calibration
what type of calibration would you use in a research environment?
electroacoustic calibration
what frequencies does a click evoked ABR threshold represent
2 -4 k Hz
normal hearing latency/intensity function
Normal hearing is a sloping curve, as intensity increases - wave V latency decreases
Tonebursts
Tonebursts (or clicks) with ipsilateral masking like a notched noise or a high-pass noise
Derived responses
who pioneered the CE Chirp
Claus Elberling
what is our best tool for calibration
our ears! :D
in what hearing loss will you not get an accurate ABR
low frequency HL
SNHL latency/intensity function
SNHL L/I functions are more steeply sloped than normal ones, but still follow a similar downward curve because some areas of the basal end can’t be excited when the sound is softer because of the hearing loss
in what instance would you use a high pass noise to increase frequency specificity
In instances such as steeply sloping sensorineural hearing loss this is helpful because you can mask out frequencies that have better hearing. These better thresholds can be compensating for the one you’re testing for and if you ipsilaterally mask with a high-pass noise or a notched noise you’re increasing the frequency specificity of the toneburst because you’re getting it’s true response/reading. It is also helpful in finding thresholds for those better frequencies in a steeply sloping hearing loss because the worse thresholds can be pulling that threshold up, but if you mask them out you get a more frequency specific reading there as well.
chirp disadvantages
need norms for chirp data and no special advantage in identifying sites of lesions
Get at least 10 normal hearing subjects
Test their puretone audiometric thresholds at 2000 Hz and 4000 Hz
Take the average of all the thresholds at 2000 Hz, take the average of all the thresholds at 4000 Hz. Find the average of those two numbers.
Using an ABR machine, find their behavioral thresholds to clicks (dB “on dial”)
Average all their ABR behavioral thresholds.
Compare the averaged puretone thresholds with the behavioral ABR thresholds. 0 dBnHL = the difference between those two averages.
You got your click evoked ABR threshold. How do you estimate the range of the threshold?
(click ABR threshold - 10) +- 10 dB = threshold estimation range at 2-4k Hz
conductive HL latency/intensity function
CHL L/I functions look similar to normal and mirror it, but they’re shifted to the right and the difference between it and a normal L/I is usually similar to the ABG
what are the differences in dB between toneburst ABR thresholds and the participant’s pure-tone audiometric thresholds at 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz?
500 Hz = 30 dB, 1000 Hz = 15 dB, 2000 Hz and 4000 Hz = 10 dB
why do chirps have a larger amplitude than clicks
Chirps utilize a time delay where each component of the basilar membrane is excited at the same time and sends synchronous signals to the brainstem. Each frequency signal reaches the brainstem at the same time, so each wave at each frequency is added together. This creates larger amplitudes, compared to a click where the frequencies’ amplitudes are measured at different times depending on when the brainstem receives the signal.
steps for electroacoustic calibration clicks
Use an ABR machine using clicks, a coupler an SLM, and an oscilloscope. SLM reading is not meaningful. Record it’s peak amplitude and what intensity “on dial”
Use a 1000 Hz reference puretone signal presented by an audiometer, coupler, SLM, and oscilloscope. Measure the dB SPL from the SLM and the amplitude.
Match the amplitude of the puretone signal to the click. If not the same, use 20log(on dial amplitude / ppe amplitude) to find the amount to add to the reading from the SLM. The ppeSPL is the missing level from the click SLM.
what degrees of HL can ABR not differentiate
severe / profound
steeply sloping SNHL latency/intensity function
Steeply sloping SNHL have an even steeper L/I function because they have good neural survival at the apex but very poor at the base
4 reasons frequency specific ABR doesn't guarantee place specificity
ABR is an onset response, it’s so quick it’s hard to maintain frequency specificity
Spread of activation in the cochlea, when intensity increases it’ll excite more of the cochlea including neurons not in your target frequency
The frequency response of a transducer, inserts are okay but bone oscillators etc. aren’t too good
Outer and middle ear have different resonance properties and enhance different frequencies
7 advantages of chirp ABR
1. better morphology
2. shorter latencies
3. larger wave V amplitudes
4. shorter test duration
5. less sweeps
6. more accurate threshold estimation
7. maximum neural synchrony
advantages/disadvantages of biological calibration
advantages/disadvantages of electroacoustic calibration
biological calibration advantages: easiest to compare with an audiogram and it doesn't require expensive instrumentation to complete
biological calibration disadvantages: you rely on subjects’ responses and could be hard to replicate
electroacoustic calibration advantages: it can be verified objectively and not dependent on the number of subjects or their degree of HL to establish nHL norms
electroacoustic calibration disadvantages: it requires fancier equipment like an SLM, coupler, oscilloscope and that there’s no direct relationship between it and an audiogram
what type of stimulus gives best threshold estimation
chirps
retro cochlear pathology L/I function
looks very similar to CHL, need other testing to differentiate
What stimulus and recording parameters will you use when eliciting a 500-Hz toneburst ABR? Why?
I would use rarefaction polarity, Blackman gating, 2 cycles = 4 ms, plateau at 0 ms, gain at 100,000, at a rate of 29.8 Hz, a time base at 20 ms, filter settings at 30-3000 Hz, and 2-4 replications of 2000 sweeps near threshold.
Why?
Rarefaction polarity: alternating can get rid of peaks you want and degrade the response
Blackman gating: less ringing in the waveform than linear, cleaner peak
Gain 100,000: EEG is a small response because it’s coming from the brainstem so you need a lot of gain - makes EEGs bigger and easier for us to read waveform
Rate of 29.8 = close to 30 but avoiding integers of 60 to not have other electrical interference
Filter settings 30-3000 Hz: With tonebursts we want to focus on lower frequencies because higher frequencies are examined in clicks. This way we can filter out the higher frequencies and focus on the lows, or the specific frequency we’re looking for.
2-4 replications of 2000 sweeps: tonebursts are noisier and have smaller responses and peaks so you have to do more sweeps than a click to get identifiable waves
explain rationale of the time delay of a chirp
low frequencies enter the cochlea first since this area of the BM takes the longest to reach maximum displacement high frequencies enter the cochlea last since they're the fastest to reach max displacement. this causes each frequency to reach the brain stem at the same time causing maximum neural synchrony