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85 Cards in this Set
- Front
- Back
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Cochlear Mechanics Overview
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Stapes generates sound wave in inner ear fluid. Inside cochlea, gets a pressure difference across BM. BM moves up and down. Organ of Corti rides with BM. Relative motion within organ of Corti results in shearing of hair bundles. Current flow through hair cells. Change of transmitter release. Signal encoded in auditory nerve.
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Pressure difference across BM
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Inward & outward movements of stapes cause pressure changes (vibrations) w/ in SM. Inward vibration of stapes is thought to result in BM moving toward ST & vice versa.
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Traveling Wave
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TW is not a sine wave moving down BM. It is characteristic motion occurring in reaction to vibrations of incoming sine wave. Dependent on properties of BM.
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Physical parameters that produce Traveling Wave (TW)
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Stiffness, mass & friction of BM.
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Basilar membrane width and stiffness
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BM is narrower at base (.15 mm) vs. apex (.5 mm); spiral lamina fills up difference. BS is stiffer at base; stiffness at apex is 1% what it is at base.
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Why does the base of the basilar membrane react before the apex?
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The base has less inertia to overcome. --> a characteristic phase/time lag from base to apex.
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Why does the TW occur?
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Phase changes along BM. All points are vibrating at the same frequency. Different parts of BM move at different times w/ different amplitudes.
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Bekesy's findings
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TW always begins at base & proceeds apically. Moves slower as it moves apically (dues to width, stiffness and fluid drag). TW moves from base to apex in ~5 ms. Amplitude grows until a certain point (dependent on frequency) then declines rapidly. Frequency selectivity.
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Problems with Bekesy's work
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1) Used cadavars.
2) Need high intensities. 3) Can only measure low frequencies. |
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Tuning Curve (TC)
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Certain point on BM has max amp for only one freq & is less for higher or lower freqs. Width of TC at some arbitrary point is considered a measure of frequency resolution.
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Bekesy's tuning curves
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Shallow, almost flat, low freq slope of 6 dB/oct. More steep high freq slope 26 dB/oct. Led to notion that BM acts like low pass filter - each location passes freqs up to its best freq. Would expect this b/c low freq sounds always begin in base and proceed toward apex.
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The TC dilemma
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How could BM envelope with its broad low freq response be responsible for our ability to perceptually discriminate very fine diffs in freq? In the 50s and 60s the tuning characteristics of the 8th nerve fibers were found to be much better in terms of frequency resolution than would be accounted for by relatively broad tuning curve of BM. Something between BM & 8th nerve fibers must be responsible for "sharpening" the tuning curve.
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Russell & Sellick (1978) measure IHCs
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IHC tuning was identical to 8th nerve tuning curves. Sharp IHC tuning was pre-neural.
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No 2nd Filter
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Freq resolution is largely determined by cochlear process at level of BM/cochlear partition. Hypothesized that there must be some process in health cochlea, called ACTIVE process, in addition to passive process related to physical properties of BM that is necessary for maintaining sharp freq tuning at BM.
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Gold (1948)
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Mathematically concluded some active energy process would be needed. Hypothesized that additional mechanical energy would have to generate a vibratory by-product (i.e. some kind of feedback loop).
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Kemp (1978)
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Recorded sounds emanating from the ear (Kemp echos = otoacoustic emissions).
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Role of Efferents?
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Kim (1984), Thompson & Thompson (1991) and Kim et al. (1995) showed that Type II afferents from OHCs go to CN, then to MSO, which implies a reflex loop w/ efferents returning to OHCs. Ultimately, rewrote aud theory of cochlear transduction.
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What did William Brownell (1983) demonstrate?
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That OHCs are able to elongate & contract in response to electrical currents.
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OHC structure and physiology
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Have developed endoplasmic reticulum & Hensen's body (modified Golgi apparatus) similar to that in muscle cells. Energy production occurs outside OHC & energy utilization occurs as direct conversion of electrical potential energy w/ in endolymph into mechanical energy by OHC. Arrangement allows OHCs to move without getting tired.
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Damage to OHC active processes
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1) Elevated thresholds
2) Broadens freq tuning 3) Linear responses near CF 4) Eliminates OAEs |
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OHC Motility
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OHCs stereocilia firmly coupled to TM so that movement of BM is directly tied to movement of stereocilia. Displacement of BM made more effective. Results in sandwiching effect between TM & IHC stereocilia at low to moderate intensities. Bending of IHC through passive processes only happens at high intensities & with broader freq resolution (fluid drag?).
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Two ways to classify OAEs
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Measurement based and mechanism based
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Measurement Based OAE classification
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Spontaneous: narrow band signals occurring in absence of external stimulation.
Evoked: occur following acoustic stimulation. Found in almost all normal ears. Classification scheme depends on properties of evoking stimulus. Transient (clicks or tone bursts), distortion-product, or stimulus frequency. |
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Mechanism Based OAE classification
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Reflection emissions: due to linear coherent reflection of traveling wave from random impedance perturbations. (E.g. SOAE).
Distortion emission: due to nonlinearities acting as sources of cochlear traveling waves. All evoked emissions are a combination of linear reflection and non-linear distortion mechanisms. |
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Stimulus-frequency (SFOAE)
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Reflection emissions at low levels; distortion emission at high levels.
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Transiently evoked (TEOAE)
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Reflection emission at low levels; distortion emission at high levels.
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Distortion product (DPOAE)
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Always initiated by non-linear distortion; may have additional reflection component.
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Measurement based. Which one doesn't belong: DPOAE, SFOAE, TEOAE, SOAE
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SOAE - because you don't put in a stimulus. Not evoked, the others are evoked.
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Mechanism based. Which one doesn't belong: DPOAE, SFOAE, TEOAE, SOAE
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DPOAE - generated by distortion. Others are not generated by distortion at low to medium levels.
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SOAEs
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Usually inaudible. Presence suggests normal cochlear sensitivity near frequency of SOAE. Most likely originates from nonlinear OHC activity at place in cochlea tuned to freq of SOAE. Emission needs to be at least 3 dB > NF. Vulnerable to same insults that damage OHCs (drugs and noise).
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Characteristics of SOAEs
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Bandwidths that are narrow ~1 Hz wide. Not measured in the presence of HL > 25-30 dB HL.
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Why is the noise in the ear canal for OAEs pre-amplified and high pass filtered?
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Because there is a lot of physiologic noise below 500 Hz.
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Prevalence of SOAEs
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Present in ~35-72% of normal ears. Multiple SOAEs in the same ear is possible. Detected in one or both ears of the same subject. More common in women. More often in right ears. Prevalence pattern similar to that of adults in neonates, infants, and children.
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Frequency Characteristics of SOAEs
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Adult ears = majority of SOAEs measured between 1-2 kHz vs. infants = 3-4 kHz. Can be measured at higher frequencies in both populations, don't see less than 500 due to filtering used. Freq of SOAE is stable over short period of time but then can shift (<1% or 10 Hz or less).
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Amplitude of SOAEs
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-12 to 20 dB SPL (adult mean amp = -3 to 0 dB SPL; infants = 10 dB SPL). Fairly stable after 24 months of age.
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TEOAEs
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Intensity at which TEOAE can be detected rarely corresponds with perceptual threshold. Use short duration tones, clicks, tone bursts, chirps or noise. Possible to evoke with bone conduction. Measure emission for ~20 ms following each click. Need good SNR.
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Chirp stimulus
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Sinusoidal signal with an instantaneous frequency that changes monotonically.
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Click stimulus
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Stimulates large portion of the cochlea. Most robust response from mid frequency range 1-3 kHz.
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Tone burst stimulus
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More specific narrow band region.
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Human hearing is most sensitive between
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1-3 kHz. This is because of ME characteristics.
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Time-domain averaging
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A common method to extract a periodic component of interest from a noisy compound signal. With the period of the interesting component determined, we often consecutively cut out some segment in length of the period from the compound signal and directly average them. In such direct averaging with digital computers, the period actually used possesses some error, called period cutting error, which is less than half a sampling interval.
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Ideal TEOAE stimulus temporal waveform
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One positive peak, one negative peak and minimal ringing afterwards.
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The longer the duration of the stimulus the ____ frequency specific it will be.
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More
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How are low to moderate level TEOAEs described based on measurement AND mechanism classifications?
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TEOAEs occur following acoustic stimulation therefore are described as evoked based on measurement classifications.
For mechanism classification TEOAEs are classified as reflection emission at low levels and distortion emission at high levels. |
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Can you choose noise as your stimulus for TEOAE testing with the ILO96 equipment?
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Yes. (For pediatric testing it could be useful. For acoustic reflex testing it can be useful for evoking a response, so perhaps that is why it was put as an option. Noise is more random than a click stimulus.)
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What are the seven stimulus parameters important in TEOAE measurement?
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1. Type
2. Spectral characteristics (spectrum) 3. Intensity 4. Stability 5. Temporal characteristics 6. Polarity 7. Rate |
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Describe the click stimulus used for measuring TEOAEs in terms of temporal and spectral characteristics.
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The click stimulus contains a broad range of frequencies and stimulates a large portion of the cochlea, with the most robust response in the mid-frequency range from 1000-3000 Hz. It has a flat frequency response. The goal as far as temporal characteristics is to get a crisp, brief waveform in the ear canal.
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Why is it important to have a flat stimulus frequency response for measuring TEOAEs? If you do not have a flat stimulus frequency response, what can you do to alter this response?
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It is not possible to record a normal TEOAE if the stimulus is not flat. The stimulus must provide constant energy across the whole frequency range of the stimulus. If the stimulus spectrum is not flat the probe must be repositions or replaced.
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What is the desired stimulus intensity for recording TEOAEs and why?
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80-86 dB peak stimulus which is equivalent to about 45 dB SPL. A higher level stimulus is used to evoke a wider range frequencies for OAEs. If lower stimulus levels are used smaller OAES can be collected and the response can be dominated by frequencies near the SOAEs. Effective in eliciting robust response from normal ears yet sensitive for detection of cochlear dysfunction in abnormal ears.
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Why should you monitor stimulus stability?
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To be sure that the desired stimulus is being presented throughout the test.
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TEOAE stimulus set
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3 positive pulses that are equal in amplitude. 4th is a negative pulse that is 3 times larger in amplitude + 10 dB SPL (out of phase). Subaverage of responses to 4 stimuli is formed; this cancels artifact.
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Stimulus polarity
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Can use alternating polarity of click stimulus to minimize linear components in averaged TEOAE, so that nonlinear response of cochlea can be measured.
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Correlation coefficient
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Examine the reproducibility between two TEOAE waveforms. Value of 1 = pure signal. Value of 0 = noise only. Correlation is directly related to SNR. Kemp recommends overall correlation coefficient exceed 0.5 before accept TEOAE as present. Can use digital filtering to examine reproducibility of individual bands.
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TEOAE stimulus rate
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Not explored very much with TEOAEs. Usually slow to moderate rates < 60 stim/sec.
Amp reduction seen when stim rate inc from 50/sec to over 1000/sec. |
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Interstimulus interval
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Interval will shorten as inc stimulus rate. Amplitude will decrease with increase stimulus rate which shortens ISI.
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Intensity of TEOAE stimulus
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TEOAE amp increases with increased intensity. Rate of growth = nonlinear; at low & moderate levels, TEOAE amp increases at rate of ~0.5 dB/dB, while at high levels growth rate is less ~0.2 dB/dB. For screening purposes, typically use ~80-83 dB peak equivalent SPL = ~45 dB >perceptual threshold. Use higher level stim to evoke wider range of OAE freqs. With lower levels, smaller OAEs collected & response can be dominated by any freq comps near self sustaining-spontaneous oscillation condition (SOAEs).
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Infants TEOAE amplitude compared to adults
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~10 dB larger. Possible reasons: 1) smaller ear canal volumes, 2) more efficient ME systems, 3) more powerful cochlear generators.
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When is TEOAE considered present?
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If signal exceeds noise by 3 dB or more (6 dB = more conservative approach).
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TEOAE Power Spectrum
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Based on difference between 2 waveforms. Can use filters to get rid of low frequency noise.
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0.3 Pa = ___ dB SPL
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83.5
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DPOAEs
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We know from psychoacoustics that if we present two tones the cochlea generates numerous additional tones because of its non linearity. 2f1-f2 is the largest. Elicited fro all nml ears. Amp is usually 60 to 70 dB below stim levels.
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DPOAEs are thought to be generated by
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Active cochlear process responsible for enhancing BM vibrations.
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F1 is ____ in frequency than F2
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lower
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Two ways to determine the presence of DPOAEs
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1) A priori decision: DP amp is compared to noise floor to determine if present. Usually use 6 dB SNR. For screening purposes how many must pass?
2) Obtain distribution of data from normal and impaired ears. Compare your results to the distribution. Levels, level difference and frequency ratio parameters must be the same. |
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Commercially available equipment allows DPOAEs to be measured from
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1-6 kHz.
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We can measure DPOAEs up to
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13 kHz (f2 of 20 kHz)
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DPOAEs are usually plotted as a function of
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The primary tone freqs. Research has shown that DP is generate in region of cochlea that maximally responds to primary tones.
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DP amplitude is strongly dependent on...
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Frequency ratio (f2/f1). Largest amp is ~1.21-1.22 (collapsed across frequencies). Varies across frequencies. 1.1 better for higher frequencies, 1.3 better for lower frequencies.
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How do you judge if DPOAEs are normal or not?
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Most objective way: compare pts DP results with distribution from nml ears. When DP response is below normal range --> pathological.
Can look at DP amp vs. frequency (frequency sweep). This does not represent true sensitivity. Can also examine DP amp input/output functions. Can see threshold just above noise floor, ~3 dB. |
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Three response forms for DPOAEs
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1) DP gram (frequency sweep): vary freq, fixed ratio, fixed levels.
2) Level sweep: fixed freq, fixed ratio, vary levels. 3) Ratio sweep (latency): fixed frequency, varied ratio, fixed level. |
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DPOAE amplitude is more dependent on which level?
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L1. This is why you want L1 to be greater than L2.
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Implications of I/O function
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Clinical implications of the shape of the DP I/O function is unclear. Optimal combination of stim levels is still unresolved. Varies depending on what you are trying to examine. Should use moderate levels (below 70 dB SPL).
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DPOAE Latency Measures
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DPs have latencies that presumably reflect the delay due to the processes that govern the mechanical transmission within the cochlea assosciated with TW & development of resonance. Phase-gradient or group-delay method (hold f2 constant and sweep f1). Slope of the phase change of the DP w/ freq is used to calculate latency. Lower freq = longer latency.
Used for 1) ototoxic monitoring, 2) identify genuine responses (rule out artifacts). |
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DPs in Normal Ears
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Mean DP SNR 19-30 dB. Moderate variability across subjects. No differences between ears. No significant race or gender differences. Amplitude decreases with age.
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What is the DP amplitude difference for infants vs. adults
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3 dB. (Term infants have greater DP amplitudes than pre-term infants).
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Advantages of DP testing
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Get frequency specific info. Can measure in those with greater hearing loss that for TEOAEs. Typically will not be able to measure in hearing losses >50 dB HL.
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Calibration for DPs
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Adjust the speaker command voltage levels as a function of freq to produce constant SPL as measured at the DP measurement mic. Standing waves is a problem. There will be a substantial increase of stimulus SPL at the TM in the 3-7 kHz freq range (as much as 20 dB diffs). Most problematic when determining if the ear is impaired. Less problematic for serial monitoring.
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DPOAEs and SNHL
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No physiological evidence that supports the expectation that nml DPs can predict pure tone thresholds directly. To distinguish betwn neural and sensory losses need to use pure tone audiometry, ABR, and OAEs. Use immittance to make sure OAEs are not weakened due to ME problems.
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Nulls and dips of DPOAEs
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Nulls or monotonic dips can be observed in the DPOAE. They can occur in a DP-gram at a single freq or in growth curves. Dips may be generated by phase cancellation betwn the acoustic components or by interactions of the 2 generators (low-level and high-level generators).
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SOAEs effect on DPOAEs
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SOAEs may enhance DPOAEs if occur w/in 50 Hz of DP freq. This is not very likely b/c SOAEs are narrow band signals. If suspect SOAE interfering, alter your freqs slightly. Prieve et al. 1997 said that those with SOAEs have larger DPOAEs.
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TEOAEs in normal ears
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Present in 99% of normal ears (hearing < 20 dB HL). When SNHL is greater than 40 dB HL TEOAEs always absent.
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Relationship between DPOAEs and Pure Tone Thresholds
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For low (<65 dB SPL) & high (>65 dB SPL) level stim, if overall thresholds are 25 dB HL then will have present DPs from 1-6 kHz.
Hearing threshold levels of 25-50 or 60 dB HL, DP amp are generally reduced or absent. Above 50-60 dB HL, DPs absent for both high & low level stimuli. |
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Differential Diagnosis with EOAEs
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Present EOAEs w/ retrocochlear disorders resulting in sev to profound SNHL. There is the possibility of differentiating betwn sensory & neural hearing loss. TEOAEs present in 47% of tumor ears (expected in majorit of those ears b/c nml hearing but in 18% there was signif HL). Similar findings for DPs. EOAE measures expected to support diagnosis of retrocochlear HL in ~1/4 of the patients (attributed to cochlear HL that often accompany 8th nerve tumors). EOAEs are extremely useful if cannot do ABR measures.
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The 6 response parameters important in a TEOAE measurement
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1) TEOAE magnitude or amplitude.
2) Background noise level. 3) TEOAE spectral characteristics (spectrum). 4) Analysis time (windowing). 5) Reliability 6) TEOAE latency |
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The four factors important for clinical measurement of OAEs
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1) Status of the external and middle ear.
2) Coupling (fit) of the probe assembly. 3) Stimulus characteristics and stability. 4) Noise. |
