sound (or the number of times per second the waveform repeats itself) which is being coded in the auditory system and then perceived as a pitch in the brain – although pitch is closely related to frequency, pitch is an auditory sensation of frequency and thus the two are not interchangeable. Research into how the auditory system is able to encode frequency information entering the ear suggests possible mechanisms taking place on the basilar membrane of the inner ear. Interestingly, studies also show how the brain is able to perceive a pitch when its corresponding frequency is not present in the sound entering the ear. This phenomenon is called the ‘missing fundamental’, as mentioned in the quote from Stainsby. The ‘missing fundamental’ has lead to further theories aiming to explain how frequency information is encoded in the auditory system and how analysis of this information can lead to the perception of a single pitch.
The Processes of Hearing
The process of hearing a pitch begins when the sound is collected by the pinna of the outer ear. The pinna acts to channel the sound waves into the ear canal and the reflections of the sound waves off the pinna help to locate the source of the sound. The resulting pressure fluctuations in the ear canal force the eardrum, or tympanic membrane, to vibrate at the same frequency as the sound. The vibrations of the membrane cause the hammer and anvil to pivot around their junction, making the stirrup move into and out of the oval window like a piston (Campbell and Greated 1987). The rapid backward and forward movement of the stirrup causes fluid to be displaced within the cochlea which then forces bulges to travel down the basilar membrane towards the helicotrema, thus setting the basilar membrane into a state of vibration. These bulges increase in height to a certain point on the basilar membrane and then rapidly die away. As the bulge moves along the basilar membrane, hairs on the tectorial membrane are bent, which causes an electrical pulse to be fired by the hair cells. The nerve cells then identify these impulses and carry the signals to the brain. It is in these nerve pulses that information about the frequency of the sound is encoded and carried to the brain which then organises the frequency information into pitch.
Pitch Perception of Pure Tones
In order to understand how the auditory system converts the information about the frequency of the sound from the basilar membrane into electrical impulses, it is best to begin by observing the processes involved when a pure tone is heard. A pure tone has energy at only one frequency and therefore follows a sinusoidal waveform. Two mechanisms have been proposed which aim to explain how the frequency of a pure tone is identified in the ear. The first mechanism is called place coding. This mechanism suggests that a pure tone of certain frequency will generate a maximum bulge at a certain point along the basilar membrane and thus specific bundles of hair cells are stimulated at that particular frequency. In order to increase the accuracy of the position of maximum bulge on the basilar membrane, the outer hair cells, once stimulated, begin a...