Question
What SP/AP ratio percentages or parameters are most often used to differentiate between normal and abnormal ECoG tracings? Does this criteria change depending upon the type of electrode used, specifically when using TM electrode?
Answer
Our norms are based on recordings from the TM. In general, an SP/AP amplitude ratio to 90 dB HL click stimuli greater than approximately 50% is considered to be enlarged. At times, we also use the SP/AP area ratio (based on a study by Tibbils and Ferraro) and the amplitude of the SP to tonebursts.
Finally, Margolis et al. like to measure the N1 latency difference between responses measured to condensation vs. rarefaction polarity clicks. A difference of greater than 0.34 msec. is considered a positive finding for endolymphatic hydrops.
Bio:
John A. Ferraro, Ph.D. Professor and Chairman Hearing and Speech Department University of Kansas Medical Center Co-Director KU Intercampus Program in Communicative Disorders Associate Dean for Research, School of Allied Health. Ph.D. in Hearing and Speech Sciences from University of Denver, post-doctoral fellowship in auditory electrophysiology at Northwestern University. Research interests are in the area of clinical applications of Auditory evoked potentials - several publications and presentations in this area, including a recent textbook entitled ''Laboratory Exercises in Auditory Evoked Potentials.''