Lecture 3 + 4 Flashcards
How many peaks does an ABR have?
5
T/F: The ABR reflects actions potentials, NOT post-synaptic graded potentials
TRUE
T/F: the peripheral auditory system has poor temporal resolution
FALSE: it is very temporally precise. Many neurons fire precisely at the same time to things like stimulus onset. Rapid onsets and changes tend to be important for this
Sinks or sources occur progressively in the extracellular space at each node of Ranvier?
Sinks
ABR occurs up to how many ms?
6 ms (typically 5.5 ms for wave V)
What direction is ABR measured?
Positive up (but used to be negative up)
What is the closed field problem with ABR?
Action potentials don’t give rise to open-fields
Why is “breaking the symmetry” helpful in ABR?
Changes in direction or impedance will break the symmetry and allow for open fields (which is important for action potentials to synchronize)
Do all ABRs look the same?
No, there is a lot of individual variation in morphology
Typically, wave ___ is larger than wave I
V
Waves __ and __ are often combined
IV and V
What ABR waves are horizontal?
I, II and III
Which ABR waves are more vertical?
IV and V
What 3 ways can we find ABR generators?
- Correlate with near field activity
- Introduce lesion (e.g., freezing)
- Source localization
Explain ABR generators and complexity
-Complexity increases as we move rostrally (up)
·Multiple simultaneously-active pathways (parallel processing)
Speed of the basilar membrane
2.8 m/s (or 2.8 mm/ms)
Speed of VIIIth nerve
22 mm/ms (2.2 cm/ms)
Cochlear filter build-up time (at CF)
0.5 ms
Synaptic delay
1 ms
Length of VIIIth nerve
2.6 cm
Where does Wave I occur?
1.7 ms
Where does Wave II occur?
2.8 ms
Where does Wave III occur?
3.9 ms
Where does Wave IV occur?
~5.1 ms
Where does Wave V occur?
5.7 ms
T/F: Wave V occurs earlier in biological females compared to biological males
TRUE
Which frequencies contribute most to the ABR? (High, low or mid)
High
Where does Wave I arise from?
-The distal portion of the 8th nerve
-Latency is the same as the CAP (~1.5-1.7 ms); Wave I IS the CAP (negative at mastoid)
Where does Wave II arise from?
-The proximal (inside) end of the 8th nerve
-Dipoles are oriented away from mastoid (mastoid negative)
Where does Wave III arise from?
-Cochlear nucleus is the main source; also proximal portion of AN
-Mostly horizontal dipole (mastoid negative)
Where does Wave IV arise from?
-Likely the superior olivary complex (and elsewhere)
-Mostly vertical dipole moving toward vertex (mastoid negative)
T/F: Wave IV is generated ipsilaterally
FALSE: Wave IV is generated contralaterally
Where does Wave V arise from?
-Likely the lateral lemniscus (NOT generated by the IC)
-Mostly vertical dipole moving toward vertex (mastoid negative)
Studies on investigating ABR generators in cats revealed which two main cells as being involved? What frequencies are they primarily driven by?
Globular and bushy cells; 2000 Hz and above (high frequencies)
What are the two categories used to measure ABR and what are they composed of?
- Amplitude measures: peak, peak-to-peak, and interpeak ratio
·Usually use peak-to-peak - Latency measures: absolute, interpeak, interaural
·Interpeak is the best latency measure
Which waves are measured from the center of the peak and which are measured from the shoulder?
-Waves I-III are measured in the center
·Measure the second peak in wave I if there are two peaks cause it’s more reliable
·Measure from the midpoint of wave III if there are two peaks
-Wave V is measured on the shoulder/edge (right past the peak)
Why is contralateral recording useful in ABR?
Because in ipsilateral recording, waves IV and V are often fused. In contralateral recording, they become easier to distinguish
Recording the ipsi and contra on ABR and will give rise to how many channel recordings?
3: ipsi, contra and horizontal (horizontal not very useful though)
ABR is a ___ response, reflects ___ and requires ___ activity
Onset, action potentials, highly synchronized
What frequency contains the peak amplitude for ABR using clicks?
3 kHz
If the peak amplitude occurs at 3 kHz, what happens at lower frequencies?
The peaks arrive later (happens to all 5 waves)
Why are tone bursts (5-10 Hz) used over clicks in ABR?
They are still fast enough to get a good signal for the ABR, but are more frequency/place specific than a click
What is windowing?
How fast we turn the sound on and off
What is the most popular window for ABR and why?
Blackman is the most popular because it’s a nice shape and has a more narrow peak
What are the two types of stimulus polarities?
-Rarefaction waves: negative electrical pulse, pushing the eardrum out (sound goes away from ear)
-Condensation waves: positive electrical pulse, pushing the eardrum in (sound is pushed into the ear)
T/F: condensation causes depolarization of the hair cells
FALSE: rarefaction is the stimulating phase and causes depolarization. Condensation causes hyperpolarization
Do rarefaction or condensation wave peaks occur earlier?
Rarefaction peaks occur a little earlier
What is the benefit of alternating polarity when doing ABRs?
It eliminates stimulus-related electrical artefact and cochlear microphonic (but gives a slight decrease in quality)
How does amplitude of the noise relate to the number of trials in an ABR?
Amplitude of the noise is proportional to the reciprocal of the square root of the number of trials
What is the best approach to remove 60 Hz line noise?
Use a stimulus rate that will put 60 Hz 180º out of phase on every other trial (40 Hz is great)
Non-coherent Averaging (Induced)
-Averaging the frequencies together rather than the waves (trials)
-Advantage is that little timing differences don’t matter
How does ABR change with intensity in normal responses?
There will be a predictable change in ABR with level; higher intensity results in higher amplitude waves
In what frequency range is low pass filtering always set in ABR?
1500-3000 Hz (1500 is harder to get precise measures but 3000 has more noise so it’s personal preference)
What are the 2 primary frequency bumps in the ABR?
500 Hz (2 ms separations allow us to see waves I, III, and V) and 1000 Hz (1 ms separations allow us to see waves I-V)
In what frequency range is high pass filtering always set in ABR?
30-150 Hz
When would you use a 30 Hz high pass filter compared to a 100-150 Hz high pass filter?
-Use 30 Hz if you want to see wave V more clearly
-Use 100-150 Hz if you want to see waves I and III more clearly
Which ABR wave (I-V) is used for estimating thresholds?
Wave V
T/F: sound level meters (SLM) have really fast integration that allow for use with ABR
FALSE: ABR occurs at a much faster rate than SLM integration (1/8th of a second)
What is 0 dB HL?
The average behavioural threshold for pure tone
What is 0 dB nHL?
The average behavioural threshold for clicks
What is the peak latency per increase in dB?
400 us (0.4 ms) per 10 dB
Why is the wave V L-I function steeper for low frequencies?
-Because the traveling wave makes it take longer to get down to the apex of the cochlea
-Across sound levels, there is a greater difference at low frequencies because more of the BM is stimulated with high sound levels (basalward shift)
Which wave’s latency is most affected by stimulus rate?
Wave V latency is most affected with high stimulus rate due to synaptic delay (wave I is barely affected)
Which waves’ amplitudes are most and least affected by intensity and rate?
Wave I amplitude is VERY affected by intensity and rate while wave V amplitude is LEAST affected by intensity and rate (wave V is robust so can still be recorded in bad conditions)
Should you use a higher or lower stimulus rate when recording thresholds from wave V?
HIGHER stimulus rate should be used (40/s) since
How is Wave V affected by noise?
Noise decreases amplitude and increases latency
T/F: biological females have higher amplitude but longer latency values for waves III-VI
FALSE: they have higher amplitude AND shorter latency values
How does Wave V latency change by gender with increasing hearing loss?
Females show little wave V latency change with increasing hearing loss, whereas wave V latency in males increases ~0.1 ms for every 20 dB drop in thresholds
What are the theories for the sex difference?
-Differences in hearing sensitivity
-Body temperature
-Head size and brain dimension
-Physiologic and biochemical properties
-Likely greater travelling wave velocity in females and at young ages
Are ABR results impacted by state of arousal?
No, even in extremely reduced states of arousal (such as narcolepsy or coma), ABR latencies and amplitudes remain intact
How is ABR affected by drugs?
ABR is generally resistant to sedatives (e.g., Chloral hydrate) and anesthetic agents (e.g., Nitrous oxide)
How does muscular artifact impact ABR?
ABR wave componentes can be completely obscured by excessive muscle artifact (often arising form neck or jaw muscles)
Why is summing repeated measures helpful in ABR?
It will increase the signal-to-noise ratio by a factor of 1.414 (~40% improvement)
What does subtracting repeated measures do for ABR?
Removes anything consistent in the response (aka it’s an estimate of background noise)
What is smoothing?
Low-pass filtering; helpful with a noisy recording
Summing responses to alternate polarity stimuli pros and cons
-Pros: reduces cochlear microphonic, reduces artifact
-Cons: reduces quality, does not improve SNR as much as summing same polarity, noise reduction should be the same as with single polarity summing
Subtracting responses to alternate polarity stimuli
-Removes most of the response
-Preserves cochlear microphonic
-Preserves artifact
