Chapter 9 - PSYB51

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Mathias Niemier

Chapter 9 – Hearing – Physiology & Psychoacoustics  Deafness deprives you of most fundamental human abilities – communication through speech Function of Hearing What Is Sound?  Sounds are created when objects vibrate  Vibrations of object cause molecules in object’s surrounding medium to vibrate as well and vibration in turn causes pressure changes in medium  Pressure chances = waves and similar to waves on pond  Sound waves travel at particular speed depending on medium, moving faster through denser substances o Speed of sound through water is 1500 m/s o Light waves move through air almost a million times faster than sound waves  When jet plane travels faster than speed of sound, place catches up to and passes fronts of sound waves it’s creating – result is that sound waves combine into shock wave, or huge pressure fluctuation – sonic boom Basic Qualities of Sound Waves: Frequency and Amplitude  Magnitude of pressure chance in sound wave – difference between highest pressure area and lowest pressure area  Amplitude: magnitude of displacement (increase or decrease) of sound pressure wave or of head movement  Intensity: amount of sound energy falling on unit area  Pressure fluctuations can be close together, or can be spread apart over longer periods  Although sound waves also have wavelengths, more typically describe patterns by noting how quickly pressure fluctuates  Frequency: for sound, number of times per second that pattern of pressure change repeats  Hertz: unit of measure for frequency. One hertz equals one cycle per second  Just as amplitude and wavelength of light waves correspond to perceptual qualities in vision, amplitude and frequency of sound waves are highly correlated with auditory characteristics  Loudness: psychological aspects of sound related to perceived intensity or magnitude  Pitch: psychological aspect of sound related mainly to fundamental frequency. Low frequency sounds correspond to low pitches and high frequency sounds corresponding to high pitches  If young, able to detect sounds that vary from 20-20 000 Hz  Humans hear across very wide range of sound intensities – ratio between faintest sound humans can detect and loudest sounds that don’t cause serious damage to ears is more than one to a million  Decibels: unit of measure for physical intensity of sound. Decibels define difference between 2 sounds as ratio between 2 sound pressures. Each 10:1 sound pressure ratio equals 20 dB, and 100:1 ratio equals 40 dB o dB = 20 log (p/0 ) o p = pressure 2 o p0 = reference pressure and typically defined in auditory research contexts to be 0.0002 dyne/cm  important thing to remember about logarithmic scales such as decibels is that relatively small decibel changes can correspond to large physical changes Sine Waves, Complex Tones, and Fourier Analysis  Sine wave/pure tone: waveform for which variation as function of time is sine function  Air pressure in sine wave changes continuously (sinusoidally) at same frequency  Period: time (or space) required for one cycle of repeating waveform  Phase: relative position of 2 or more sine waves. For sounds, phase is relative position in time  Sine waves aren’t common, everyday sounds, because new vibrations in world are so pure  Complex tones: sound wave consisting of more than one sinusoidal component of different frequencies  All sounds, no matter how complex, can be described as combination of sine waves  Fourier analysis: mathematical theorem by which any sound can be divided into set of sine waves. Combining sine waves will reproduce original sound  Spectrum: representation of relative energy (intensity) present at each frequency  Harmonic spectra: spectrum of complex sound in which energy is at integer multiples of fundamental frequency  Fundamental frequency: lowest frequency component of complex periodic sound. All other harmonics have frequencies are integer multiples of fundamental  Shape of Fourier spectrum is most important quality that distinguish different sounds  Timbre: psychological sensation by which listener can judge that 2 sounds with same loudness and pitch are dissimilar. Timbre quality is conveyed by harmonics and other high frequencies Basic Structure of Mammalian Auditory System Outer Ear  Pinna: outer, funnel like part of ear. Only mammals have pinnae  Ear canal: canal that conducts sound vibrations from pinna to tympanic membrane and prevents damage to tympanic membrane. Extends about 25 mm into head o Length and shape enahce sound frequencies between about 2000-6000 Hz o Main purpose is to insulate structure at end, tympanic membrane, from damage  Tympanic membrane: the ear drum – thin sheet of skin at end of outer ear canal. Tympanic membrane vibrates in response to sound  Common myth that puncturing eardrum will leave you deaf – most cases, damaged tympanic membrade will heal itself Middle Ear  Outer ear: external sound gathering portion of ear, consisting of pinna and ear canal  Tympanic membrane is border between outer and middle ear  Middle ear: air filled chamber containing middle bones, or ossicles. Middle ear conveys and amplifies vibration from tympanic membrane to oval window  Ossicles: tiny bones of middle ear – malleus, incus and stapes o Malleus: one of ossicles. Receives vibration from tympanic membrane and attached to incus o Incus: middle ossicle. Connects malleus and stapes o Stapes: one of ossicles. Connected to incus on one end, and presses against oval window of cochlea on other end  Inner ear: hollow cavity in temporal bone of skull, and structures within cavity – cochlea and vestibular canals  Ossicles, which are smallest bones in human body, amplify sound vibrations in 2 ways o Joints between bones are hinged in way that makes them work like levers – modest about of energy on one side of fulcrum becomes larger on other end. Action increases amount of pressure change by about 1/3 o Concentrating energy from larger to smaller surface area – tympanic membrane, which moves malleus, is about 18x as large as oval window, which is moved by stapes  Amplification provided by ossicles is essential to ability to hear faint sounds because inner ear, is made up of collection of fluid-filled chambers  Because it takes more energy to move liquid than air, fluid creates impedance mismatch  Middle ear has 2 muscles o Tensor tympani: muscle attached to malleus; tensing tensor tympani decreases vibration o Stapedius: muscle attached to stapes; tensing stapedius decreases vibration o Both are smallest muscles in body o Main purpose is to tense when sounds are loud, restricting movement of ossicles and muffling pressure changes that might be large enough to damage  Acoustic reflex: reflex that protexts ear from intense sounds, via contraction of stapedius and tensor tympani muscles Inner Ear  Cochlea: major structure of inner ear. A spiral structure of inner ear containing organ of Cortj. Rolled up, it’s the size of baby pea, about 4 mm and uncoiled, about 35 mm. filled with watery fluids in 3 parallel canals o Tympanic canal: one of 3 fluid filled passages in cochlea. Extends from round window at base of cochlea to helicotrema at apex. Called scala tympani o Vestibular canal: one of 3 fluid filled passages in cochlea. Extends from oval window at base of cochlea to helicotrema at apex. Calles scala vestibule o Middle canal: one of 3 fluid filled passages in cochlea. Middle canal is sandwiched between tympanic and vestibular canals and contains cochlea partition. Called scala media  Helicotrema: opening that connects tympanic and vestibular canals at apex of cochlea  3 canals of cochlea are separated by 2 membranes o Reissner’s membrane: thin sheath of tissue separating vestibular and middle canals o Basilar membrane: plate of fibers that forms base of cochlear partition and separates middle and tympanic canals in cochlea  Cochlear partition: combined basilar membrane, tectorial membrane, and organ of Corti, which are together responsible for transduction of sound waves  Vibrations transmitted through tympanic membrane and middle ear bones causes stapes to push and pull flexible oval window in and out of vestibular canal at base of cochlea.  Movement of oval window causes waves of pressure changes – “travelling waves” to flow through fluid in vestibular canal  Because cochlea is closed system, pressure changes can’t spread out in all directions as they do in atmosphere – instead, displacement or “bulge” forms in vestibular canal and travels from base of cochlea down to apex  Round window: soft area of tissue at base of tympanic canal that releases excess pressure remaining from extremely intense sounds  Because vestibular and tympanic canals wrapped tight around middle canal, vestibular canal bulges out it puts pressure on middle canal  Organ of Corti: structure on basilar membrane of cochlea that is composed of hair cells and dendrites of auditory nerve fibers  Hair cells: cells that support stereocilia that transducer mechanical movement in cochlea and vestibular labyrinth into neural activity sent to brain stem; some hair cells also receive inputs from brain. Arranged in 4 rows that run down length of basilar membrane – one row of about 3500 inner hair cells and 3 rows with total of about 10 500 hair cells  Auditory nerve fibres: collection of neurons that convey info from hair cells in cochlea to (afferent) and from (efferent) the brain stem. Collection also includes neurons for vestibular system  Stereocilia: hairlike extensions on tips of hair cells in cochlea that initiate release of neurotransmitters when they are flexed. On inner hair cell, arranged as if posing for group photo, in several nearly straight rows with shorter stereocilia in front and taller ones in back. On outer hair cell, stands in rows that form shape of V or W  Tectorial membrane: gelatinous structure, attached on one end, that extends into middle canal of ear, floating above inner hair cells and touching outer hair cells. Taller stereocilia of outer hair cells are embedded in tectorial membrane, and cilia of inner hair cells are nestled against it  Like photoreceptors in retina, hair cells are specialized neurons that transducer one kind of energy into another form of energy.  Cochlea has only 14000 hair cells and hair cells must be capable of responding so fast that we can detect time differences of as little of 10 millionths of a second in order to know direction from which a sound arrives  Tip link: tiny filament that stretches from tip of stereocilium to side of neighbor o When stereocila connected by tip links bends together when deflected by shearing motion of tectorial membrane o When stereoceilim deflects, tip link pulls on taller stereocilium in way that opens ion pore somewhat like opening a gate for just a tiny fraction of a second – permits potassium ions to flow rapidly into hair cell causing rapid depolarization o Depolarization leads to rapid influx of calcium ions and initiation of release of neurotransmitters from base of hair cell to stimulate dendrites of auditory nerve  Opening of ion pores that results from direct connection between stereocilia via tip links is only known example of mechanoelectrical transduction (MET), which is responsible for both extreme speed and sensitivity of hair cells  MET also sensitive – ion pores open when deflection as little as 1 nm, roughly diameter of single atom  Firing of auditory nerve fibers finally completes process of translating sound waves into patterns of neural activity  Summary of process o Air pressure wave is funneled by pinna through auditory canal to tympanic membrane, which vibrates back and forth in time with sound wave o Tympanic membrane moves malleus, which moves incus, which moves stapes, which pushes and pulls on oval window o Movement of oval window causes pressure bulges to move down length of vestibular canal, and bulges in vestibular canal displace middle canal up and down o Up and down motion forces tectorial membrane to shear across organ of Corti, moving stereocilia atop hair cells back and forth o Flexing of stereocilia initiates rapid depolarization that results in release of neuro transmitters into synapses between hair cells and dendrites of auditory nerve fibers – and neurotransmitters initiate action potentials in auditory nerve fibers that are carried back to brain  Coding of Amplitude and Frequency in Cochlea o As amplitude of sound wave increases, tympanic membrane and oval window move father in and out with each pressure fluctuation – result is that bulge in vestibular canal becomes bigger, which causes cochlear partition to move father up and down, which causes tectorial membrane to shear across organ of Corti, which causes hair cells to bend farther back and forth which causes more neurotransmitters to be released, which causes auditory nerve fibers to fire action potentials more quickly o Larger the amplitude, higher firing rate of neurons that communicate with brain o Different pairs of cochlear partition are displaced to different degrees by different sound wave frequencies o High frequencies cause displacements closer to oval window, near base of cochlea; lower frequencies cause displacements nearer apex o Place code: tuning of different parts of cochlea to different frequencies, in which info about particular frequency of incoming soundwave is coded b place along cochlear partition that has greatest mechanical displacement o Cochlear tuning to frequency is caused by way structure
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