Question
How or when can we as clinical audiologists add cortical Auditory Evoked Potentials (AEP) testing to our test batteries? Is it something that is or will become available for clinical use?
Answer
Clinical audiologists already using the ABR can probably easily record later auditory evoked potentials by opening the response window on their equipment. However, there are several setting differences between recording the ABR and recording later cortical potentials. The first difference is in the number of electrodes needed for the recording of the later potentials. In research a 32 or 64 electrode caps or nets are used to record the later AEPs, in the clinic fewer single electrodes can be used at midline and on each side of the head. But, in addition to a ground electrode and a reference electrode, eye monitoring electrodes have to be used because eye blinks significantly affects the morphology of the late responses and they need to be rejected before the signals are averaged. Secondly, because they are larger in amplitude than the ABR response, the late responses do not need to be amplified as much, they are typically amplified 1000 times after being converted from analog to digital signals. And finally to minimize noise in the recording, responses are also filtered, but filtering for cortical AEP is very different from filtering for the ABR, usually a high pass filter set at 100 Hz is used for the recording of the later potentials. (See Luck 2005 for an introduction to AEP technique). Another difference involves the mode of stimulation of the later potentials. There are three major cortical AEP classified by latency from the P1-N1-P2 complex with latencies between 80ms and 200ms to the late potentials such as the Mismatch Negativity (MMN) and P300 with latencies between 150ms and 1000ms (See Stapells, 2002 for more details). The P1-N1-P2 response is an obligatory cortical AEP, passive recording of this response can be done (typically subjects are watching a silent video during testing). This response is always present in a healthy auditory system when subjects are awake (with some differences in morphology in children). It can be elicited by the onset of a sound such as a click or a tone, or it can be elicited by a change in a stimulus. The other two cortical AEPs, the MMN and the P300, are obtained with oddball paradigm presentations: where a standard stimulus is presented most of the time and a deviant stimulus is presented occasionally (usually 10-20% of the time). The MMN can be recorded in passive listening conditions;this response is automatic but it is not always present (see Naatanen et al. 2007 for a review). The other potential, the P300 is also elicited using an oddball paradigm but in this case the recording is not passive;subjects' participation is required (typically clients are asked to count the deviants).
Some clinical audiologists already use the P300 in their Auditory Processing test battery or the obligatory P1-N1-P2 in the assessment of hearing sensitivity. Cortical AEP potentials will probably become more common in clinical practice in the next few years when more information about specificity and sensitivity of these measures is available. Advantages of this type of recordings include very good temporal resolution and information about the auditory system at a higher level than the ABR. However, because these potentials are generated at such a high level they are influenced by other factors than auditory factors such as attention and cognitive factors. In addition the between subjects variability is high. As a result clinical interpretations are still difficult and at this time no clinical norms are commonly used. However, the study of AEP in clinical populations is a very active area of research where results might have direct implications on the clinical assessment of the auditory system, in the assessment of Auditory Processing Disorders (APD) in particular.
References
Luck, S. J. (2005). An Introduction to the event-related potential technique. Cambridge, MA: The MIT Press.
Naatanen, R., Paavilainen, P., Rinne, T., & Alho, K. (2007). The mismatch negativity (MMN) in research of central auditory processing : A review. Clinical Neurophysiology, 118, 2544-2590.
Stapells, D. (2002). Cortical event-related potentials to auditory stimuli. In J. Katz (Ed.), Handbook of clinical audiology, fifth edition (pp. 378-406). PA: Lippincott, Williams & Wilkins.
Christine Rota-Donahue teaches Audiology and Hearing Science at Lehman College of the City University of New York and is pursuing her Doctorate in Speech-Language- Hearing Sciences at the CUNY Graduate Center. After more than 20 years in clinical practice in both Audiology and Speech-Language Pathology, she is currently studying the use of electrophysiological measures in the diagnosis of pediatric Auditory Processing Disorder.
Editor's note: For more information on this topic, please see Ms. Rota-Donahue's article on the use of the P1-N1-P2-N2 complex to measure frequency discrimination in children. Click here to view this article or here to register for the CEU text course.
In addition, refer to Dr. Lightfoot's recent article on the use of the N1-P2 cortical auditory evoked potential to estimate hearing threshold. Click here to view the article or here to register for the CEU text course.
