How Your Brain Understands What Your Ear Hears

amplitude: The displacement of a wave. In the case of a sound wave, the greater the amplitude of the wave, the greater the intensity, or pressure, of the sound. The extent to which air particles are displaced in response to the energy of a sound.

APD: See auditory processing disorder.

auditory cortex: The area of the brain (in the temporal cortex) that connects fibers of the auditory nerve and interprets nerve impulses in a form that is perceived as sound.

auditory nerve: The eighth cranial nerve, which connects the inner ear to the brainstem and is responsible for hearing and balance.

auditory processing disorder (APD): Reduced or impaired ability to discriminate, recognize, or comprehend complex sounds, such as those used in words, even though the hearing is normal (such as coat/boat or sh/ch).

basilar membrane: Found in the organ of Corti, it is the cellular membrane in which the hair cells are embedded. The basilar membrane moves in response to pressure waves in the cochlea, initiating a chain of events that results in a nerve impulse traveling to the brain.

brainstem: A region of the brain that connects the spinal cord to higher levels of the brain, such as the cortex.

cochlea: Snail-shaped structure in the inner ear that contains the organ of hearing. The cochlea is a coiled, fluid-filled cavity responsible for converting vibrational energy from the middle ear into nerve impulses that travel to the brain.

cochlear duct: See scala media.

cochlear implant: A medical, electronic device that bypasses the damaged structures in the inner ear and directly stimulates the auditory nerve. An implant does not restore or create normal hearing. Instead, under the appropriate conditions, it can give a deaf person a useful auditory understanding of the environment and help him or her understand speech. The implant is surgically placed under the skin behind the ear. An implant has four basic parts: a microphone, which picks up sound from the environment; a speech processor, which selects and arranges sounds picked up by the microphone; a transmitter and receiver/stimulator, which receives signals from the speech processor and converts them into electric impulses; and electrodes, which collect the impulses from the stimulator and send them to the brain.

conductive hearing loss: A type of hearing loss that results from dysfunction of the outer or middle ear (such as a punctured eardrum or buildup of ear wax) that interferes with the efficient transfer of sound to the inner ear; characterized by a loss in sound intensity.

critical period: A period of time during an organism’s development in which the brain is optimally capable of acquiring a specific ability, provided that appropriate environmental stimuli are present. Humans as well as some animals are known to have a critical period during which language is acquired.

decibel (dB): A unit used to convey the intensity of sound.

ear canal: A component of the outer ear that leads to the tympanic membrane (eardrum) of the middle ear. The ear canal is lined with wax and hairs that prevent small foreign material from traveling deeper into the ear.

endolymph: A fluid in the labyrinth, the organ of balance in the inner ear.

eustachian tube: A small tube that connects the middle ear with the back of the throat. It allows the air in the middle ear to be refreshed periodically.

frequency: The number of times a sound vibrates per unit of time. Frequency is expressed in hertz (Hz), a unit of measurement equal to one cycle per second.

gene: The functional and physical unit of heredity. Genes are segments of DNA found along a chromosome. They typically encode information used to produce a specific protein. Human DNA is organized into 46 chromosomes—23 from the father and 23 from the mother. The study of mice with hereditary hearing loss has enabled researchers to begin understanding the role that DNA and genetics play in human hearing disorders.

hair cells: Found in the organ of Corti in the cochlea of the inner ear, these are the specialized receptors of hearing. The name refers to stereocilia, bundles of hairlike projections jutting upward from the cells. When the stereocilia are moved by sound vibrations, the hair cells translate this mechanical stimulation into an electrical nerve impulse that is carried to the brain by the auditory nerve.

hertz (Hz): A unit of frequency equal to one cycle per second.

impact noise: A short burst of sound.

imprinting: The process by which young individuals of a species acquire irreversible behavior patterns of that species. With respect to hearing, imprinting involves the ability of the brain to distinguish and process the sounds and rhythms of the first language or languages the young hear.

incus: The center bone of the series of three small bones, or ossicles, of the middle ear. Sometimes called the anvil.

infrasonic: Sounds with frequencies below 20 Hz and, therefore, beyond the range of human hearing.

intensity: The amplitude of a sound wave. Sound intensity, which is expressed in decibels, is measured in relation to an accepted reference, such as the threshold at which an average person can hear a sound.

inner ear: The most interior portion of the ear, made up of two interconnected parts: the vestibular system, a balance organ, and the cochlea, a hearing organ.

loudness: Our perceived impression of the intensity, frequency, and duration of a sound.

malleus: The first bone in the series of three small bones, or ossicles, of the middle ear. Sometimes called the hammer.

Ménière’s disease: Inner ear disorder that can affect both hearing and balance. Ménière’s disease can cause episodes of vertigo, hearing loss, tinnitus, and the sensation of fullness in the ear.

midbrain: A region of the brain that relays sound input to the auditory cortex.

middle ear: The part of the ear that includes the eardrum and ossicles and ends at the round window that leads to the inner ear. An air-filled space connected to the back of the throat by the eustachian tube.

NIHL: See noise-induced hearing loss.

noise-induced hearing loss (NIHL): Irreversible hearing loss caused by exposure to very loud impulse sounds, such as an explosion, or to less-intense sounds for an extended period of time. Loud noise levels damage hair cells of the inner ear.

organ of Corti: The sensitive organ of hearing within the cochlear duct. The organ of Corti contains specialized cells called hair cells that transduce sound vibrations into electrical impulses.

ossicles: The three smallest bones in the human body. The ossicles consist of the malleus, incus, and stapes (known also as the hammer, anvil, and stirrup, respectively), found in the middle ear. They are part of the system that amplifies sound vibrations that enter the middle ear.

ossicular chain: The three bones that make up the ossicles of the middle ear (the malleus, incus, and stapes).

otitis media: An inflammation of the middle ear, usually associated with a buildup of fluid related to a viral or bacterial infection. The obstruction can cause hearing problems, which may arise when the fluid interferes with the ability of the ossicles to conduct sound vibrations to the inner ear.

otosclerosis: An abnormal growth of bone in the middle ear, which prevents structures within the ear from working properly, causing hearing loss.

ototoxic: Any substance that damages auditory tissues, including a special class of antibiotics, called aminoglycoside antibiotics, that can damage hearing and balance organs for individuals who are sensitive.

outer ear: The part of the ear composed of the pinna and the ear canal.

oval window: An opening in the bony wall that separates the middle ear from the inner ear.

perilymph: A fluid, nearly identical to spinal fluid, that fills the cochlea.

perilymph fistula: The leakage of inner ear fluid into the middle ear. It is associated with head trauma, physical exertion, or exposure to severe pressure, but it can also occur without apparent cause.

phonemes: The basic sound elements of a spoken language.

pinna: The outer ear, which is composed of skin and cartilage. The pinna focuses sound waves into the middle and inner ears. Having two pinnae helps animals determine the location of a sound. In some animals, the pinna serves additional functions, such as heat dissipation.

pitch: The perception of a sound based on its frequency.

round window: An opening in the cochlea that allows pressure from sound waves to be released.

scala media: Also called the cochlear duct, this region between the upper and lower chambers of the cochlea contains the organ of Corti.

scala tympani: The lower chamber of the cochlea.

scala vestibuli: The upper chamber of the cochlea.

sensorineural hearing loss: Hearing loss caused by damage to the hair cells or nerve fibers of the inner ear.

sensory integration: The involuntary process by which the brain assembles a picture of our environment at each moment in time using information from all of our senses. Children with learning disabilities or autism have difficulties with sensory integration. (See the Web site of Sensory Integration International, The Ayres Clinic, http://www.sensoryint.com/faq.html.)

sound: Vibrational energy. A pressure disturbance propagated through a medium and displacing molecules from a state of equilibrium. The auditory perception of this disturbance. Something heard by the ears.

sound intensity: The magnitude of a sound, measured against a standard reference in units known as decibels (dB). Intensity refers to the amplitude of a sound.

sound waves: The longitudinal progressive vibrations in an elastic medium by which sounds are transmitted.

stapes: The final bone in the series of three small bones, or ossicles, of the middle ear. Sometimes called the stirrup.

stereocilia: Hairlike extensions jutting from one end of the inner ear’s hair cells into the cochlear fluid.

subsonic: See infrasonic.

tectorial membrane: Found in the organ of Corti of the cochlea, this sheet of cells lies above the stereocilia of the hair cells. Movement of the basilar membrane (to which the hair cells are attached) causes the stereocilia to move against the tectorial membrane, initiating a nerve impulse that travels from the hair cell to the brain.

temporal lobe: A region of the brain that contains the auditory cortex, which is necessary for interpreting sounds.

tinnitus: The term for the perception of sound when no external sound is present. The sensation of ringing, roaring, buzzing, or clicking in the ears or head. An ailment that is associated with many forms of hearing impairment and noise exposure.

transduction: A process by which energy is converted from one form to another.

tympanic canal: See scala tympani.

tympanic membrane: The eardrum. A structure that separates the outer ear from the middle ear and vibrates in response to sound waves. These vibrations are transferred to the small bones in the middle ear.

ultrasonic: Sounds with frequencies above 20,000 Hz and, therefore, beyond the range of human hearing.

vertigo: The illusion of movement. A sensation that the external world is revolving around an individual (objective vertigo) or the individual is revolving in space (subjective vertigo). May be caused by an inner ear dysfunction.

vestibular canal: See scala vestibuli.

vestibular system: The system responsible for maintaining balance, posture, and the body’s orientation in space. This system also regulates locomotion and other movements and keeps objects in visual focus as the body moves. Located next to the cochlea, the vestibular system consists of three semicircular canals oriented in different planes. Movement of fluid within the canals responds to movements of the head and visual information, allowing the brain to process an animal’s current state of balance.