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HESP630 Final > Auditory Steady-State Response > Flashcards

Auditory Steady-State Response Flashcards

1
Q

Describe the background of the ASSR.

A
  • Became clinically available in 2001
  • However, used in research since the late 1970s
  • Early studies used lower modulation rates (35-55 Hz)
  • 40 Hz modulation rate seemed promising
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2
Q

Why did the 40 Hz modulation rate seem promising? Why wasn’t it?

A
  • Good estimation of behavioral thresholds in NH and HI adults at low and high frequencies
  • BUT highly affected by sleep and sedation so unstable in infants and young children
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3
Q

What are similarities between the ASSR and ABR?

A
  • Utilize surface electrodes
  • Evoked potential
  • Not affected by patient state
  • Need a quiet test environment
  • Requires neural synchrony
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4
Q

What are differences between the ASSR and ABR?

A
  • Provides objective response detection
  • Allows for simultaneous testing of right and left ears
  • Can test up to 4 frequencies at once
  • Can assess profound hearing loss
  • Presents modulated pure tones
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5
Q

How does the ASSR provide objective response detection?

A
  • Frequency-specific information for 250-8000 Hz
  • The objective detection algorithm determines the presence of a response for the clinician, eliminating the need for subjective judgment of waveforms
  • Automated algorithms assess amplitude and phase coherence of response
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6
Q

How does the ASSR assess profound hearing loss?

A
  • Use of non-transient signals (transient signals used in ABR)
  • Tones can also be presented at higher levels with ASSR
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7
Q

What are advantages of the ASSR?

A

1) Good predictor of thresholds? (better for severe/profound HL but not so much for NH)
2) Objective
3) No spectral splatter (as with transient tone bursts)
4) Many frequencies can be recorded at once

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8
Q

Why is neuronal adaptation a concern with steady-state stimuli?

A
  • Steady signals will be ignored by the nervous system and firing will stop
  • Have to manipulate the stimulus to make the sound more interestion
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9
Q

What is the solution to neuronal adaptation?

A

-Modulate the steady-state tone (AM, FM, Mixed modulation)

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10
Q

Describe amplitude modulation.

A
  • Changes the intensity of the carrier frequency over time
  • Inferior colliculus is most sensitive to 100% modulation
  • Modulation depth = amplitude
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11
Q

Describe frequency modulation.

A
  • Changes the frequency of the carrier frequency over time

- Modulation rate = frequency

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12
Q

Describe mixed modulation.

A
  • AM + FM

- Gives the most robust response

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13
Q

What is a carrier frequency (Fc)?

A
  • Higher tone, the frequency being tested

- Area that is being activated on the tonotopically arranged basilar membrane

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14
Q

What is a modulation frequency (Fm)?

A
  • Lower tone, where the response is found

- Rate at which the tonotopically stimulated area will be activated

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15
Q

Describe the basic procedure of the ASSR.

A
  • Stimulus is presented
  • Cochlea responds at Fc
  • Response recorded at Fm
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16
Q

How is the ASSR recorded at Fm?

A
  • The evoked response will result in a peak of neural activity the rate of stimulation
  • Response is judged as present at the frequency region stimulated by the carrier frequency if there is a significant increase in neural activation at a rate corresponding to the modulation frequency
  • ASSR is present if the carrier frequency is presented at a rate (i.e. modulation frequency) that is sufficient to cause overlapping of transient responses resulting in a sustained response
17
Q

Describe the ASSR of a 1000 Hz carrier tone with 100% AM at a modulation frequency of 85 Hz.

A
  • Area of BM activation: 1000 Hz
  • Rate at which this area of the BM is activated: 85 Hz
  • Therefore, the ear hears at 1000 Hz but the brain responses at 85 Hz
18
Q

How can the ASSR can up to 4 frequencies at once?

A
  • Use different modulation frequencies for the respective carrier frequency
  • Response spectrum will be able to differentiate them
19
Q

What 2 things are necessary to get a neural response for the ASSR?

A
  • Stimulated area of the BM is intact

- CN VIII phase-locks

20
Q

What is an event-related potential (ERP)?

A
  • Neural response that phase-locks to the modulation envelope
  • Embedded within the ongoing EEG
21
Q

What neural response generators are responsive to AM and FM signals?

A
  • Primary auditory cortex
  • Inferior colliculus
  • Cochlear nucleus
  • CN VIII
22
Q

What modulation frequencies correlate to the different levels of the auditory system?

A
  • Thalamus/cortex (LLR) < 20 Hz
  • MLR: 20-60 Hz
  • Brainstem: > 60 Hz
23
Q

What is the optimal ASSR modulation frequency?

A
  • > 60 Hz
  • Cortex is less sensitive to modulation frequencies > 40 Hz
  • Brainstem is less affected by conscious state, cognition, etc. so it’s better for objective hearing screening
24
Q

Describe the ASSR automatic response detection algorithm in the frequency domain?

A
  • Calculates SNR to ensure response is above the noise floor

- Compares the variance of the noise (FFT amplitude at a “distant” frequency vs. variance of FFT amplitude at AM rates)

25
Q

What is the F-test?

A
  • Statistic to determine if power of response at the rate of stimulation > than EEG noise at surrounding frequencies
  • Response is significantly different from noise when F-ratio reaches p < .05
26
Q

Describe Intelligence Hearing System’s phase analysis?

A
  • No phase coherences = phase absent

- Phase coherence = response present (measure of variability of phase)

27
Q

What is phase?

A
  • Displacement from baseline (aka # cycles per second)
  • Measured at a particular point (time) of a cycle
  • Therefore, it’s a relative measurement
  • In radians or degrees
28
Q

Describe measurement/analysis of phase coherence.

A
  • R varies from min of 0 (perfect incoherence, phase evenly disperse) to a max of 1 (perfect coherence, all phases identical)
  • Higher R values indicate a lower probability the phase is randomly changing from epoch to epoch
  • Statistical significance of phase coherence values was determined using the Rayleigh test of circular variance
29
Q

Describe the relationship between ASSR and age.

A
  • Responses at 50 dB SPL are more easily detected at 3-15 weeks of age than in the first few days after birth
  • Comprehensive frequency-specific testing of hearing using steady-state responses will likely be more accurate if postponed until after the immediate neonatal period
  • Amplitude of the response is increased by as much as 20-25%, mostly for low- and high-frequencies

HESP630 Final (9 decks)

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