The human brain perceives pitch through two primary mechanisms reflected in place theory and frequency theory. Each mechanism describes how sound waves are interpreted as specific pitches by the brain, offering insights into the intricate processes of auditory perception.
Place theory, or place coding, suggests that different pitches are heard because various sound waves activate specific locations along the cochlea's basilar membrane. The brain determines the pitch of a sound by identifying the precise place on the membrane, generating the neural signal. High-frequency sounds cause significant vibrations near the beginning of the cochlea's membrane, whereas low-frequency sounds vibrate more of the membrane and are less localized. This theory effectively explains the perception of high-pitched sounds, particularly those between 5,000 and 20,000 Hz, but does not adequately address low-pitched sounds.
Frequency theory, also known as temporal coding, offers another explanation for pitch perception. According to this theory, the brain reads pitch by monitoring the frequency of neural impulses traveling up the auditory nerve. The entire basilar membrane vibrates in response to an incoming sound wave, triggering neural impulses to the brain at the same rate as the sound wave. For instance, a sound wave with a frequency of 100 waves per second generates 100 pulses per second along the auditory nerve. This method is effective for frequencies up to 100 Hz, matching the maximal firing rates of many neurons.
Volley theory is a variation of frequency theory that explains pitch perception for tones between 100 and 5,000 Hz. In this theory, sets of neurons fire at their highest rate, but slightly out of sync with each other, allowing them to achieve overall firing rates that match frequencies up to 5,000 Hz.
