| acoustic energy | The variations in air pressure produced by the vibration of an object.
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| acoustic reflex | Muscular contractions within the middle ear that damp sound vibrations by stiffening the eardrum and restricting the movements of the ossicles.
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| acoustics | The branch of physics concerned with sound.
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| adaptation | A reduction in the responsiveness of neurons, produced by prolonged stimulation.
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| amplitude | The property of sound waves that is related to the magnitude of the change in air pressure produced by a sound source. This property, sometimes referred to as intensity, is related to loudness. See intensity.
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| anvil (incus) | The middle member of the three ossicles within the middle ear; it relays sound vibrations from the hammer to the stirrup.
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| auditory canal | The hollow cavity leading from the pinna to the eardrum, which in humans is about 2.5 cm long and 7 mm in diameter.
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| auditory nerve | The bundle of nerve fibers innervating the cochlea and carrying information from the ear to higher stages of the auditory system. Also known as the eighth cranial nerve.
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| basilar membrane | The thin sheet of tissue separating the tympanic canal and the cochlear duct.
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| binaural | Listening with two ears. See monaural.
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| binaural cues | Sources of sound information for localizing a sound source by comparing the sounds received by the two ears.
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| cilia | In the case of hearing, cilia are tiny hair-like structures that extend into the fluid of the inner ear; their movements are caused by pressure waves in that fluid bend the cilia and trigger electrical signals. In the case of olfaction, cilia are tiny tufts of thin hairs projecting out of each olfactory receptor cell and extending through the mucous layer into the nasal cavity; thought to be the site where odorous molecules trigger electrical changes in the olfactory receptor cell.
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| cochlea | A coiled, fluid-filled chamber in the inner ear containing the specialized organ for hearing, the basilar membrane.
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| cochlear emissions | Sounds that are generated entirely from within the cochlea.
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| cochlear nucleus | A structure receiving input from the auditory nerve; its cells exhibit a high degree of frequency tuning.
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| cortical deafness | Impaired hearing that results from damage to the auditory cortex.
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| decibel (dB) | A unit for expressing sound amplitude.
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| delay line | Key element in a theory designed to explain binaural cells' sensitivity to interaural time differences. The delay line retards the arrival of signals from one ear relative to those from the other ear.
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| eardrum (tympanic membrane) | The thin, oval membrane that covers the end of the auditory canal and separates the outer ear and the middle ear; it vibrates when sound waves strike it.
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| echoes | Reflected sound waves.
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| efferent fibers | In the auditory system, neural projections from the brain back to the cochlea.
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| Eustachian tube | The opening connecting the middle ear and the throat, which maintains air pressure within the middle ear at nearly the same value as the air pressure in the outside environment.
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| frequency tuning curve | A graph describing the sensitivity of an auditory neuron to tones of various frequencies.
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| hair cells | The ears' receptors. See inner hair cells, outer hair cells.
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| hammer (malleus) | The outermost of the three ossicles within the middle ear; one end of the hammer is attached to the eardrum, and the other end relays sound vibrations to the anvil.
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| Hertz (Hz) | A unit for expressing the frequency with which the intensity of a sound or a light varies over time.
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| inner hair cells (IHCs) | The approximately 3,500 flask- shaped structures situated along the length of the basilar membrane of the human ear. See outer hair cells.
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| interaural intensity difference (IID) | The difference in the intensity of sound arriving at the two ears; one of the sources of information for sound localization. See interaural time difference.
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| interaural time difference (ITD) | The difference in the time of arrival of a sound wave at the two ears; one of the sources of information for sound localization. See interaural intensity difference.
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| medial geniculate nucleus | A structure in the thalamus that is part of the auditory system.
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| monaural | Listening with one ear. See binaural.
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| noise | A complex sound whose many constituent frequencies combine to produce a random waveform.
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| organ of Corti | The receptor organ for hearing, situated within the cochlear duct.
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| ossicles | A series of three tiny bones that transmit sound vibrations from the eardrum to the oval window. See anvil, hammer, stirrup.
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| outer hair cells (OHCs) | The approximately 12,000 cylindrical structures situated along the length of the basilar membrane of the human ear. See inner hair cells.
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| oval window | The small opening into the inner ear, which is covered by a thin membrane and which receives vibrations from the eardrum via the ossicles.
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| pinna | The part of the ear projecting from the side of the head; by influencing the frequency composition of sound waves entering the ear, this prominent structure plays a role in sound localization.
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| place theory | The idea that different portions of the basilar membrane vibrate in response to different sound frequencies. See frequency theory.
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| primary auditory cortex | The cortical region in the temporal lobe that receives afferent auditory signals from more peripheral auditory structures; sometimes called A1; neurons in this cortical area are organized tonotopically.
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| pure tones | Sinusoidal variations in sound pressure, such as those produced by striking a tuning fork.
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| resonant frequency | The frequency at which a given object vibrates when set into motion.
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| round window | The thin membrane that covers a small opening into the middle ear; displacement of this membrane compensates for pressure variations within the cochlea.
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| sonar (SOund NAvigation Ranging) | A technique whereby sound reflected from object surfaces is used to detect those objects and judge their distances from the sound source. See echolocation.
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| sound pressure level (SPL) | A reference level for sound intensity.
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| stirrup (stapes) | The innermost of the ossicles within the middle ear; attached to the oval window, it receives sound vibrations from the anvil and sets the oval window into vibration.
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| tectorial membrane | An awninglike layer of tissue arching over the hair cells within the inner ear.
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| threshold intensity | The minimum sound intensity necessary to elicit a neural response from an auditory neuron.
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| tinnitus | An annoying, persistent ringing in the ears. tip of the nose effect The phenomenon in which an odor seems familiar though one cannot name it.
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| tonotopic organization | The orderly layout of preferred frequencies over the length of the basilar membrane.
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| traveling wave | Up and down movements of the basilar membrane produced by fluctuations in fluid pressure within the cochlea.
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| Zwicker tone | The illusory perception of a tone produced by prior exposure to broadband noise missing acoustic energy at the frequency corresponding to the illusory tone.
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