Auditory Brainstem Response ghfghhfrh.pptx

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1. Auditory Evoked Potentials

2. Points to be discussed… Auditory Evoked Potentials Auditory Brainstem Response Test procedure Instrumentation Characteristics of ABR Factors affecting ABR Clinical Application

3. Introduction Evoked potentials (EPs) are bioelectric potentials recorded using electrodes placed on body. They occur in response to sensory stimulation. Evoked potentials have very small amplitude and often much smaller than ongoing EEG. It can be recorded using electrodes placed on the head . In AEP, the potentials are elicited by giving external auditory stimuli through ear phones.

4. Classification Of AEP On Basis of Latency

5. Short latency potentials: These are cochlear potentials called electrocochleographic (ECOG) potentials and auditory brainstem responses (ABR). Which occur within and first 10 ms after the presentation of transient stimulus. Middle latency potentials (MLR) which occur at latencies from 10 to 50.

6. Long latency potentials: These are event-related potentials, which occur at latencies between 100 and 300 (or 500) ms. ( Picton , Hill Yard & Kreutzer et al. 1974)

7. Classification on basis of Event related potentials

8. Evoked potential: occur in response to physical stimulus and may be either exogenous or endogenous in nature. Emitted potential: occur in the absence of any physical stimulus in relation to decision or preparatory processes.

9. Near Field Vs Far Field Recordings Near Field Electrodes are placed at the site of generation. Invasive procedure Amplitude of the responses are good Can be recorded at threshold also Far Field Potentials are recorded at the location far away Non invasive procedure Less amplitude Responses are seen at intensities above threshold

10. LLR MLR ABR EcochG

11. Neural Synchrony Simultaneous responses from a large number of neural units, or synchronous neural discharge are best elicited by an electrical pulse (which sounds like a brief click). A pulse of click is characterized by an abrupt or rapid onset and a broad frequency bandwidth theoretically containing all frequencies.

12. A pulse causes stimulation of a broad portion of the cochlear portion simultaneously, which in turn cause a response from a large number of neurons. More the neurons that discharge within a brief time, the larger the amplitude of the peaks.

13. Auditory Brainstem Response

14. Introduction The Auditory brainstem response (ABR) is a complex to particular types of external stimuli which represents neural activity originating from VIII cranial nerve and auditory portions of brainstem.

15. A normal ABR waveform is characterized by five to seven vertex positive peaks that occur in the time period of 1.4 to 8.0 ms after onset of a stimulus. Responses are usually displayed with positive peaks reflecting activity towards the vertex. These peaks are labelled with Roman numerals I - VII

16. First recorded by J ewett and Williston (1971)

17. Neural Generators of ABR

18. Instrumentation

19. Introduction A wide range of sophisticated computer based systems is available for available for clinical recording of evoked potentials. The components common to most of these systems are stimulus generator electrodes, amplifier, filters, and a signal averager with artifact rejection, response display, response processing, and a means to print test results.

20. STIMULATING SYSTEM DISPLAY PRINTER AVERAGER FILTER PHYSIOLOGIC AMPLIFIER And PREAMPLIFIER ELECTRODES PATIENT TIME LOCKED TRIGGER TRANSDUCERS STIMULUS AMPLIFIER ATTENUATOR STIMULUS GENERATOR RECORDING SYSTEM

21. Stimulus Generator The most effective and widely used stimulus for neurological applications is a pulse or acoustic transient (commonly referred to as a click ). The stimulus has an essentially instantaneous onset and is of brief duration (usually 0.1 ms).

22. These pulses are broadband stimuli that are shaped by the frequency response of the ear phones and, when transduced through TDH-39 supra-aural earphones, ER-3 insert earphones, or comparable earphones, provide maximal stimulation in the 2000 to 4000 Hz frequency range. Thus, brief pulses reflect activity primarily from more basal portions of the cochlea.

23. Stimuli should be able to be presented independently to the right and left ears, and most systems allow binaural presentation as well. Stimulus level is adjusted with one or more attenuators.

24. The polarity of the stimulus can be selected as condensation (positive onset phase), rarefaction (negative onset phase), or alternating.

25. Clicks Short duration stimuli with instantaneous rise time.

26. The frequency of clicks is determined by the transducer.

27. Hence, click presented through head phones will have more high frequency energy (2-4 KHz). This will elicit ABR with short latencies. Click presented through bone vibrator will have more low frequency energy (500 Hz-2 Khz )

28. Tone burst Short duration tonal stimulus. Has a rise time, plateau and fall time.

29. Shorter the rise time-less frequency specific Longer rise time – frequency specific (poor ABR). Tone bursts of 250Hz, 500Hz, 1KHz, 2KHz and 4KHz are used.

30. Transducers Insert earphones such as Etymotic Research ER-3 phones are recommended when acquiring ABRs for several reasons Separation of the stimulus artifact from the onset of the response through 0.9 ms delay line in the earphones makes Wave I of the ABR more visible in most instances (Hood & Morehouse, 1985)

31. Earphones prevent ear canal collapse Increase interaural attenuation Are comfortable for long periods of time (which may assist in reducing patient artifact)

32. Can attenuate surrounding environmental noise more efficiently than other earphones types. Use of ‘ear dome’ type of earphones is discouraged because they tend to produce collapse of ear canals.

33. The absolute latencies of the waves will be delayed by appx . 0.9 ms with the ER-3 insert earphones in comparison to ABRs obtained with supra aural earphones.

34. Trigger To extract a time locked waveform from background noise, the computer must know when the stimulus occurs and when to begin a recording epoch. A trigger pulse that marks the beginning of the recording epoch can be set to occur in conjunction with the stimulus.

35. Usually the recording epoch begins with the onset of the stimulus, and the trigger pulse is coupled with the onset of the stimulus, but certain circumstances may warrant placing the trigger before or even after the stimulus. For example, beginning the recording epoch a few milliseconds before the onset of the stimulus allows recording of a pre stimulus period, which may be useful in evaluating noise levels.

36. When using AEPs to estimate hearing sensitivity, a bone conduction transducer is also necessary. The Dynamic Range of the bone conduction oscillator is less than that for air conduction, as is true in pure tone audiometry .

37. Electrodes ABR is recorded by attaching electrodes to the surface of the scalp. Montages of either three or four electrodes are generally used for recording either one or two channel ABRs, respectively.

38. Two channel recording : Two channel recordings, using four-electrode montage, are recommended for neurological applications in order to obtain ipsilateral and contralateral responses. One electrode is placed at vertex which is equidistant between the nasion and the inion on the saggital plane.

39. electrodes are also placed at each of the ears, either on the mastoid or on the lateral or medial surface of earlobe.

40. The fourth electrode is ground electrode, which can be placed at any location of the body, but is usually attached to the forehead. The vertex electrode position is referred as Cz , the left ear as A1, the right ear as A2 and the forehead as FPz

41. These electrode positions may also be labeled according to vacuum tube convention, where the vertex electrode is referred to as G1 electrode and the ear level electrodes as G2.

42. Single Channel recording : In one-channel recordings, three electrodes are used, with attachment at the vertex and each ear. Recordings are obtained between the electrodes at the vertex and the ear receiving the acoustic stimulus, with the other ear electrode used as the ground.

43. When the ear is tested, the ear electrode that acts the ground is switched. Because electrode connections and/or designation of the recording versus ground electrodes change with stimulus presentation modes, care must be taken to avoid electrode routing errors.

44. Electrode application and electrode Impedance Electrodes are first applied by cleaning the surface of the skin where the electrode is to be placed and then placing the conductive electrode gel or paste on the skin and on the electrode. The electrodes should be securely attached with tape.

45. Electrode impedance is measured to measure the impedance between any two electrode sites, though each site is generally compared to the ground electrode. Impedance should be below 5000 ohms for all electrodes, and fairly equal impedance (within about 2000 ohms) for all electrodes is desirable.

46. Fairly equal electrode impedance facilitates the efficiency of common mode rejection system, which serves to minimize background interference. Electrode impedance should be checked after the earphones and patient are positioned for testing and periodically during testing because changes in impedance may affect the quality of the recordings.

47. Pre-Amplifier Pre-amplifiers are the second stage of the recording system and the attachment point for the electrodes. The pre-amplifiers are either near the attachment point of the electrodes or connected to the electrodes through an electrode input box and cable.

48. When the pre-amplifier are near the point of electrode attachment, some advantage in the signal-to-noise ratio may be obtained because the low amplitude signals will need to travel less distance.

49. Because of the low amplitude of the ABR (on the order of 0.1 to 1.0  V), a companion amplifier and pre-amplifier may be necessary to provide amplification of the signal by about 100,000 times. Amplification of the neural activity places the response into a range that can be processed by the signal averager.

50. Signal Averaging The signal averager converts the converts the analog electrical activity from the amplifier (the physiologic response) into a series of numerical (digital) values. These digital values are processed by the computer to generated the summed or averaged response.

51. Signal to noise ratio Enhancement with Signal Averaging and Time locking When recording the early potentials, it is necessary to distinguish a low amplitude response from higher amplitude background noise. This is accomplished by averaging a large number of responses together and time-locking the onset of stimulus with the onset of the computer analysis sweep.

52. Time locking allow the evoked potential of interest to be summed while the background noise, because of its random nature, averages toward zero and thus is attenuated. The signal to noise ratio is a function of the number of sweep that are averaged together.

53. Filtering In each channel of the measuring instrument, the preamplifier is followed by an analog filter. The evoked potential signal of interest and the background noise each has its own spectrum. Most often, the noise bandwidth exceeds that of the signal of interest. Because the noise and signal occur simultaneously, their spectra are superimposed.

54. Therefore, both signal and the noise contribute many of the same frequencies in the resulting spectrum. An operator may set the filter pass band to reject or reduce noise above and below the anticipated spectral range of response signal. This filtering improves the SNR by reducing the noise. Filtering, however, cannot remove the noise that shares spectra with the signal.

55. Filters may be either low-pass, high-pass or they may be band-pass. In reported clinical applications of EPA, only band-pass filters have been used.

56. Notch filter : Special analog filters designed to suppress specific noise components, notably 60 Hz (50 Hz in India) power line pick up are available in most commercial AEP system. These are often called notch filters, a special case of band stop type. Prior to using any such filter clinically, the amount of distortion that it causes should be measured under the exact, intended conditions of use.

57. Common Mode Rejection Evoked potential recordings of interest are recorded between 2 electrodes. By using a differential amplifier, the voltage at each of the two electrodes in a pair (e.g. Cz and A1) is subtracted from each other. Consider, for example, recording obtained between Cz and A1. If the Cz electrode is selected as the vertex positive upward electrodes,

58. then voltage propagated toward that electrode is slightly different than voltages at the A1 electrode, which is positioned at the left ear.

59. The electrical activity contains both the desired signal related to the response of the neural pathway to the acoustic stimulus, and unrelated electrical noise from the body. The unrelated and outside electrical activity is common to both electrodes and has the same values.

60. When the voltages at each of the electrodes (i.e. Cz and A1) are subtracted from each other, electrical activity common to both electrodes is cancelled (called Common Mode Rejection) and the remaining voltage is that which actually differs between the electrodes ideally the target potential.

61. The term Common Mode Rejection Ratio (CMMR) refers to how well the differential amplifier eliminates the activity common to the inverting and non-inverting leads. Typical CMMRs in contemporary equipment are –90 to –100 dB. The negative sign verifies that the output is smaller than the input. For common mode rejection to work most efficiently, the impedance should be similar at two electrodes.

62. Artifact Rejection The need for filtering noise in a recording can be reduced by using an artifact rejection reject circuit. These are standard in most evoked potential systems. Each successive digitized trace first goes to a buffer where it is examined for any voltages that exceed some pre set level.

63. If all voltages are at or below a pre set level, then the digitized voltages are dumped to the memory unit for averaging with prior and succeeding traces. Conversely if excessive voltage is found at any address in the analysis window or epoch, then that sample in the buffer is erased instead of being forwarded to the averaging memory. This process prevents spuriously large potentials that are essentially noise from dominating and distorting an AEP.

64. Display Instruments Display Storage Plotter

65. Test Procedures Test Environment : Evoked potentials should be acquired in a quiet test environment. A sound treated room with appropriate acoustic isolation is desirable when recording responses to low intensity stimuli but may not be necessary when testing is performed using only high intensity stimuli.

66. Insert ear phones are helpful in attenuating external sounds and are highly desirable for testing patients of all ages.

67. Patient Considerations : Recording of early-evoked potential are best obtained when the patient is quiet and relaxed in order to avoid artifacts related to muscle responses. Patients are usually placed in a reclining position with a good support to the neck and are often encouraged to close their eyes, relax and sleep during the recording process.

68. An exception to testing with eyes closed occurs when the patient has spontaneous nystagmus , which produces artifacts related to eye movements. Because spontaneous nystagmus often increases with eyes closed, instruction of these patients to keep their eyes open and fixed on a nearby object will usually decrease the interference of this muscle activity.

69. Test Protocol Stimulus – Air conduction clicks Stimulus Intensity – 75 dBnHL Stimulus Rate – 27.7/s No. of Sweeps – 1500 -2000 Filter Bandwidth – 100 – 3000 Hz

70. Characteristics of ABR Absolute Latency: The time interval between the stimulus onset and the peak of a wave form referred to as the latency of the response. This latency is more precisely, the absolute latency of a peak because it is related to the stimulus rather than to other peaks in the response.

71. Wave I – 1.6 ms ± 0.2 ms Wave III – 3.7 ms ± 0.2 ms Wave V – 5.6 ms ± 0.2 ms

72. Interpeak Latency The time between peaks in the ABR is referred to as interwave latency intervals, interpeak latencies, interwave latencies. The interpeak latencies used in clinical interpretation of ABR waveforms are those for wave I – III, wave III – V, and wave I – V.

73. Wave I – III : 2.0 ms ± 0.4 ms Wave III – V: 2.0 ms ± 0.4 ms Wave I – V: 4.0 ms ± 0.4 ms

74. Interaural Latency Differences Interaural latency differences compare the absolute latencies of wave V obtained from stimulation of the right versus left ears at equal intensity levels. When the peripheral hearing sensitivity is similar in both ears the latency of wave V should differ by no more than 0.2 ms to 0.4 ms between two ears.

75. Differences in wave V latencies between the ears that exceed 0.2 ms or 0.4 ms are reported to exceed normal limits.

76. Latency Intensity Function As the intensity of the stimulus decreases the latencies of the peaks of the ABR increases and response amplitude of the peak decreases. These latencies increases occur slowly for intensities from 90 to 60 dBnHL and then increase more rapidly at lower levels. Wave V : 0.3 ms per 10 dB

77. In conductive HL, longer than normal latencies because actual intensity of the stimulus reaching the inner ear is decreased. In auditory nerve or brainstem disorders, the latency of wave V is generally prolonged at all intensities.

78. Rate changes Increasing the rate at which stimulus are presented results in latency and amplitude changes in the ABR. High stimulus rates can be employed to evaluate neural synchrony and recovery and use of higher rates may sensitize testing to subtle neural disorders.

79. When the stimulus rate is increased from about 10 stimuli per second to 100 stimuli per second. Wave V latency increases by approximately 0.5 ms in normal individuals.

80. Amplitude Abnormal ABR ranges in amplitude from 0.1 to 1.0 µV. As the stimulus intensity decreases response amplitude decreases. The lower amplitude earlier peaks (e.g. Wave I and III) may become obscured in the background noise first with remaining visible at the lowest intensities.

81. The wave V/I amplitude ratio is obtained by dividing the peak to peak amplitude of wave V by the peak to peak amplitude of wave I. Wave V/I ratio = ≥ 1.0

82. Factors Affecting ABR

83. 1. Effect of Non-pathologic factors 2. Effect of Stimulus factors 3. Effect of Acquisition/Recording factor    

84. Non Pathologic Factors 1 ) Age and Gender 2) Body temperature 3) Attention and State of arousal 4) Drugs 5) Muscular artifacts

85. 1. Age and Gender (i) Infancy and childhood Age effects ABR wave form is incomplete at birth. Generally only three major components waves I, III, V are observed. Interwave latency value i.e. I-III, III-V, and I – V are initially prolonged.

86. The wave I-V latency interval, for example is normally about 5.00 msec at full term birth. During the 1 st 18 months after birth, other wave components emerge and waves III and V progressively shorten in latency. Age must be considered when interpreting ABR findings in children under the age of 18 months.

87. ABR can be recorded first approximately at 27-28 weeks of conceptional age ( Galambos and Hecox, 1978). At this time (i.e. well before the normal termination of pregnancy at 40 weeks), wave I may be relatively more prominent than later waves because, the peripheral auditory system matures before the auditory CNS does. ( Montandon , Engel 1981 et al).

88. There is some evidence that wave I amplitude in new born may, be up to twice as big as the amplitude in adults (Hecox, Cone and Blaw 1981). Proximity of the recording electrode to the cochlea due to relatively small head dimensions is offered as an explanation for the large wave I amplitude (Jacobson et al 1981)

89. Same investigators reported that new born wave I latency is prolonged from 0.3 sec msec to over 1 msec in comparison to adult values (Gold stein, Jacobson, Johnson, Cox, Hack and Metz 1981, 1982).

90. Gender effects:- Distinct ABR differences female Vs male adults are well appreciated. The issue of ABR gender differences in infant and childhood however remain unresolved.

91. Some investigators have reported to gender differences in new borns ( Ctockyard and Westmoreland 1979) Other found short latency in female Vs male preterm infants (Cox, Hack and Metz 1981) but the difference was small and inconsistent in comparison to the striking gender effect for adults.

92. Amplitude was significantly greater for females than for male only for wave I and only at one intensity level (70 dBHL ). Finally, statistically significant gender different in ABR latency have been reported for older children.

93. (ii) Advancing Age: Gender effects : Throughout adulthood, females show shorter latency values and larger amplitude than males for later ABR (III, IV, V and VI) waves. Because the effect is negligible for the wave I and more pronounced for later waves, interwave intervals are significantly shorter (0.12 to 0.30 ms less) for females.

94. Amplitude is significantly larger for females although clinical importance for this finding is minimal. The two part theory is that interwave latencies will be shorter if the distance between the generators for each of the waves is shorter and amplitude will be larger if the recording electrode is relatively closure to the wave generator.

95. Age effects : Most of the studies on ABR and age suggest that latency increases over the age range of 25 to at least 55 years, on the order of approximate 0.2 msec over that age span in ABR. Wave I-V interval also appears to increases significantly over than age range of 60-86 years implying brainstem involvement.(Allison et al, 1983)

96. Delayed synaptic transmission associated with age related loss of neurons and changes in neuron membrane permeabilitiy and contributing to decreases amplitude and increases latency, has been suggested ( Johannsen and Lehn 1984)  

97. 2. Body temperature:- Body temperature is an important variable for the AER measurement of the normal temperature 37°C is verified at the time of testing, and then there is no need for further account for temperature in the interpretation of the AER results. Temperature exceeding + 1°C from this value (i.e. below 36°C above 38°C) must be considered as a possible factor in AER outcome.

98. Effects of ABR : An initial effect of hypothermia may be the selective loss of auditory sensitivity for high frequency signals, as estimated electro physiologically ( Mamley and Johnstan , 1974). With severe hypothermia (body temperature less than 14 – 20 °C), the ABR disappears ( Rosenblum , Ruth and Gal 1985).

99. In hyperthermia, However,it has been consistently found that a I-V latency decrease of 0.5-0.6 msec over the temperature range of 38 through 42°C in young male and female patients with no CNS pathology.

100. 3. Attention and state of arousal: Most available clinical evidence indicates no difference in ABR wave from recorded in awake Vs the natural sleep state for moderate to high stimulus intensity levels (Jewett and Willinston 1971, Terkidsen 1985)or for low intensity stimuli close to auditory threshold. Sleep state is best verified and quantified by the EEG recordings.

101. Even extremely reduced state of arousal, such as narcolepsy(Helleckson et al,1979) and metabolic coma (Hall and Haargadine, 1984) have no serious effect on ABR latency or amplitude. Attention likewise has little or no effect on the short latency responses. (Kuk and Abbas, 1989)

102. 4. Drugs:- Potentially ototoxic drugs, which affect AERs by causing a peripheral hearing impairment, are simply categorized and listed such as Gentamicin, Neomycin, Kanamycin, Quinine, and Streptomycin.

103. Chloroquine, a medical treatment for rheumatoid arthritis, appears to produce significant delays in absolute latency values for ABR waves III and V and abnormal wave I-III, I-V but not III-v latency intervals (Bernard 1985).

104. Chloral Hydrate is the oldest synthetic “sleeping drug” and most popular sedative for quieting children for AER measurement. Published reports confirm that chloral hydrate does not affect ABR (Mokotoff et al, 1977).

105. 5. Anesthetic agents:- Anesthetic agents produce differential effect on AERs. Responses dependent on the end organ, eight nerve and primary leminiscal sensory pathways are not seriously influenced by anesthesia. Unfortunately much of the information in the relationship between the anesthesia and AERs was obtained from animals experiments Vs from clinical experiment with humans (Galambos 1963, Smith 1987)

106. Halothane appears to cause slight delay in ABR interwave latencies, without altering wave morphology (Cohen and Britt 1982) and Isoflurance may prolong ABR absolute and interwave latencies.( Stocard et al,1980) Fentanye has no apparent effect on ABR.

107. Alcohol – Acute ingestion at alcohol, ABR waves (III through VII) are increased but amplitudes are not affected. Chronic alcohol abuse, according to animals and clinical research, is associated with significantly prolonged ABR latency values. During alcohol withdrawal, ABR latency values (for later waves) may be usually decreased.

108. 6. Muscular artifacts ABR have components can be completely obscured by excessive muscle artifact, often arising from neck and jaw muscles. As the Ecochg muscle artifact may be mistaken for wave components of the ABR, even after customary averaging and filtering.  

109. Effect of Stimulus factors:- Acoustic stimuli are necessary for the generation of all AERs. They are 1)      Frequency 2)      Duration 3)      Intensity 4)      Rate 5)      Polarity 6)      Transducer 7)  Masking sound effects auditory response

110. 1. Type and frequency The click evoked ABR is very useful and clinically practical for estimation at auditory functioning in the 1000-4000 Hz regions. For hearing screening purpose it may be adequate. However information an auditory sensitivity across the audiometric range, especially the speech frequency region (500 Hz through 3000-4000 Hz) is extremely important for rational audio logic management of hearing impaired patient, for fitting of hearing aids.

111. Because of the pressing clinical need for an electrophysiologic technique to assess auditory sensitivity at different frequencies, a tremendous research effort has been directed towards developing a method for recording frequency specific ABRs.

112. The most obvious approach for generating an ABR reflecting hearing sensitivity at a specific frequency is to use brief tone stimulation, such as tone burst. The tone burst ideally has energy as a single pure tone frequency (.e.g. 500 Hz) under all presentation conditions, including high stimulus intensity levels, and it contains no energy at other frequencies.

113. The ideal stimulus activates the BM of the cochlea where there are neural units with this characteristic frequency, even in patients with cochlear pathology. The ABR can be obtained using tone burst that have a rapid rise time while still maintaining same frequency specificity (Picton, Havel and Smith 1979) There is a trade-off behavior frequency specificity and normal synchrony in that tone bursts with longer rise times will be more frequency specific, but all general poor neural synchrony which will affect the quality of the response.

114. High frequency stimuli elicit shorter latencies than lower frequency stimuli because high frequencies stimulate the more basal portions of the BM. This generation earlier response because the traveling wave moves from the base to the apex. Low frequency stimuli are analyzed and in a less synchronous manner than high frequency stimuli resulting in less distinct wave forms.

115. Shorter duration signal with relatively narrow spectra and same tonality can be used for the AEP generation. One type of narrow spectrum signal used for threshold EPA is tone pip. The tone pip is generated by ringing an electric filter with a brief rectangular electrical pulse

116. The main activity of tone pip has the periodicity of the resonant frequency of the filter. The pips usually have unequal rise and fall times. The tone pip has no plateau; its magnitude begins to diminish after the max magnitude is attained. Tone pips are used infrequently for tonal threshold EPA mainly because of wide spectral spread of acoustic energy.

117. Filter settings: A crucial acquisition parameter to consider in recording frequency specific ABRs is filter settings. A high pass setting of 30 Hz or lower is essential in order to encompass the low frequency portion of the ABR spectrum, which is prominent for a low frequency stimulus. (Hyde, 1975; Picton et al,1981).

118. Raising this cut-off frequency results in decreased wave V amplitude, response distortion, elevated response threshold and reduced response detectability.

119. 2. Duration The standard pulse duration utilized in clinical ABR testing is 100 microseconds or 0.1 msec. Because the ABR is an onset response, the duration of the stimulus, should not alter the response (Hecox, Galambos 1976).

120. There are special applications where a stimulus with a different duration may be desirable. Examples include testing higher frequency regions of the cochlea to monitor ototoxicity effects and testing patients who exhibit severe hearing losses across the standard audiometric frequency range and normal or nearly normal high frequency between 8000 Hz and 16000 Hz (Berlin, Wexler, Jerger 1978)

121. A 1000 microsecond pulse creates a null in the spectrum at 1/pulse width or 10000 Hz, whereas a 50 microsecond pulse is recommended with a suitable transducer to assess responses in frequency regions of the cochlea responsive to stimuli above 8000 Hz. In contrast to duration, the rise time of the stimulus has a marked effect on the ABR that is related to reduced synchrony due to fewer neurons firing simultaneously.

122. As raise time increase, latency increases, amplitude decreases, and the morphology deteriorates. Rise time greater than 5 msec may fail to generate a brainstem response. Synchronous firing of many neurons, which is the general physiologic under pinning of the ABR, is very dependent on an abrupt stimulus onset.

123. The two practical consequences of this principle are that the ABR is not heavily dependent on stimulus duration (George et al 1984) and almost instaneous onset (almost always 0.1 msec -100 msec) click stimulus is routinely used in clinical ABR recordings , although click duration as short as 20 msec and as long as 100 msec (Yamada, Yagi et al 1975) have been reported to be used.

124. Studies of the effect of stimulus duration an ABR have yielded mixed results. One of the earlier reports by Hecox and Galambos (1976) described alteration in the ABR to changes in the duration (on time) and interburst interval (of time) of white noise burst stimuli in six normal hearing female subjects.

125. These investigators found an increase in the ABR wave ‘V’ latency (0.5 msec) and decreased amplitude as the duration was increased from 0.5 to 30 msec but these changes were not observed when the stimulus off time was lengthened. On the basis of this observation, the authors concluded that the wave V component of the ABR changes were due to response recovery processes, not duration.

126. Gorga et al 1984 estimated ABR behavioral thresholds for 2000 Hz tone burst stimuli ranging in duration from 1 to 512 msec. They demonstrated that stimulus duration does not affect ABR threshold for normal or hearing impaired subjects whereas behavioral threshold decreased (improve) on the order 10-12/decade of time for normal subjects.

127. In another study of click duration on ABR in normal hearing subjects, Beattie and Boyd (1984) analyzed latency for wave, I , III and V of duration of 20, 50, 100 and 200 and 400 msec. It has been found that there were no latency differences within 25-100 msec range, but latency did increase by about 0.10 msec from duration of 100-200 and by 0.20 msec over the 100-400 msec duration range.

128. Duration of the stimulus and amplitude of the spectrum are systematically related. Funasaka (1986) investigated the effect of 3 KHz tone burst with duration 5, 10, 20 and 30 msec on ABR. Subjects were young adults. Rise and fall times were constant at 1 msec. Interstimulus intervals ranged form 80 to 140 msec. As the stimulus duration was lengthened there were increase in latency and amplitude of waves V and VI. Wave III latency remains unchanged, but amplitude decreased .

129. The author argues that these effects are not a function of the recovery process limitation; instead, the duration differentially affects the slow wave (frequency) component of ABR and not the fast component. . ABR latency increase directly with stimuli rise time, beginning with instaneous (0 msec ) onset stimuli, at least for normal hearing subjects (Hecox, Galambos et al 1976).

130. When the rise time exceeds 5 msec identification of earlier ABR wave components, such as wave I becomes difficult. This is due to the reduction in the number of neural units that fire synchronously (Spoendlin, 1972) also, because the traveling wave is slower; there is an increased contribution of the more apical regions of the cochlear to the ABR.(Kiang,1975)

131. References Handbook of Clinical Audiology, 2 nd e.d., Jack Katz