Hearing Science PDF

Hearing Science Anatomy and Physiology of the Auditory and Vestibular Systems ​ Hearing is a result of the peripheral and central nervous systems working together Vestibular system: is responsible for the ability to perceive changes in head movement (acceleration and deceleration...

Hearing Science Anatomy and Physiology of the Auditory and Vestibular Systems ​ Hearing is a result of the peripheral and central nervous systems working together Vestibular system: is responsible for the ability to perceive changes in head movement (acceleration and deceleration) and the orientation of the head with respect to gravity. And maintain our sense of balance. Temporal Bone: houses/supports and protects most of the structures of the auditory periphery. Provides a framework of support for the outer, middle, and inner ears as well as the 7th and 8th cranial nerves. Outer Ear Pinna: has a funnel effect that results in a slight enhancement of frequencies around 5 kHz. ​ Pinna Effect: More efficient collection of higher frequencies due to the ridges and recesses. Whereas wavelengths of lower frequencies are essentially larger than the pinna and can pass around the structure ​ Head Shadow Effect = a directional hearing effect. (Sound being presented on right side has direct access to right ear but head interferes with access to the left ear, so sound reaching left ear would result in lower intensity.) ​ Aids in localization by creating complex sound resonance that changes as the location of the sound source changes. External Auditory Meatus/ Ear Canal: 2.5-3 cm in length and 0.75 in diameter, originates at the concha and ends at the TM ​ Covered by epidermal lining and contains hair follicles and glands that secrete a substance that leads to production of cerumen ​ The EAM is sensitive due to being innervated by the Trigeminal(5th), Facial (7th), and glossopharyngeal(9th) cranial nerves. ​ EAM is an acoustic resonator → Peak Resonance = 3000 to 4000 Hz Middle Ear: (ME is an air-filled cavity that acts as a filter which alters transmission of some LOW frequency sounds through the system). Tympanic Membrane: 8-10 mm in diameter. Made of three layers; Epidermal (outer), Fibrous (middle), and Membranous (mucosal lining). The fibrous layer's thinner area is called the Pars Flaccida (most elastic portion of the TM). Pars Tensa is stiffer, containing more fibers. ​ Annular Ligament: Lines rim around the TM , and anchors membrane to wall of ear canal ​ Umbo: located in center of TM and marks point of attachment for malleus and TM Medial Wall of the Cochlea: Has two openings; ​ Oval Window: (Superior) → connected to Stapes ​ Round Window: (Inferior) → compresses fluid and relieves pressure from the oval window ​ Ossicular Chain: transmits vibrations from the eardrum to the cochlea. The structure of the chain serves to dampen high intensity sounds and stabilize middle ear transmission. ​ Eustachian Tube: primary function is to allow fresh air into the ME cavity and balance air pressure. Happens when you open your mouth or swallow. ○​ Protects ME from fluid from nose or mouth as the structure remains closed at rest. Transformer Action: sound travels through air at LOW impedance (a measure of resistance to movement). In hearing, sound changes in air pressure and must be directed to a fluid-filled system (the cochlea) where the impedance is HIGH. Without help from the transformer action sound would be reflected back out to the outer ear. ​ Three Mechanisms ○​ Area ratio advantage between TM and stapes footplate ○​ Lever advantage from ME bones ​ The way the malleus and incus interact ○​ Buckling advantage from concave structure of TM Acoustic Reflex: ​ Serves to attenuate high intensity sounds and help protect the inner ear. The higher the intensity of the acoustic signal above reflex threshold, the greater the contraction of the AR. (contraction of stapedius muscle due to stim) ​ Measures a change in stiffness when it bilaterally contracts stapedius muscle. ○​ Normal AR is initiated at 70-90 dB HL from 500, 1k, and 2kHz ○​ Amplitude is largest for bilateral stimulation ​ Acoustic Decay → detects retrocochlear lesions → Positive decay = BAD → Stapedius muscle innervated by facial nerve (CN 7)/ Tensor Tympani innervated by trigeminal nerve (CN 5) Inner Ear: Cochlea: a bony snail shaped shell, spiral is from 2.2-2.9 turns ​ Located in petrous portion of temporal bone ​ Fluids: Endolymph and perilymph ​ Membranes: Basilar, Tectorial, Reissner’s ○​ Basilar and Reissner's divide cochlea into 3 ducts  ​ Basal portion of the cochlea (HFs) Narrow/ stiffer ​ Apical portion of the cochlea (LFs) Wide/ less stiff ​ Promontory: boniness between oval and round window, serves as protection Membranous Cochlea and Auditory nerve fibers (Inside) ​ 3 Ducts ○​ Scala Vestibuli (Superior) → Oval window, Stapes Footplate = Perilymph ○​ Scala Media (Cochlear Duct) = Endolymph ○​ Scala Tympani (Inferior) → Round Window = Perilymph ​ Endolymph = High in potassium, low in sodium ​ Perilymph = High in Sodium, low in potassium ○​ Perilymph and endolymph (cochlear fluids) need to be separated, why? ​ Whenever you want charged particles to move you have to have a gradient by keeping the difference between charged ions stable by keeping them separated ​ Helicotrema: where the scala tympani and vestibuli communicate ○​ Scala media communicates with the saccule through the ductus reuniens ​ 2 fluid filled channels: → key roles in inner ear function ○​ Vestibular Aqueducts: ​ Endolymphatic channel ○​ Cochlear Aqueducts: ​ Contains perilymph ​ Basilar Membrane: ○​ Supports the Organ of Corti (actual end organ for hearing) ​ Thicker and and stiffer at the base ​ Stria vascularis = battery of the cell → maintains the electrical potential ​ Outer hair cells = cochlear amplifiers, sharpen the peak which makes sounds clearer ○​ Without OHC’s would not be able to hear soft sounds ○​ OHC are mobile, do their dance when they're activated ○​ IHC’s are primarily responsible for encoding and connecting sound to the brain ​ IHC’s = 95% afferent (carry nerve impulses towards brain) ​ OHC’s = 95% efferent (carry nerve impulses away from brain) ​ You can still hear in general without OHC’s ​ Act of potassium entering cell = depolarization (excitation) ○​ After depolarization → calcium channels open ○​ Hearing aids can give back the amplification that OHC’s once gave but are not able to sharpen the tuning and that is why they complain of things still not sounding “clear” What is the difference between damaging IHC’s vs OHC’s? → When inner hair cells are damaged, it directly impacts the transmission of sound information to the brain, leading to significant hearing loss, particularly in understanding speech, while damage to outer hair cells primarily affects the amplification of sound, resulting in a less severe hearing loss and potentially causing distortion at higher sound levels; ​ Essentially, inner hair cell damage is more detrimental for clear sound perception, while outer hair cell damage can lead to reduced sensitivity to quieter sounds. ​ IHC = Transmission of info ​ OHC = amplification Vestibular Apparatus: ​ 3 semicircular canals ○​ Utricle ○​ Saccule Basilar Membrane movement and Stereocillia deflection: ​ Rarefaction stimulus causes an upward movement in the BM → which leads to stereocilia deflection away and depolarization of the hair cell. ○​ Rarefaction = depolarization ​ Resting phase = no movement of BM ​ Compression stimulus results in BM movement downward → leads to stereocilia deflection toward limbus and hyperpolarization of hair cell ○​ Compression = hyperpolarization OAE’s → Otoacoustic Emissions = sounds generated by the inner ear in response to sound. They are a sign of cochlear health, OAE’s are generated either spontaneously or in response to acoustic stimulus. ​ OHC motility is linked to OAE’s ○​ If OHC’s are damaged, OAE’s are usually absent → SNHL (cochlear damage) ​ OAE intensity range = 30-40 dB ○​ So a loss of 30-40 or more will result in absent OAE’s ​ SO damage to IHC, auditory nerve or central mechanisms will NOT compromise OAE’s ​ OAE’s measure OHC/Cochlear function NOT neural function ​ OHC’s are often damaged by high intensity sounds, IHC’s often survive ○​ IHC’s play a major role in transduction and coding of high intensity acoustic signals Cochlear potentials associated with hair cell depolarization: ​ Endocochlear/ Resting potential ​ Cochlear microphonic = summation of hair cell responses from along the BM to a sound stimulus ○​ CM mimics the stimulus → Alternating polarity cancels out CM ​ Summating potential = direct current potential generated by hair cells Auditory Nerve: ​ Connects the cochlea to the brainstem ​ Is apart of the 8th cranial nerve ​ All auditory nerves course to the CPA and then input to the cochlear nucleus in the brainstem ○​ Type 1 fibers connect to IHC’s (90%) ​ Myelinated ○​ Type 2 fibers connect to OHC’s Tonotopic Organization: ​ The position of a neuron indicates the frequency of sound it is most sensitive to; essentially, it's a map of sound frequencies across the auditory pathway, from the cochlea to the brain. A&P of the balance system ​ Balance system is comprised of multiple sensory systems ○​ Vestibular system ○​ Visual system ○​ Somatosensory/proprioceptive system ​ Information is integrated at the level of the brain stem in the cerebellum ○​ With significant influence from the cerebral cortex *** balance system as opposed to vestibular system. Do not confuse these 2 terms. ○​ The balance system is comprised of 4 elements: ​ Vision ​ Inner ear ​ Musculature ​ Cerebellum Ocular Motor Control and Perceptions of Motion ​ The membranous structure of the vestibular system is housed within the petrous portion of the temporal bone. ○​ It is secured to the bony labyrinth by connective tissue and bathed in perilymph. ​ The membranous structure has 2 groups of specialized sensory receptors ​ The first group is comprised of: 3 semicircular canals (SCC) ○​ Lateral (a.k.a. horizontal) ○​ Posterior ○​ Superior ​ Each of the SCC’s originates from the utricle and terminates in a dilated end (the ampulla). ○​ The ampulla also attaches to the utricle ​ The second group is comprised of: The 2 otolithic organs ○​ Utricular macula ○​ Saccular macula ​ They come together at roughly right angles ○​ The 2 horizontal SCCs (left and right) are in parallel planes ○​ The two superior and posterior canals are in roughly orthogonal planes to one another ​ The SCCs (cont’d) ○​ So they’re paired like this: ​ Right horizontal SCC → Left horizontal SCC  ​ Right superior SCC → Left posterior SCC ​ Right posterior SCC → Left superior SCC ​ The otolithic organs also function in pairs: ○​ 2 utricular maculae in approximately the horizontal plane ○​ 2 saccular maculae in the vertical plane ​ Contained within each SCC ampulla and otolithic organ is an arrangement of hair cells ○​ These constitute the neuroepithelial transduction mechanism for the vestibular end organs ​ The hair cells are situated on a mound of supporting cells in the ampulla called the crista ampullaris ​ Covering the hair cell projections (stereocilia and kinocilium) within the ampulla is a gelatinous membrane, the cupula Industrial Audiology Physical Health Impact of Noise Exposure: ​ Hearing loss, Tinnitus, Increased blood pressure, Stress reactions, sleep disturbance ​ Cardiovascular disease → * can be caused by increased stress * Mental Impact of Noise Exposure: ​ Sense of isolation, Annoyance, Interference with concentration, Communication challenges, Difficulty learning, Behavioral problems Current OSHA Requirements: What monitoring is required? ​ The hearing conservation program requires employers to monitor noise exposure levels in a way that accurately identifies employees exposed to noise at or above 85 decibels (dB) averaged over 8 working hours, or an 8-hour time-weighted average (TWA). ​ The exposure measurement must include all continuous, intermittent, and impulsive noise within an 80 dB to 130 dB range and must be taken during a typical work situation. ​ This requirement is performance-oriented because it allows employers to choose the monitoring method that best suits each individual situation. ​ Employers must repeat monitoring whenever changes in production, process, or controls increase noise exposure.  ○​ These changes may mean that more employees need to be included in the program or that their hearing protectors may no longer provide adequate protection. ​ Employees are entitled to observe monitoring procedures and must receive notification of the results of exposure monitoring. ○​ The method used to notify employees is left to the employer’s discretion. ​ Employers must establish and maintain an audiometric testing program. This must include: ○​ Baseline audiograms ○​ Annual audiograms ○​ Training ○​ Follow-up procedures ○​ All testing must be provided at no cost to all employees who are exposed to an action level of 85 dB or above, measured as an 8-hour TWA Types of audiometric assessment required by OSHA: ​ There are two types of audiograms required in the hearing conservation program: baseline and annual audiograms An STS is an average shift in either ear of 10 dB or more at 2,000, 3,000, and 4,000 hertz Acoustics *A sound source needs to have mass and elasticity* Mass: defined as the amount of matter in a substance ​ → Therefore gasses, liquids and solids all contain some amount of mass ​ → weight is related to mass but is a different concept Weight: Gravitational force exerted on a mass by earth mars and moon Elasticity: An object’s ability to resist change to its shape and volume ​ → Steel has much more elasticity than a piece of paper ​ → A volume of air can return to its former volume after being compressed *Sound perception is relied on the air molecular vibration to create sound pressure change* Sound Intensity: Amount of energy transmitted per second over an area of one square meter (Unit: watt/ m^2) Hearing Level Threshold: The lowest intensity at each frequency that a person with normal hearing can be expected to hear 50% of the time. Sensation Level (SL): describes the level of a sound relative to the threshold of the subject ​ → If a person’s threshold is 23 dB HL, then 33 dB HL is 10 dB SL to this person dB SPL: ​ → Animal Studies ​ → OAE’s HL: Audiogram ​ → SRT ​ → SDT SL: ​ → WRS Psychoacoustic (?) Define: 1.​ Acoustic Impedance: a.​ how much a medium resists sound waves passing through it 2.​ Active Mechanism: a.​ A process in the cochlea that amplifies sound, primarily mediated by the outer hair cells. 3.​ Apex: a.​ The inner tip of the cochlea where low frequencies are processed. 4.​ Basilar Membrane: a.​ A membrane within the cochlea that vibrates in response to sound, with different locations responding best to different frequencies. 5.​ Base: a.​ The beginning of the cochlea where the oval window is situated and high frequencies are processed 6.​ Characteristic Frequency (CF): a.​ The frequency that elicits the greatest response at a particular point on the BM or from a specific auditory nerve fiber. Synonymous with Best Frequency (BF) 7.​ Cochlea: a.​ The spiral-shaped, fluid-filled organ in the inner ear responsible for converting sound vibrations into electrical signals. 8.​ Combination Tones: a.​ Distortion products perceived as additional tones when two pure tones are presented simultaneously. 9.​ Compound Action Potential: a.​ The combined electrical response of a group of auditory nerve fibers to a sound stimulus. 10.​Compressive Nonlinearity: a.​ A nonlinear response where the output increases less than proportionally to the input, compressing a large input range into a smaller output range. 11.​Conductive Hearing Loss: a.​ Hearing loss due to a reduced efficiency of sound transmission through the outer or middle ear. 12.​Dead Region: a.​ A portion of the BM where inner hair cells are non-functional, leading to a complete absence of transduction and neural response for specific frequencies. 13.​Dynamic Range: a.​ The difference in level between the loudest and quietest sounds that a person hears. 14.​Efferent Nerve Fibers: a.​ Nerve fibers that carry information from the brain to the cochlea. 15.​Envelope: a.​ The curve that outlines the peaks of a waveform, especially relevant in describing the traveling wave on the BM. 16.​Evoked Otoacoustic Emissions: a.​ Sounds emitted from the ear in response to an acoustic stimulus, reflecting active processes within the cochlea. 17.​Frequency-Threshold Curve (FTC): a.​ Synonymous with tuning curve, a plot depicting the sound level required to elicit a threshold response from an auditory nerve fiber at different frequencies. 18.​Hair Cells: a.​ Sensory cells within the cochlea that transduce mechanical vibrations into electrical signals, including inner and outer hair cells. 19.​Inner Hair Cells (IHCs) a.​ The primary sensory hair cells in the cochlea, responsible for sending auditory information to the brain. 20.​Input-Output Function: a.​ A graph illustrating the relationship between the magnitude of a stimulus (input) and the magnitude of the response (output). 21.​Middle Ear Reflex: a.​ A reflex activated by loud sounds that causes contraction of muscles in the middle ear (stapedius), reducing sound transmission primarily at low frequencies. 22.​Organ of Corti: a.​ The sensory organ within the cochlea that contains the hair cells and associated structures responsible for hearing. Supported by BM 23.​Ossicles: a.​ The three small bones in the middle ear (malleus, incus, and stapes) that transmit vibrations from the eardrum to the oval window 24.​Otoacoustic Emissions: a.​ Sounds produced by the cochlea that can be detected in the ear canal. 25.​Outer Hair Cells (OHCs): a.​ Hair cells in the cochlea that amplify the motion of the BM, contributing to sensitivity and sharp tuning. 26.​Oval Window: a.​ A membrane-covered opening in the cochlea that receives vibrations from the stapes. 27.​Passive Mechanism: a.​ The mechanical properties of the BM and surrounding structures that contribute to frequency tuning, operating in a largely linear manner. 28.​Phase Locking: a.​ The tendency of auditory nerve fibers to fire action potentials in synchrony with the phase of a low-frequency sound stimulus. 29.​Presbycusis: a.​ Age-related hearing loss. 30.​Rate vs. Level Functioning: a.​ A plot of a neuron's firing rate as a function of sound level at a specific frequency. 31.​Reissner’s Membrane: a.​ One of the membranes that divides the cochlea lengthwise. 32.​Retrocochlear Hearing Loss: a.​ Hearing loss caused by damage to the auditory nerve or other parts of the auditory pathway beyond the cochlea. 33.​Round Window: a.​ A membrane-covered opening in the cochlea that allows for pressure equalization when the oval window vibrates. 34.​Saturation: a.​ The point at which a neuron's firing rate no longer increases with increasing sound level. 35.​Sensorineural hearing Loss: a.​ Hearing loss caused by damage to the cochlea or the auditory nerve. 36.​Spontaneous Firing Rate:​ a.​ The rate at which an auditory nerve fiber fires action potentials in the absence of sound stimulation. 37.​Stereocilia:​ a.​ Small hair-like projections on the top of hair cells that are deflected by sound-induced vibrations, leading to transduction. 38.​Stria Vascularis: a.​ A structure in the cochlea that plays a vital role in maintaining the electrical potential necessary for hair cell function. (Battery) 39.​Tectorial Membrane:​ a.​ A gelatinous membrane that lies above the hair cells and interacts with their stereocilia during sound stimulation. 40.​Threshold: a.​ The lowest sound level at which a response is elicited from an auditory nerve fiber or is perceptible to the listener. 41.​Tonotopic Organization: a.​ The orderly arrangement of neurons according to their characteristic frequency, reflecting the frequency-to-place mapping on the BM. 42.​Traveling Wave: a.​ The pattern of vibration that moves along the BM from the base to the apex in response to sound. 43.​Tuning Curve: a.​ A graph showing the sensitivity of an auditory nerve fiber or a point on the BM to different frequencies, often by plotting threshold as a function of frequency. 44.​Two Tone Suppression: a.​ The reduction in the response to one tone caused by the simultaneous presentation of a second tone, particularly relevant when the second tone is outside the excitatory area of the first tone.  Cochlear implants What is a cochlear implant? ​ Most successful sensory prosthetic device ​ Bypasses the cochlea to stimulate the auditory nerve directly Why bypass the cochlea? ​ With severe to profound loss we no longer have the strong connection and neural firing ​ So instead we use electrical stimulation to stimulate the neural population Basic CI Components: ​ Electrodes ideally in scala tympani ​ 12-22 electrode contacts ​ Info is sent across the hair and skin ​ The CI provides electrical stimulation to the cochlear nerve ​ The stimulation from the CI encompasses the entire speech frequency range Difference from Hearing Aids: ​ Hearing aids ○​ Non-surgical management of HL ○​ Stimulates the auditory nerve with acoustic energy ○​ Mechanically stimulates sensory hair cells ​ Cochlear implants ○​ Surgical management of HL ○​ Stimulates the auditory nerve with electrical impulses ○​ Bypasses disordered cochlear hair cells Benefits of CI: ​ Detection of speech and environmental sounds at comfortable loudness levels ​ Enhancement of lip-reading ​ Enhancement of speech perception skills (detection, discrimination, identification, comprehension) ​ Improvement in speech production and vocal quality after experience and training ​ Detection → discrimination → identification → comprehension How is sound processed? ​ Sound is separated into chunks of spectral information (channels) ○​ A simplified example: four channels, 200-503 Hz, 503-1265 Hz, 1265-3181 Hz, and 3181-8000 Hz ​ The temporal envelope is kept, the fine structure is thrown away ○​ The temporal fine structure is replaced by an electrical pulse train ​ This stream of electrical pulses is sent to an electrode in the CI electrode array ○​ Lower frequency channels toward the apex of the cochlea ○​ Higher frequency channels toward the base of the cochlea → end up with pulses of electrode stimulation that follow the envelope → cochlea is on octave spacing so the frequency gets doubled ​ Logarithmic spacing → these devices are designed for primarily speech understanding so things like music sound extremely distorted Putting it all together: 1.)​The microphone picks up sounds from the environment 2.)​The speech processor amplifies, filters, and digitizes the sound into digital code 3.)​These coded signals are sent to the transmitting coil via the cable 4.)​The transmitting coil delivers processed signal across skin via electromagnetic induction/radiofrequency transmission 5.)​The internal receiving coil receives electromagnetic signal and transmits information to the internal stimulator 6.)​The internal stimulator generates electrical pulses proportional to magnitude of original signal 7.)​Pulses delivered to designated electrode contacts, providing electrical current to auditory nerve fibers Impedance ​ In order to maintain the voltage difference between two endpoints, a circuit must contain a component that impedes flow ○​ We need both voltage and impedance (opposition to current) ​ Electric current is directly proportional to voltage ○​ As voltage increases, current increases → Think a kink in the hose of our water analogy Electrical Stimulation Parameters ​ Need to code ○​ Intensity ○​ Frequency ○​ Time CI stimulus ​ Rectangular, symmetrical, biphasic electrical pulses (charge balanced) ○​ Like other implantable neuroprostheses, alternating current is used for safe stimulation with no net charge ​ Pulses can be altered in two ways to affect perceived loudness ○​ Increase/Decrease pulse amplitude (height) ○​ Increase/Decrease pulse duration (width) ​ Each phase has a pulse width of 10 -100 microseconds ○​ 1 microsecond is 1/1,000,000th of a second Electrodes ​ Electrode array ○​ Electrode leads (wires) ○​ Intracochlear electrodes contacts (active/stimulating electrodes) ○​ Extracochlear ground electrodes Stimulation type ​ Monopolar stimulation (Most common → Default) ○​ Stimulation is provided to an active intracochlear electrode and an extracochlear electrode serves as the ground or return electrode ​ Default stimulation type in all 3 manufacturers ​ Benefits ○​ Broader field of electrical stimulation recruits greater number of cochlear nerve fibers, allowing for sufficient loudness at lower charge levels ○​ Better battery life Bipolar stimulation ​ Stimulation if provided to an active intracochlear electrode and the neighboring intracochlear electrode serves as the ground or return electrode ​ Potential benefits ○​ Narrower field of electrical stimulation should theoretically lead to less channel interaction and greater frequency resolution ​ Limitations  ○​ Recruits fewer cochlear nerve fibers, resulting in need for increased current level ○​ Reduced battery life What is a speech processing strategy or signal coding strategy? ​ The way the input signal is converted to an electrical code ​ How the system translates the ○​ Frequency ○​ Temporal ○​ Amplitude...cues of acoustic signals into electrical stimulation How would you represent speech with an electrical signal? ​ Frequency, intensity, temporal cues ​ What are the challenges? ○​ Speech is really variable, hard to account for all listening situations DECISION TO IMPLANT purports the decision hinges upon two questions: → Relative to the outcome possible with optimized hearing aids, will a cochlear implant improve the patient’s communication abilities and/or hearing performance? Relative to the outcome possible with optimized hearing aids, will a cochlear implant improve the patient’s quality of life? If yes, then we need to consider: → Safety/medical contraindications Realistic expectations and support system EAR CHOICE Implantation of poorer ear ​ Keep residual hearing in better ear Implantation of better ear ​ Sometimes appropriate if poorer ear not expected to be successful Example: adult congenitally deaf in one ear, progressive HL in better ear