Frequency Composition of Auditory Middle Responses

Abstract

Power spectral analysis and digital filtration of the auditory middle responses (AMR) to click stimuli in six adult subjects with normal hearing were performed.
The spectral analysis revealed that the dominant frequency components of the AMR were located at 30 to 50Hz. Frequency ranges below 30Hz in the spectrograms probably resulted from the spontaneous activities of the brain. These frequencies often disturbed the detection of the main peaks of AMR, especially Pa and Nb. A small peak at 100-140Hz in the spectrograms was supposedly due to the auditory brain stem responses (ABR) and the earliest part of the AMR, that is, a negative-positive deflection which followed the ABR.
After the low frequency brain activities were completely eliminated with high pass digital filtration at 30Hz, Na-Pa-Nb components and a positive peak with latency of 60-70msec were constantly recognized. On the other hand, when high pass filter was set at 40Hz, the positive peak at 60-70msec disappeared and Nb was followed by two successive positive peaks appearing at 50-55msec and 80-85msec after the onset of stimulus.