김 진 욱 Chap 14. O VERVIEW OF A UDITORY E VOKED P OTENTIALS (3/3)

Inha University Microwave and Avionics Lab / 12 Contents Noise Reduction Normative Aspects of the ABR ABR Recording Parameters 1

Inha University Microwave and Avionics Lab / 12 Noise Reduction Noise : Any unwanted electrical activity Biologic source : muscle activity, EEG Nonbiologic source : bioamplifier, periodic 60-cycle noise Filtering ABR is in the frequency regions from 100 to 3000Hz selectively eliminating the electrical activity below 100Hz and above 3000Hz will reduce the background noise Four basic types of filters 2

Inha University Microwave and Avionics Lab / 12 Noise Reduction Other aspect of a filter Cut-off Freq : the freq at which the voltage at the filter output is reduced to 70.7% of the input Rejection rate : how fast the voltage is reduced for a filter outside the passband The spectrum of the AEP The response is less than one-tenth the amplitude of the background noise, making it virtually impossible to discern the AEP Assumption for using time-domain signal averaging to improve the SNR to make the response visible The AEP is the same in response to a constant stimulus The noise is random The AEP and the noise are independent 3

Inha University Microwave and Avionics Lab / 12 Noise Reduction General rule : the SNR increases in proportion to the square root of the number of sweeps. Let us have a AEP that we sum over 16 stimulus presentations Average amplitude : 16 /16 sweeps = 1 If background noise is 4, SNR = 1 /4 = 0.25 Let us sum together the response to 16 noise time epochs Average noise : 16 /16 sweeps = 1 SNR = 4 /4 = 1 Relationship between number of sweeps and improvement in SNR 4

Inha University Microwave and Avionics Lab / 12 Noise Reduction Recommendations for the number of sweeps to obtain an AEP As a“ballpark(in close proximity) figure”for threshold estimation, the SNR should be on the order of 1 or more, while for determination of peak latencies and amplitudes, an SNR of 2 or greater is desirable For one subject, to a low-level stimulus Response amplitude : 0.1, background noise : 10, SNR : 0.01 SNR improvement for 2000 sweeps Hence SNR is 0.447(< 1), recommended number of sweeps is inadequate For a different subject, to a high-level stimulus Response amplitude : 0.6, background noise : 3, SNR : 0.2 SNR improvement for 2000 sweeps Hence SNR is 8.94(> 1), which is a much larger SNR than is reqired An understanding of the underlying principles of signal averaging can lead to propitious modifications of these guidelines 5

Inha University Microwave and Avionics Lab / 12 Normative Aspects of the ABR Subject variables that affect the ABR include core temperature, gender, and age A decrease in core temperature Lead to an increase in ABR peak latencies Increases in interwave intervals A decrease in peak amplitudes Females show Shorter response latencies, smaller interwave intervals, and greater ABR peak amplitudes than males Shorter cochleae than males → result in greater traveling-wave velocity, shorter ABR peak latencies and greater amplitude in human female 6

Inha University Microwave and Avionics Lab / 12 Normative Aspects of the ABR Age has an influence on ABR peak latencies, interwave intervals, and amplitudes In the infant, the latencies and the interwave intevals are prolonged relative to adult values On the other end of the aging spectrum, ABR peak latencies appear to increase and peak amplitudes decrease with advancing age The ABR is sensitive to such variables as stimulus rate, polarity, spectrum, and whether presented monaurally or binaurally A conceptual model of this process would suggest that a rarefaction stimulus should be the most effective stimulus ABRs to binaural stimuli are larger than those to monaural stimuli 7

Inha University Microwave and Avionics Lab / 12 Normative Aspects of the ABR The click stimulus is broadband One does not selectively stimulate limited regions of the cochlear partition Lower-frequency tonebursts produce longer-latency ABRs, reflecting the increased travel time to apical cochlear regions For higher-level tonebursts, it appears that the ABR to low- frequency toneburst may acutally arise from more basal (higher-frequency) cochlear regions 8

Inha University Microwave and Avionics Lab / 12 Normative Aspects of the ABR ABRs from one subject in response to varying levels of click stimuli As level decreases, response amplitudes decrease, and latencies increase Waves Ⅱ and Ⅳ tend to be difficult to identify at moderate click levels, and wave Ⅰ disappears at moderate-to-low levels Near threshold, wave Ⅴ is often the only observable wave 9 L= Ⅰ L= Ⅲ L= Ⅴ (85) L= Ⅴ (75) L= Ⅴ (65) L= Ⅴ (55) L= Ⅴ (45) L= Ⅴ (35) L= Ⅴ (25)

Inha University Microwave and Avionics Lab / 12 Normative Aspects of the ABR 10 ABRs from a normal-hearing young adult to an 85dB nHL click stimulus for click rates varying from 25 to 100Hz With increasing rate, there is an increase in peak latencies, and increase in interwave intervals, and a decrease in peak ampitudes For this higher click level, figure shows a decrease in wave Ⅴ amplitude with rate The optimal parameters must take into account the age of the subject, the stimulus parameters, and the clinical purpose of the test L= Ⅴ (25Hz) L= Ⅴ (50Hz) L= Ⅴ (75Hz) L= Ⅴ (100Hz)

Inha University Microwave and Avionics Lab / 12 ABR Recording Parameters ABR recording parameters for two cliniclal purposes : neurodiagnosis and threshold estimation For neurodiagnosis The click level is fairly high in intensity to optimize the detection of wave Ⅰ, especially in those with high-frequency hearing loss The rate is listed at 10 to 20 Hz, to keep response acquisition time reasonable but optimize the chances of obtaining a reliable wave Ⅰ Two suggested recording montages First suggested montage : only three electrodes are required Second montage : requires four electrodes (vertex, both mastoid, forehead) Many people feel insecure applying an electrode over the fontanelle near the vertex in infants, and the first montage avoids this problem 11

Inha University Microwave and Avionics Lab / 12 ABR Recording Parameters 1000 to 2000 sweeps are good suggested values to obtain a suprathreshold response For threshold estimation Need to modify this suggested protocol in several ways Stimulus level would be decreased in a threshold search The optimal stimulus rate should be increased to somewhere in the neighborhood of 30 to 50 Hz If using toneburst stimuli, then the time window should be increased to 15 to 20 ms in adults or 20 to 25 ms in infants 12