Are there any rules when you should use masking when testing ABR, and how do you know how much masking to use? How does the masking noise affect the result?
The easiest way to determine when to mask during ABR testing, is to examine the collected ABR. For air conduction, if the absolute latencies of waves I and V are normal, there is no need to mask. This is because the crossover of the stimulus to the non-test ear takes time. This time delay would in turn delay the absolute latencies of the waveforms. The latencies would also be delayed due to the inter-aural attenuation (IA) of the stimulus. (The IA of the click is 65dB.) Therefore, a normal ABR can only be elicited from the stimulated ear. If, however, the latencies are abnormal, masking is needed.
For bone conduction, masking is not required if wave I is present at normal latencies. This is because wave I is a near-field potential and a normal wave I can only be recorded from the electrode near the stimulated ear. If, however, wave I is absent or delayed for bone conduction, masking is needed.
Most ABR systems offer white noise as a masker which is an appropriate choice because the click is a broadband stimulus. The amount of masking noise needed would be enough to elevate the hearing threshold in the non-test ear so cross over is prevented. For example, if a click is presented at 95 dB nHL via insert earphones and the IA is 65 dB, the signal will be reaching the non-test ear at 30 dB. Enough masking noise (without over-masking) that elevates the threshold in that ear above 30 dB would be adequate—approximately 40-50 dB. Masking noise does not affect the latency or amplitude of the response.
Karen Morris, M.S., CCC/A is the Manager of Audiology Services for GN Otometrics. She received her Master's degree in Audiology from Purdue University. She has worked as a clinical audiologist for over 20 years in a variety of settings and has extensive clinical experience that includes evoked potentials and vestibular assessment. Prior to joining GN Otometrics, Karen worked for Bio-logic for 7 years most recently as the Technical Support Manager and Sales Specialist.
For more information about GN Otometrics, visit http://www.otometrics.com/ or the GN Otometrics web channel on AudiologyOnline.