How is ASSR similar to the ABR?
- Surface electrodes
- Evoked potential
- Patient state
- Unlike middle and late cortical potentials, ASSR and ABR are not affected by sleep state - Quiet test environment
- Neural synchrony
- Not required for every evoked potential - Both are closely correlated with behavioral auditory thresholds
How is ASSR different from ABR?
- Objective response detection
- Determines presence of a response for the clinician, eliminating need for subjective judgment of waveforms - Up to 4 frequencies simultaneously
- Simultaneous R & L testing
- Assess profound hearing loss
- Modulated pure tones
- Can obtain frequency-specific information for 250-8000 Hz
Why can’t you assess profound hearing loss with the ABR?
- Stimuli are transient
- Can’t be presented at high stimulus levels
What are the advantages of ASSR?
Good predictor of thresholds
- Better than click-ABR for severe/profound hearing losses
- But not so good for normal hearing
Objective
- Automated algorithms to assess amplitude and phase coherence of response
No spectral splatter (as with transient tone bursts)
Many frequencies can be recorded at once
What stimuli are used in ASSR?
- Steady-state
- Stimulus is constantly on
Concern: neuronal adaptation
- Steady signals will be ignored by the nervous system
- Have to manipulate the stimulus
Solution: modulate the steady-state tone
- Amplitude modulation
- Frequency modulation
- Mixed modulation
What is amplitude modulation?
- Changes the intensity of the carrier frequency over time
- Use 100% modulation because you get the best response to that rate at the upper part of the brainstem
Modulation rate= modulation frequency
ASSR is not limited to one carrier/modulator combination
- Can test 4 frequencies simultaneously
- As long as you use different modulation frequencies for the respective carrier frequencies, the response spectrum will be able to differentiate them
What is frequency modulation?
- Changes frequency of the carrier frequency over time
- Depth of frequency modulation defines the frequency range of the stimulus
20% FM for a 1000 Hz tone will cover a 200 Hz range, above and below 1000 Hz
What is mixed modulation?
Amplitude and frequency modulation
Yields most robust response
What is the Fc vs. Fm?
Carrier frequency (Fc): higher tone
- Area that is activated on the tonotopically arranged basilar membrane
- The frequency being tested
Modulation frequency (Fm): lower tone
- Rate at which the tonotopiclaly stimulated area will be activated
- Where the response is found
*Cochlea responds at Fc, but response is recorded at Fm
When is a neural response recorded?
- Area of the BM is intact
- 8th nerve phase-locks
- If the ear has hearing at the Fc (e.g., 500 Hz), the brain responds at the Fm (e.g., 85 Hz)
What are some of the neural generators that respond to AM and FM signals?
- 8th cranial nerve
- Cochlear nucleus
- Inferior colliculus
- Primary auditory cortex
How do neural generators respond to different modulation rates?
<20 Hz
- Thalamus or cortex (late latency response)
20-60 Hz
- Middle-latency response
> 60 Hz
- Brainstem activity
Describe the conclusions from Winter (2002).
Results
- 90 Hz: the largest activity occurred in the brainstem
- 40 Hz: the initial brainstem component remained and significant activity also occurred in the cortex
- 12 Hz: responses were small but suggested combined activation of both brainstem and cortex
Conclusion:
The whole auditory system is activated by modulated tones, but the cortex is less sensitive to modulation frequencies greater than 40 Hz
What is the automatic response detection algorithm?
Frequency domain
- Calculate SNR
- Compares the variance of the noise
What is the f-test?
Statistic to determine if power of response at the rate of stimulation, than EEG noise at surrounding frequencies
Response= significantly different from noise when F-ratio reaches p<0.05
What is phase?
Phase: a particular point (time) of a cycle
- A relative measurement
Reference: arbitrary zero or unshifted signal (relative phase)
- Expressed as an angle (radians) or degrees
What is phase analysis?
Response present, phase coherence
- A measure of the variability of phase; it varies from a minimum of 0 to a maximum of 1
Response absent, no phase coherence
Higher values of phase coherence indicate a lower probability that the phase is randomly changing rom epoch to epoch
What results are yielded from ABR and ASSR when the subject has normal hearing?
ABR
- Accurate estimation
ASSR
- May over-estimate thresholds if patient is unsedated
What results are yielded from ABR and ASSR when the subject has a conductive HL?
ABR
- Wave I can indicate whether we have ear specific findings, i.e., do not always have to mask
ASSR
- Masking required for bone condition tone burst testing
What results are yielded from ABR and ASSR when the subject has SNHL?
ABR
- Accurate to moderate HL
ASSR
- Can distinguish between moderate to profound HL
What results are yielded from ABR and ASSR when the subject has neural/auditory neuropathy?
ABR
- Can distinguish between sensory & neural HL (wave I/cochlear microphonic)
ASSR
- Cannot distinguish between cochlear/peripheral and retrocochlear HL
What are the ABR test characteristics vs. ASSR test characteristics?
ABR
- High level of skill required
- Better fro estimating normal hearing
- More online decision making flexibility
- Absolute quiet not required
- ABR more accurate <6-8 weeks of age
ASSR
- Automated; avoid interpretation bias
- Test at high dB levels: can ear be stimulated? (important for CI candidacy)
- Many frequencies recorded at once
- Better representation of steeply sloping hearing losses
- Patient needs to be still
