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ALR / Cortical Evoked Response Audiometry

What is ALR / Cortical ERA?
Auditory Late Responses (ALRs) are longer latency components that for most part are generated in higher regions of the auditory CNS, including the auditory cortex. Cortical Evoked Response Audiometry (CERA) refers to the technique of measuring ALRs for the purpose of assessing ones hearing abilities. The responses are typically measured using surface electrodes placed on the scalp of an individual.

Why ALR?
ALR/Cortical ERA is traditionally used to help determine the degree of hearing loss in adult populations. Compared to traditional Auditory Brainstem Responses (ABRs), or behavioral audiometry, ALRs demonstrate audibility at the cortex without the need for the listener to play an active role in the procedure. This is a key advantage in various medico-legal scenarios or in cases where the individual is unable or unwilling to provide accurate behavioral responses to a sound.
Other advantages include the relative robustness of the ALRs to myogenic activity, the high frequency specificity of the tonal stimuli due to their longer duration, and is much closer to the behavioral audiometric pure tone compared to traditional ABR octave wide stimuli, and the ability to calibrate the stimuli according to international standards (BS EN ISO 389 as for pure tone audiometers) and not the standard 389-6 used for Tone burst/Tone pips & Chirps .
The key ALR waveforms observed in CERA are the P1, N1, and P2 responses. The ALR latency typically ranges from 50 – 300 ms and N1-P2 amplitude ranges from 0-20 μV. (see Figure 1, showing P1, N1 and P2 responses to a 2 kHz toneburst stimuli, used to provide a typical threshold recording from the left ear. In this case, hearing sensitivity is shown to be within the normal audiometric range.)

Figure 1 ALR responses Right and Left

How to test
Patient Preparation
is very important. Patient arousal and attention state has a significant effect on the amplitudes of the ALR. The ALR waveform changes as a person becomes drowsy or falls asleep. When a patient is asleep the N1 amplitude is smaller and the P2 amplitude is larger. However, when the subject is listening for a change or paying close attention to stimuli the N1 increases in amplitude and in subjects who are not attending to the stimuli, the N1 can become difficult to measure at low intensities (Näätänan and Picton, 1987). The response also habituates quickly, an affect which is more apparent nearer to the threshold of the listener, so it is important to limit the number of stimulus presentations within each ‘run’. Typically, between 15 and 20 presentations might be made per run and a number of successive runs are then merged into a grand average in order to reveal the ALR.
The patient is typically instructed to sit quietly during the procedure, maintaining passive attention for example by reading or watching a close-caption movie with the sound muted. It is not advised to perform ALR and under sedation (Crowley & Colrain, 2004).

Electrode Placement:
It is possible to obtain ALR with a standard 2-channel electrode montage, with an active vertex electrode referenced to either right or left mastoid. However, since the ALR has generators orientated towards the fronto- central regions of the scalp then response strength may be greater when recorded from a point on the midline that is slightly forward of the vertex position.

Setting up the Eclipse
The Eclipse comes with a pre-programmed protocol for ALR testing (license), and is ready for immediate use. Protocols can be created or modified easily to fit your clinic needs. Consult your Eclipse Additional Information to learn how to create or modify a protocol.

Protocol Settings:
In CERA, ALRs should be measured using tone burst (250 Hz – 4 kHz) stimuli at intensity levels between 0 and 100 dB HL to establish threshold.

Summary of parameters for ALR 

    P1, N1, P2
Subject State Awake and quit adults, (and older children)
Eyes Eyes open
Condition Attend or ignore conditions
Stimuli  Recordings Types of stimuli Tone burst, speech vowels or consonant vowel combinations
Inter-onset interval 1-2 sec
Stimulus duration  50 – 80 ms (including 10-15 ms onset/offset ramps)
Presentation  Typically insert or supra-aural headphones
Intensity Starting at 60-80 dB HL
Recordings Reference electrode Right/Left mastoid (optimally linked electrodes using the jumper cable)
Filtering 1-30 Hz
Analysis time Pre stimuli -100ms
Post stimuli 700ms or more
Sweep 50-300 [comprised of 10-20 sweep sub averages]
Waveform reproducibility Set within the latency range 30 –270 ms
Measurements Adult
Children
Infants
Measures 
P1, N1, P2
P1, N200-250
Reliable components
Baseline to peak amplitude, peak latency
Use latency window established using grand mean data
Response presence   Determined by   Replicable components
Response amplitude should be ≥ 2.5 times larger than the residual noise.
Residual noise amplitude should be < 1.5 μV (as determined by average difference between replicates)

Interpretation of the ALR result
Typically the ALR threshold recording is started at 60dB HL and increased or decreased by 20dB based on the response. 5 or 10 dB steps are typically used when close to threshold.

It is recommended that the amplitude of the response is taken as the peak-to-peak amplitude between N1 and P2, and a lower value of 2.5 μV is recommended before a response can be clearly identified. The Waveform Reproducibility function can also be used to indicate the response reliability. This function provides a measure of the correlation between curves held in the A and B buffer when there is a response present.

The residual noise in the trace can be estimated by measuring the ‘average gap’ between replicates (for example, between rarefaction and condensation responses displayed in the A-B buffer).

See Figure 2, which shows the A and B curves separate for each trace. Over the duration of the epoch (-150 ms to + 750 ms) the average gap between traces can be observed to be below 1 μV, and the Waveform Reproducibility for the highlighted curve is shown to be high (73%) indicating reliable response.

Figure 2 ALR showing A&B buffer for repeatability

Electrophysiological Threshold Estimation and correction factors
An ALR threshold at 20dBHL at 2kHz would be considered within the range of normal hearing. Applying a typical correction factor would estimate the behavioral threshold to be 13.5dBHL at 2 kHz.
For masking the ALR, please follow the pure tone audiometry guidance’s.

Lowest level response >5uV: interpolate
Lowest level response <5uV: is threshold

ALR threshold behavioral correction factors to estimated hearing thresholds.

  500Hz 1000Hz 2000Hz 4000Hz
Stimuli (dB HL) 50 60 65 65
Mean Correction (dB)* -6.5 -6.5 -6.5 -6.5
d(B) estimated hearing level eHL 43.5 53.5 58.5 58.5

*Reportings from Liverpool UK, Variance 95% within ±10dB. Note occasionally very poor response > 20dB error

Reporting
Choose the Report Icon

When complete, choose Save and Exit.


References
Crowley, K.E. & Colrain, I.M. (2004) A review of the evidence for P2 being an independent component process: age, sleep and modality. Clin Neurophysiol.(115(4) 732-44.

Näätänen, R. & Picton, T. (1987) The N1 wave of the human electric and magnetic response to sound: A review and an analysis of the component structure. Soc. For Psychophysiological Research, Inc. 24 (4) 375-425.

October 2015
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