Question
Would MMN be a useful tool for diagnosing CAPD in children in the age group who cannot perform on dichotic digits test?
Answer
Although the MMN does appear to have potential for diagnosis of CAPD in hard-to-test populations, including young children, the jury is still out on that issue.
For readers unfamiliar with the MMN, it stands for ''Mismatch Negativity Response'' and is a difference-wave response elicited primarily by neurons in the thalamus and cortex to small differences in acoustic stimuli. It is pre-attentive and does not require any active participation on the part of the listener. Further, it can be elicited by very minute differences in speech stimuli. Research from Nina Kraus's Auditory Neuroscience Laboratory at Northwestern University has indicated that some children with auditory-based learning disorders have difficulty behaviorally discriminating speech contrasts that have rapidly changing spectro-acoustic parameters - such as /da/ versus /ga/ - and also exhibit absent MMNs to this contrast. Conversely, these same children exhibit both behavioral and neurophysiologic responses to speech contrasts that do not require rapid spectro-temporal acoustic processing, such as /ba/ versus /wa/. As such, the MMN may illuminate fundamental fine-grained auditory discrimination difficulties in some children without the need for active participation.
However, many recent studies in the literature have found that the MMN may be difficult to obtain even in normal subjects. Further, there have been reports that it is frequently absent even in normals, leading to difficulties interpreting the response. Moreover, most electrophysiologic labs do not, at present, have MMN-to-speech capability. Because of these concerns, the clinical use of the MMN is under much scrutiny at this time.
Finally, the Dichotic Digits and similar tests assess far different processes than the fine-grained acoustic discrimination represented by the MMN. Thus, even if a child exhibited normal MMNs to speech contrasts, that would not rule out a CAPD affecting other types of skills not represented by MMN or other electrophysiologic measures.
For this reason, although I do use electrophysiologic measures (including MLR and cortical potentials to speech and nonspeech stimuli) with young children, I still feel that a multidisciplinary approach in which patterns of difficulties and behaviors across a variety of learning, language, and communication assessments is the best way to determine likelihood of a CAPD in a young child, as well as assisting in hypothesizing as to the type of CAPD exhibited and what deficit-specific management measures should be taken.
BIO: Teri James Bellis, Ph.D. is a professor of audiology at the University of South Dakota and author of the book ''When the Brain Can't Hear: Unraveling the Mystery of Auditory Processing Disorder'' (Pocket Books/Simon & Schuster, 2002). An expert in the field of CAPD, Dr. Bellis publishes and lectures nationally and internationally on the subject. Currently, she is revising her text ''Assessment and Management of CAPD in the Educational Setting'' (Singular Publishing, 1996) for re-release in February of 2003.