Lecture 7 Flashcards
1
Q
What is a steady state response?
A
- A response that has a steady amplitude and phase relationship to the stimulus
- The stimulus response is on the whole time (unlike ABR, which is a transient response)
- Looking at an ongoing response to a stimulus (an amplitude modulated sound)
2
Q
Sometimes, all steady state responses are referred to as ____
A
Frequency following responses
3
Q
What are 5 examples of steady state responses?
A
- the cochlear microphonic
- frequency-following responses
- envelope-following responses
- steady-state evoked potentials
- the auditory steady-state response
4
Q
Explain speech evoked responses
A
- Speech also has fluctuations
- Can hear the same activity going on in the brain that is in the original sound
- SSRs are encoding the fluctuations that are happening in the actual sound (our brains follow the frequencies of the signal)
5
Q
What is the maximum firing rate of a neuron?
A
- The response gives out at 1500-2000Hz (mostly a low frequency response)
- Neurons can only fire a maximum time/second
6
Q
How do we look at a bunch of neurons?
A
The volley principle
7
Q
Who started the work with steady state far field potentials?
A
- David Regan was the first to do detailed steady-state work with scalp (far-field) potentials in the visual system
- Director of the center for research in vision and hearing
8
Q
What type of response is an ABR? Explain.
A
- Transient: happens once after a stimulus (looking at what happens after the transient response is played)
- Transient responses are triggered by an event (ABR by envelope of click or tone burst)
- Once transient responses are started, they do their own thing (ABR is path up the brainstem)
9
Q
What is a transient response looking at?
A
- Looking at what happens after the transient response is played
- Looking at travel time
- Get an ABR triggered at a certain point
10
Q
What do steady state responses follow?
A
- Steady-state responses follow the stimulus (i.e., they have a constant amplitude and phase relationship to the stimulus)
- You are getting a response that looks like the stimulus (the tone is steady and the response fluctuates with the stimulus)
- Ongoing response to an ongoing stimulus
11
Q
What is more difficult to determine with a steady state response?
A
- More difficult to determine the timing (hard to determine where it began because there is always some delay between stimulus and response)
- Whereas with ABR, timing is very important
12
Q
Explain the path of a sound wave
A
- In the air, we have longitudinal waves of compression (air particles pushed together) and rarefaction
- As speaker diaphragm pulls back, low pressure wave approaches the ear drum, eventually pulls it out (rarefaction)
- The auditory system is physically moving in motion with the sound
13
Q
Explain the temporal coding of sound
A
Light green: basilar membrane pulled upwards (rarefaction)
- Tip links are stretched,
- Ion channels open
Dark green: basilar membrane pushed downwards (condensation)
- Tip links are loose
- Ion channels closed
14
Q
Firing rate synchronizes with ____
A
Hair cell movement
15
Q
What is the receptor potential?
A
- Receptor potential: measure the potential in the IHC (receptor potential fluctuates as the cilia move back and forth)
- Not a sinusoidal fluctuations and is somewhat displaced on the axis (as the tip links open you have a larger change in receptor potential than when they shut)
16
Q
Receptor potentials follow the ____
A
Stimulus
17
Q
How fast do the hair cells go?
- Where do you see DC potential?
- What is the DC shift?
- What is the AC shift?
- Where does the RP get smaller?
A
- Start to see a DC potential at higher stimulus (less of what is going on is following the stimulus as you increase in frequency)
- The DC shift is the sustained portion
- The AC portion is the steady state portion (the portion fluctuating with the stimulus)
- Receptor potentials get smaller at high frequencies but is still there
18
Q
What is phase locking?
A
Tendency for the AN to fire at a certain phase
19
Q
Explain AN rate coding and phase-locking at low frequencies
A
- At LFs, the AN may fire at phases of the signal (the same timing)
- The nerve has the tendency to fire at certain times (phase locking)
- There is rate and phase locking happening
20
Q
Explain AN rate coding and phase-locking at high frequencies
A
- Phase locking breaks down at high frequencies (neurons can’t fire that fast, and there is some jitter with each firing)
- Very hard to track high frequency signals
- Only rate coding happening (no phase locking)
21
Q
Inner Hair Cells connect to ____ Type I SG Fibres
A
5-20
22
Q
What is the volley principle?
A
- The central auditory system ‘listens’ to many neurons, so individual neurons do not need to fire on every cycle
- All the neurons can’t fire every time, but enough of them will (will get a good representation of the AN)
23
Q
What happens as the IHCs move back and forth? When are individual neurons more likely to fire?
A
- As this hair cell moves back and forth, we will get a receptor potential that fluctuates (not a pretty sinusoid, more depolarization than hyperpolarization)
- Individual neurons are more likely to fire when ion channels open
24
Q
What are period histograms?
A
- AN firing follows waveforms very precisely
- Depolarization = the AN fires
- Hyperpolarization = the AN isn’t firing
25
What is half wave rectification?
- Half wave rectification = the AN doesn’t fire during hyperpolarization
- This distorts the signal
26
Phase-locking in the AN vs CN
- Phase-locking is more precise in the cochlear nucleus (e.g., bushy cells) than in the auditory nerve
- A much cleaner representation of the frequency (more precise in the brainstem)
- Why? Each AN fiber isn’t that precise, but when they work together, they are very precise
27
What is a frequency following response? Where does this response get better?
- A response that follows the frequency
- The response gets better as you go farther up the brainstem
- A far-field (scalp) electrophysiologic response that has the same frequency components as the stimulus
28
Where is a FFR coming from?
Research suggests sources in the midbrain (IC), but there are likely contributions from the cortex (for very low frequencies), and of course, from the lower brainstem and auditory nerve
29
Can FFR be used to test hearing directly?
- Unfortunately can’t be recorded near threshold (and high-levels are less place-specific)
- You need fairly high levels and low frequency to get a good FFR (70 dB)
- At softer levels, you will not get a good FFR
30
What is the highest synchronization of the cochlea?
3kHz
31
What is the highest synchronization of the brainstem?
1.5kHz
32
What is the highest synchronization of the cortex?
0.3kHz (but typically up to 40Hz)
33
What do we do about artifact and the CM with FFR?
- Because the response looks just like the stimulus, it is hard to separate from artifact (because artifact looks just like the stimulus too)
- We usually combat these by recording to alternate polarities, and averaging an even number of trials
34
What does alternating polarity do to the FFR?
- Stimulus is not perfectly rectified in IHC transduction (since AN is hyperpolarized during condensation phase)
- Alternating polarity should almost eliminate FFR, and double the frequency of residual activity
- It’s going to fire in one half of the cycle, but not the other half (you get a smaller version of the signal at twice the frequency of the stimulus)
35
How useful are FFRs?
FFRs to the components in the stimulus spectrum have limited utility for audiology (discovered before ABR, but less useful; less useful clinically)
36
What are 4 cons to FFR?
- Only reflect encoding of low frequency information
- Easily confused with stimulus artefact and the cochlear microphonic
- Alternating polarity eliminates artefact and CM, but also distorts the FFR
- Can only get super well at high levels
37
What is a brilliant alternative to the FFR?
- The auditory steady state response (ASSR)
- This is a clinically useful alternative to the FFR (a clever way of getting the response that doesn’t have all the issues of the FFR)
38
How were SSRs discovered?
- Galambos (1981) discovered that presenting stimuli at 40 Hz produced a large steady-state response (not 40 Hz tones, but higher frequency tones presented 40 times per second)
- Presented a high frequency sound (2K) at 40 times per second (found that the response had a 40Hz shape)
39
What are SSRs similar too?
Steady state responses may be similar to transient responses... it may be a series of them, just overlapped.
40
Cortical columns synchronize very well at ____Hz
40
41
Why was the birth or SSRs important?
- You could elicit the transient response (ABR/MLR) with a tone burst at any frequency (e.g. 4 kHz) – wouldn’t get an FFR at 4K, too high
- Therefore, you could elicit a 40 Hz response with repeated tone bursts at any stimulus frequency
- FFR frequency limit is not an issue
42
A x kHz tone burst, presented 40 times per second creates...
- Displacement primarily at the x kHz cf. of the cochlea
- The STIMULUS frequency (carrier)
- Which is reflected in a 40 Hz response
- The PRESENTATION RATE
- A 2000Hz tone burst will go to the 2000Hz characteristic frequency of the cochlea (carrier frequency)
- What we are looking for from the brain is the presentation rate (40Hz) – you will also see it at the harmonics of that
43
FFR vs ASSR
FFR
- Follow the frequency of the stimulus (a continuous 500 Hz tone = a small 500 Hz response)
SSR
- Follow the presentation rate or the “envelope” (the turning on/off or up/down of a stimulus)
- Not following the frequency, but the presentation rate (how many times you present)
44
500Hz played 40 times per second gives you...
- 500Hz FFR
- 40Hz ASSR
45
What is another name for a SSR?
Envelope following response (EFR)
46
Repetition rate vs. modulation rate
- what is ASSR a response too?
- Amplitude modulating (turning up and down) is the same as turning on and off
- The ASSR is a response to the modulation rate (i.e., the envelope) or repetition rate of a stimulus… these are all the same thing
- Modulation rate = envelope rate = presentation rate
47
Why is 40Hz SSR used?
Works best for infants
48
Noise floor goes down with increase in ____
Frequency
49
The ____ doesn’t lock well at high frequencies, but the ____ does
Cortex, brainstem
50
The cortex only locks good up to ____Hz (above this, they get smaller and you are looking at ____ responses)
40, brainstem
51
____ can be the carrier
Any frequency
52
With ASSR, the response is not to the ____, but to the ____
Stimulus frequency, modulation (envelope or presentation rate)
53
How do you make an amplitude modulated stimulus?
1. Present a short stimulus over and over again at a specific rate
- The stimulus is called the ‘carrier’ and the presentation rate is the ‘envelope’
2. Turn a stimulus up and down at a specific rate
- This is done by combining multiple tones
- The tones are the ‘carrier’ and the modulation rate is the ‘envelope’
54
What happens when two tones are presented (carrier vs modulation)?
- 130 and 140 Hz
- 130 will have 13 peaks
- 140 will have 14 peaks
- Go up and down 10 times a second because they are separated by 10Hz (it will beat at the difference frequency)
- In 100 ms (1/10th of a s), one has 13 cycles, and one has 14
- These will create maximal displacement in the 130 and 140 Hz places in the cochlea
- But they fall within the same critical band, so the tones will interact, creating an amplitude modulation at 10 Hz
- This is a temporal pattern (a 10 Hz amplitude modulation) occurring in the 130-140 Hz region (the carrier frequencies)
- e.g., 130 Hz and 140 Hz will beat at 10 Hz, but there is no 10 Hz ‘tone’—130 and 140 Hz are the only two frequencies present
- The 10 Hz envelope isn’t part of the stimulus, it’s what arises when the stimulus components add together in the ear
- The modulation rate doesn’t affect where it goes on the basilar membrane (the 10Hz isn’t really there but the auditory system feels it there, that is just where it is beating)
55
120 and 140 will fluctuate at ____Hz
20 Hz (it beats at that part of the cochlea 20x per second)
56
What introduces the envelope frequency into the nervous system?
Nonlinearities in cochlear movement and in inner hair cell transduction
57
The tones have to be fairly close to ____
Beat together
58
What happens when we use 3 tones?
- Sinusoidal amplitude modulation (SAM)
- Just like a beat, except there is one tone above and below the carrier frequency (each at half amplitude of center component)
- 480+500+520 is a 500 Hz tone that is modulated at 20 Hz
- Carrier = 500 Hz
- Modulation = 20 Hz (getting a SSR at 20Hz)
- Will also see a beat at 40Hz (diff between 480 and 520)
- Will also get an FFR at the frequencies
- Will also see a beat at the harmonics
- All of the sound energy is at 480, 500 and 520 (a FFR could be recorded at these frequencies)
an ASSR will be recorded at 20 Hz
59
What is so good about SAM?
- Carrier frequency determines where on BM (what frequencies are being tested) e.g., 500 or 4000 Hz
- Modulation frequency/rate is the frequency that we record at the scalp
- A sinusoidal modulation gives us a bigger response (modulation goes up and down a little sharper)
60
an amplitude modulation that occurs when ____
Tones interact
61
Explain playing 1008, 1082, and 934Hz
- Its 3 steady pure tones, the combination goes up and down 76 times per second (that’s the difference between the tones) because they add together in your ear
- Its like turning 1008Hz up and down 76 times per second
- The sum of the components is modulated (the sum isn't present in the signal, it is just 3 tones)
62
The carrier frequency is the ____
Frequency of the tone itself
63
The modulation frequency is the ____
Difference between the tones
64
Why is ASSR better than FFR?
- no problem with what?
- where are responses larger?
- how does artifact affect them?
- No problem with the 1500 Hz limit in the brainstem
- Responses are larger at low frequencies (this is where the brain loves to time lock)
- Easy to separate from artefact!
65
Why do we hear the hum and not 3 specific tones?
- These non-linearities induce energy at the modulation frequency (when they overlap on the basilar membrane)
- Because of the distortion that the ear is doing
66
What are 4 different types of modulation?
1. Tone bursts
2. Beats
3. SAM
4. SIN3
67
Explain using a tone burst
There is a lot of splatter with each tone burst
68
Explain using a beat
A beat is very focused in frequency, but has a very small response
69
Explain using SAM
SAM (3 tones) gives a larger response, but is less focused in frequency
70
Explain using SIN3
SIN^3 gives a larger response, but is less focused in frequency
71
An exact trade-off between how large the brain ____ will be and how ____ focused it will be
Response, frequency
72
The less ____ the larger the ____ will be
Frequency focused, response
73
What gives the largest response?
SIN3 or tone bursts (poor frequency focused)
74
What gives the smallest response?
Beats (highly frequency focused)
75
What is the most frequency focused response?
Beats
76
What is the lowest frequency focused response?
Tone bursts or SIN3
77
How is amplitude modulation described?
1. AM Rate (or Frequency)
- “80 Hz AM”
2. AM Depth
- 100% AM (signal goes completely off an on) – this is what we use for thresholds
- 50% AM (signal goes to half its amplitude) – diagnostic measure
3. Shape of modulation
- Exponential (rises and falls more quickly)
- Sinusoidal (rises and falls smoothly)
78
How is frequency modulation described?
1. FM Rate (or Frequency)
- “80 Hz FM” (it goes up and down in FREQUENCY 100 times/sec)
2. FM Depth
- 10% FM (signal goes up and down in frequency by 10%)
- A larger depth gives a bigger response
79
How is mixed modulation and IAFM described?
1. Both AM and FM
- Mixed = AM and FM at same rate
- IAFM = independent AM and FM (different rates)
2. Described by
- AM Rate /Frequency
- AM Depth
- FM Rate /Frequency
- FM Depth
80
Explain a carrier of 1kHz and modulation at 81Hz (100% AM, 25% FM)
- Sound goes in
- Activation at 1kHz region of the basilar membrane
- SSR at 81Hz
- If we get 81Hz it tells us that they encoded 1kHz
81
What is the source os a 39Hz tone?
Brainstem and cortex
82
What is the source of an 88Hz tone?
- Mostly brainstem (loss of what is coming from the cortex)
- Once you go above 40 Hz, the cortex isn’t involved very much
83
420+500+580
a) carrier
b) modulation rate
c) FFR
d) ASSR
e) primary source
f) how to get more responses from the cortex?
a) Carrier = 500 Hz
b) Modulation rate = 80 Hz
c) FFR at 420, 500, 580 Hz
d) ASSR at 80 Hz
e) Primary source brainstem
f) Slow the modulation rate (460, 500, 440 Hz) – modulation rate around 40Hz (from cortex and brainstem)
84
40 vs 80Hz
40 Hz response
- Unreliable in infants
- Smaller in sleep
80 Hz
- Reliable in infants
- About ½ or third size of adult in first few months
- No significant changes afterwards
- Not affected by sleep
85
What 2 things happen with decreasing intensity?
- Amplitude decreases
- Latency increases
86
Can you present multiple stimuli to one ear?
- 4 stimuli presented simultaneously to one ear
- All modulated at a slightly different rate
- You know have different modulation frequencies in the brain response so you can test them at the same time
87
Can you present multiple stimuli to both ears at the same time?
- 4 stimuli to the left ear and 4 stimuli to the right ear
- Can also test both ears at the same time (but use different modulation frequencies in the ears)
88
What has to be down to test multiple stimuli?
- This is only true at soft levels (< 60 dB)
- At high levels, the tones interact on the basilar membrane, and the 4 or 8 frequency response is less than the individual responses
- They start to overlap on the basilar membrane at high levels
89
How is an ABR measured?
Peaks measured from the averaged brain waveform (after averaging the response many many times in a very short window)
90
How is an ASSR measured?
- Frequencies measured from the averaged brain waveform
- Since we’re looking at frequencies, we need to record a longer time window… why? Better frequency resolution
91
What is frequency resolution equal to?
Frequency resolution is equal to 1/T, where T is the length of the time window, in seconds
- 1 second time window = 1/1 = 1 Hz resolution (80 from 81)
- 2 second time window = ½ = .5 Hz resolution (80 from 80.5 and 79.5)
- 10 second time window = 1/10 = .1 Hz resolution
92
Key factors of an ABR
a) interstimulus interval
b) recording window
c) click
d) average
- Interstimulus interval (75ms for 13.3s)
- Recording window (10ms)
- Click (100us)
- Average = 2000 clicks (about 150s or 2.5min)
93
Key factors of an ASSR
a) epoch length
b) sweep length
c) what is happening the whole time?
d) average
- Epoch length (1s for rejecting noisy epochs)
- Sweep length (16s; 1/16Hz)
- Modulated tone(s) on the entire time
- Average = 10 sweeps (160s or 2.6min)
94
What are 2 ways to reduce noise?
1. Averaging (noise decreases with more averaging)
2. Improving frequency resolution (noise decreases with more bins)
95
What matters most?
How long you record
- Consider switching from 240 one-second to 80 three-second averages
- Improved resolution decreases noise in each bin to 58%
- Decreased number of averages increases noise in each bin by 73%
- i.e., they cancel: .58 ×1.73=1
96
Does sweep length matter?
- Length of a short sweep
- Length of a long sweep
- What determines how wide the frequency resolution is?
- What is there a direct trade off between?
- Short sweeps 15 frequencies (-7 & + 7) vs long sweeps 120 frequencies (-60 & + 60)
- Noise is random, so should be distributed across many frequencies
- What determines how wide the frequency resolution is? The length of the sweep that is analyzed. Longer sweeps, more narrow frequencies.
- Direct trade-off between length of sweep (frequency width) and number of averages
97
Accurate vs fast recording
- Accurate recording: 6-7 minutes (asymptotes after this)
- Fast recording: 3.5 minutes
98
Thresholds improve with ____
Averaging
99
What frequency is is best to test thresholds at?
High frequencies (low frequencies are harder to test)
100
How do you test for significance?
SNR = response level (corrected) / noise level
101
How do you determine noise level?
- Response occurs at a known frequency
- Noise is usually average level in adjacent frequency ‘bins’
- Typically, the noise level is the area with no stimulus
102
What are 2 ways to recognize signals in noise?
1. Hotelling's T2 test
2. F test
103
How does Hotelling's T2 test work?
- If a response is related to the stimulus, there should be a relatively consistent phase lag between the two
- If a response is not related to a stimulus, the phase relationship should be random
- Plot the 95% confidence interval of the mean
104
How does the F test work?
- F test for hidden periodicity
- F is a ratio of variances (or power)… both are mean-squared values
- Power of response at modulation frequency to average power of response in adjacent frequency bins
105
Is Hotelling's T2 or F test better?
- F test is equivalent to the T2 test when you have one more adjacent frequency bin than the number of T2 data points
- They are the same
106
Significance testing is important at ____ levels
Soft
107
Explain BC ASSR thresholds
- Found BC ASSR thresholds to be lower (better) in infants than adults
- Use age-specific normative thresholds
108
Artifact can be a significant problem at levels ____ dB HL and higher
40
109
How do we get rid of artifact?
- Use alternating polarity
- Use sharp anti-aliasing filters (to reject the noise) – remember, we only need to record frequencies < 100 Hz, so the LP filter can be placed above this
110
ASSR Versus ABR
- High correlations between thresholds were found (higher correlation in the high frequencies)
- Especially high correlations in the high frequencies
- On average, less time needed for ASSR (can get more frequencies faster)
111
The FFR is a response that occurs at the same frequency as the ____
Stimulus
112
The ASSR is a response that occurs at the same frequency as a ____
Stimulus modulation
113
____ requires less expertise
ASSR
