Hearing

Hearing is the reception and processing of sounds. The outer ear is the obvious part of the hearing system. The shape of the ear conducts sound waves into the ear canal. The position of the ears, creates phase differences, depending on the source of the sound. Phase and volume differences are used by the brain to determine the location of the sound source.

Sound is propagated by pressure waves in air. The frequency of wave oscillation is pitch. The pitch determining frequency of sound waves is very low compared to radio waves, for example. The centre pitch frequency for human speech and singing is 440 cycles per second (Hertz), the A above middle C on the piano. Higher frequency components of sounds are harmonics that determine the timbre characteristics of the sound.

A meaningful sound can be compared with a molecular message that locks into a receptor and activates a response. Sound receptors are built into every brain to detect distant features of the environment that require identification and response. The reception of sight and sound signals together are important to identify and localize events out there. A novel sound triggers an orienting response; stop, look and listen carefully to identify the source of the sound. An unexpected loud sound triggers a startle response that is composed of orienting and fight or flight arousal.

Cochlea

Sound waves transverse the ear canal and deflect the tympanic membrane which causes the small bones of the middle ear to produce a fluid wave in the cochlea. The organ of Corti in the cochlea converts the fluid waves into nerve impulses. The organ of Corti is suspended on a membrane that supports hair cells that convert membrane fluctuations into nerve impulses.

Hair cells are tuned to different frequencies, so that the output from the cochlea is divided into frequency bands. Outer hair cells adjust the gain of the cochlear output.

According to Corey: ”Hair cells respond to the vibration of the basilar membrane by pushing back on it, exerting force with just the right amplitude and phase to amplify the vibration, especially for faint sounds, by 100-fold or more. The movement of the basilar membrane is amplified from hundredths of nanometers to around a nanometer for the quietest perceptible sounds, from a nanometer to several nanometers at conversational level, but not much at all for loud sounds. In a healthy ear, movements of hair cells show a compressive nonlinearity… outer hair cells in each region of the long organ of Corti only amplify sound of a particular frequency, so that each region is exquisitely tuned to a characteristic frequency (CF) and not to other frequencies.

Inner hair cells then sense the amplified vibration, and send a frequency-specific signal to spiral ganglion neurons of the eighth cranial nerve. Sound signals enter the brainstem and are directed toward the temporal lobe cortices.

Go to Sound Processing in the Brain

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