Hearing and Auditory Perception

As the structure of the eye is conducive to converting light energy into neural signals, so is the structure of the ear conducive to converting sound energy into neural signals. Like light, sound travels in waves. The frequency of the sound waves (the number of vibrations in a time period) determines the pitch of a sound (the perception of the sound as being high or low), while the amplitude (intensity) of the waves determines the volume (loudness). The visible portion of the ear is called the outer ear or pinna. The middle and inner portions of the ear are surrounded by the temporal bone and not visible.

The pinna acts like a funnel to direct sound waves into the ear canal. Sound waves traveling in the ear canal hit the tympanic membrane, or eardrum, causing it to vibrate. The vibration of the tympanic membrane causes additional vibrations in the three bones of the middle ear—the hammer (malleus), anvil (incus), and stirrup (stapes). The stirrup connects to the inner ear, or cochlea, which is the site of transduction. In the cochlea, mechanical energy from vibrations is converted into neural signals. Within the cochlea is a fluid-filled chamber called the organ of Corti. Within the organ of Corti is the basilar membrane, which is the location of the hair cells. A hair cell is a sensory receptor cell for the auditory and vestibular systems. Cochlear hair cells are activated by the movement of the fluid within the organ of Corti. The axons, the projections of neurons that conduct electrical impulses, of all the cochlear hair cells form the auditory nerve. Signals from the auditory nerve are sent to the sensory relay of the brain, the thalamus. From the thalamus, auditory information is sent to the primary auditory cortex, within the temporal lobes of the brain.

Auditory perception allows people to discriminate among several features of sound, including pitch, volume, and location of the source. Pitch is the lowness or highness of a tone and is determined by the frequency of the sound wave. The higher the frequency, the higher the pitch. There are two theories of pitch perception: place theory and frequency theory. Place theory proposes that the activation threshold, the minimum amount of stimulus needed for a receptor to react, may be different for hair cells at different locations along the basilar membrane, such that a higher-frequency sound is needed to activate hair cells at the farther end of the basilar membrane. Hair cells at the lower end of the basilar membrane are activated by lower frequencies. As a result, pitch perception is determined by which of the cochlear hair cells are activated. Frequency theory holds that all hair cells along all parts of the basilar membrane can be activated by any sound frequency. According to frequency theory, hair cells will send neural signals at a rate that matches the frequency of the sound. Pitch perception is then a result of the brain interpreting the firing rate of the neurons in the auditory nerve as a particular frequency/pitch of the sound.

Similar to the way that the brain can calculate the difference, or disparity, between where an image falls on the retina in the left and right eye to aid depth perception, the brain can also calculate the difference between when a sound arrives at the left and right ear to aid in sound localization. A sound from directly above, behind, or in front of a person will reach the ears at the same time. A sound from the left, however, reaches the left ear a few milliseconds before the right ear (even though there is no conscious awareness of this difference). The brain uses the timing disparity to perceive the location of the sounds source. Additionally, because the head blocks part of the sound coming from the left, the intensity of the sound in the right ear is slightly less than that in the left ear. This sound shadow in the right ear provides further information to the brain about the location of the sound.