Auditory Steady-State Response Flashcards
Describe the background of the ASSR.
- 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
Why did the 40 Hz modulation rate seem promising? Why wasn’t it?
- 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
What are similarities between the ASSR and ABR?
- Utilize surface electrodes
- Evoked potential
- Not affected by patient state
- Need a quiet test environment
- Requires neural synchrony
What are differences between the ASSR and ABR?
- 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
How does the ASSR provide objective response detection?
- 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
How does the ASSR assess profound hearing loss?
- Use of non-transient signals (transient signals used in ABR)
- Tones can also be presented at higher levels with ASSR
What are advantages of the ASSR?
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
Why is neuronal adaptation a concern with steady-state stimuli?
- Steady signals will be ignored by the nervous system and firing will stop
- Have to manipulate the stimulus to make the sound more interestion
What is the solution to neuronal adaptation?
-Modulate the steady-state tone (AM, FM, Mixed modulation)
Describe amplitude modulation.
- Changes the intensity of the carrier frequency over time
- Inferior colliculus is most sensitive to 100% modulation
- Modulation depth = amplitude
Describe frequency modulation.
- Changes the frequency of the carrier frequency over time
- Modulation rate = frequency
Describe mixed modulation.
- AM + FM
- Gives the most robust response
What is a carrier frequency (Fc)?
- Higher tone, the frequency being tested
- Area that is being activated on the tonotopically arranged basilar membrane
What is a modulation frequency (Fm)?
- Lower tone, where the response is found
- Rate at which the tonotopically stimulated area will be activated
Describe the basic procedure of the ASSR.
- Stimulus is presented
- Cochlea responds at Fc
- Response recorded at Fm
How is the ASSR recorded at Fm?
- 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
Describe the ASSR of a 1000 Hz carrier tone with 100% AM at a modulation frequency of 85 Hz.
- 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
How can the ASSR can up to 4 frequencies at once?
- Use different modulation frequencies for the respective carrier frequency
- Response spectrum will be able to differentiate them
What 2 things are necessary to get a neural response for the ASSR?
- Stimulated area of the BM is intact
- CN VIII phase-locks
What is an event-related potential (ERP)?
- Neural response that phase-locks to the modulation envelope
- Embedded within the ongoing EEG
What neural response generators are responsive to AM and FM signals?
- Primary auditory cortex
- Inferior colliculus
- Cochlear nucleus
- CN VIII
What modulation frequencies correlate to the different levels of the auditory system?
- Thalamus/cortex (LLR) < 20 Hz
- MLR: 20-60 Hz
- Brainstem: > 60 Hz
What is the optimal ASSR modulation frequency?
- > 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
Describe the ASSR automatic response detection algorithm in the frequency domain?
- 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)
