Sound and the Neurophysiology of Hearing Today’s Topics...

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Unformatted text preview: Sound and the Neurophysiology of Hearing Today’s Topics • • • • • • Some Factors Affecting Pitch Perception Sound Intensity and the Decibel Scale The Ear Theories of Pitch Perception Auditory Pathway Auditory Cortex Factors Affecting Pitch Perception: Presbycusis Top range of healthy human hearing: approximately 20 Hz – 20,000 Hz Presbycusis: age-related hearing loss • Occurs gradually as people get older • Hearing loss usually greater for higher frequencies (>2 kHz) • Usually occurs after age 50, but can begin much earlier (as early as age 18) • Estimates: 1/3 of adults over 65, 40-50% of adults over age 75 Common symptoms: • difficulty hearing female and children's voices • overall loss of speech clarity – particularly for consonant sounds, which are generally made up of higher frequencies and have less energy than vowels. • Usually occurs in both ears equally Primary Cause: degeneration of hair cells and neurons of inner ear (we'll talk about these later in this chapter!) Virtual Pitch Most sounds – even individual musical tones - are complex sounds consisting of many component frequencies. But we still experience it as a SINGLE tone! Normally, our perception is that the tone has the pitch associated with the fundamental frequency. It's then the overtones that contribute to the timbre of the tone. “Virtual Pitch” Phenomenon Sometimes we can hear sound that is missing the fundamental frequency, yet still PERCEIVE that missing fundamental by hearing the rest of the overtones. The idea that we must actually HEAR the fundamental frequency to perceive it as the pitch of the tone is not correct. Our brain is somehow able to INFER what the fundamental must be, when we hear the rest of the overtones! Virtual Pitch Demonstration Let’s listen to these melodies – each with different component frequencies. Pure tones Harmonics 1 – 10 (same fundamental is present) Harmonics 4 – 10 (but without fundamental present) Loudness Perception Sound Intensity • The greater the amplitude of a sound wave, the louder it will be experienced. • Our ears are VERY sensitive to sound intensity. • Sound wave intensity is the amount of energy transported by the wave past a given area per unit of time (Watts/meter squared). The energy transported by a sound wave is directly proportional to the square of the amplitude of the wave. The range of intensities that we are sensitive to is HUGE – the scale of intensities necessary to describe that range would go from 1 to about 1 quadrillion (10 15)! So, to make sound intensity numbers easier to deal with, we actually use a special scale called the DECIBEL scale. Decibel Scale Decibel scale • Logarithmic scale (not linear!) • A 10 dB increase represents a TENFOLD increase in intensity. • Imagine that we give the faintest discernable sound (for humans) a value of 1. • Sound intensity in decibels is given by: I I 10 log10 1 I0 Where I1 is the intensity of the sound, I0 is the reference sound (i.e. the faintest discernable sound) Intensities of Musical Instruments Instrument Sound Intensity rock music peak 150 dB symphonic music peak 120 – 137 dB amplified rock music at 4-6 ft. 120 dB timpani & bass drum rolls 106 dB piccolo 95 – 112 dB average Walkman on 5/10 setting 94 dB clarinet 92 -103 dB piano fortissimo 92 – 95 dB french horn regular sustained exposure may cause permanent damage 90 – 106 dB oboe 90 – 94 dB trombone 85 – 114 dB flute 85 – 111 dB cello 82 – 92 dB violin 84 – 103 dB chamber music in small auditorium 75 – 85 dB fortissimo singer 3 ft. away 70 dB normal piano practice 60 – 70 dB 90 – 95 dB Sound Intensity and Hearing Loss SAFE levels of sound waves are for human hearing, OSHA (the Occupational Safety and Health Administration): • • • No more than 85-90 DB over an 8-hour period. Prolonged exposure over this level can lead to hearing loss. Rock concerts regularly exceed 110 dB. Hearing loss in musicians is more common than it is in the rest of the population: • • • About 61% of adult musicians between 27-66 have hearing loss. About 22% of youth musicians aged 18-22 have hearing loss. About 16% of child musicians age 8-12 have hearing loss. While age-related hearing loss affects mainly high frequencies (higher than the range of most music), hearing loss due to prolonged exposure to loud music affects the frequencies at 4000 Hz and lower, which is in the musical frequency range. The Outer (External) Ear Outer (External) Ear • Auditory Canal – tube allowing passage of sound waves from environment to ear drum (tympanic membrane) • Pinna • Visible ear “flap” • Gathers sound waves and funnels them into auditory canal • Helps with sound localization The Tympanic Membrane and Middle Ear Tympanic Membrane (Ear Drum) Very thin membrane that vibrates along with sound wave. Middle Ear Consists of three tiny bones (ossicles) The ossicles amplify and transfer vibrations from tympanic membrane to inner ear (to the cochlea). The Ossicles Ossicles: Malleus (hammer) • Handle connected to eardrum • Head connected to incus Incus (anvil) • Middle ossicle, connects malleus and stapes Stapes (stirrup) • Contains flat oval bone called the footplate, that is attached to cochlea’s oval window. a) Tympanic membrane (eardrum) e) Malleus f) Incus g) Middle ear h) Stapedius muscle i) Stapes j) Stapes footplate and oval window Tensor Tympani and Stapedius Muscles Tensor Tympani / Stapedius: • Help protect the ear from loud sounds by dampening vibrations delivered to inner ear (cochlea) Tensor Tympani • • • Attaches to malleus When it contracts, it pulls on the malleus, tensing the ear drum and dampening the vibration of the ossicles – this reduces the amplification of sounds Main function is to reduce noise made by chewing, speaking, yawning, singing Stapedius • • • Attaches to stapes When it contracts, it pulls the stapes away from oval window, reducing amplitude of vibrations Main function to reduce loud environmental sounds (but also chewing, talking, singing…) The action of the muscles is known as an acoustic reflex. Contraction occurs when sounds reach somewhere between 70-100 dB, and reduce sound level reaching cochlea by about 20 dB. Cochlea Cochlea: • Main structure of the inner ear • Filled with fluid • Amplified vibrations transferred to cochlea’s oval window, which vibrates fluid inside the cochlea as well • When stapes pushes on oval window, fluid in scala vestibuli pushes on Reissner’s membrane • When stapes pulls back from oval window, cochlear duct (scala media) moves back upward • As oval window vibrates, cochlear duct moves up and down at same frequency as oval window • This motion creates waves in basilar membrane, which is attached to scala tympani side of organ of corti Cochlea Organ of Corti: • • • • • Basilar membrane has hair cells embedded in it (the tectorial membrane rests on top of the hair cells) When organ of corti and basilar membrane vibrate at any point along the cochlea’s length, it produces a shearing force on the cilia of the hair cells. This causes these cilia to deflect (bend), which results in the hair cells producing neural impulses. These impulses are then sent down the auditory nerve to the brain. Transduction has occurred! Transduction & Sensory Encoding Transduction – conversion of one form of energy to another With regards to sensation, transduction means converting stimulus energies (sights, sounds, etc), into neural impulses that our brain can interpret The auditory system needs a way to represent the various aspects of the original sound wave stimulus (i.e. frequency, amplitude) sensory encoding – encoding various aspects of stimulus into patterns of neural impulses that can be interpreted by the brain These neural patterns “represent” the stimuli being detected Encoding for Pitch The way we discriminate pitches currently combines two theories: - Place theory - Frequency theory Perceiving Pitch: Place Theory Place Theory - different parts of the basilar membrane are responsive to different frequencies - high frequencies produce large vibrations near the beginning of the membrane - lower frequencies produce large vibrations near the end of the membrane Problem: At lower frequencies, there stops being a single place on the membrane that deforms maximally. Perceiving Pitch: Time (Frequency) Theory Frequency Theory - whole basilar membrane vibrates - nerve impulses are triggered at same rate as frequency of sound wave Problem: Neurons can only fire at a maximum of 1000 times/second (if that!), so this doesn’t explain how we detect frequencies over 1000 Hz. Volley Principle Volley Principle - higher rates of firing can be accomplished if neural cells work together and alternate firing. Simplified Pathway of Auditory Signals Simplified auditory pathway Impulses created by hair cells exit ear and travel along auditory nerve Signals then travel to cochlear nuclei in the brainstem (around junction of pons and medulla) Signals then sent to both ipsilateral and contralateral superior olivary complex (in pons) Signals proceed to inferior colliculus (in midbrain) Signals go to medial geniculate nucleus (in thalamus) Signals finally arrive in the auditory cortex Auditory Cortex A Few Basics of the Auditory Cortex Located near upper parts of temporal lobes Auditory Cortex located deep in lateral (Sylvian) fissure, in an area called Heschl’s gyrus Auditory cortex on each side of brain receives information from both left and right ears 3 main regions Primary Auditory Cortex (“Core” or “A1”) Reacts to simple aspects of sound (sound onset/offset, frequency, etc) This is where we become aware of sound (any sound will activate this area) Arranged tonotopically – mapping of locations from basilar membrane to surface of cortex Belt and Parabelt (secondary regions) Respond little (if at all) to simple tones, only more complex tones Appear to respond differentially to “what” a sound is (e.g. speech) and “where” it is Lateral belt seems to process “what” information Posterior belt seems to process auditory spatial info (“where”) Auditory Cortex ...
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  • Fall '19
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