Auditory brain stem response and middle latency response in hearing test. (II) Investigations in hearing-impaired adult subjects (author's transl)

Abstract

The auditory brain stem response (BSR) and middle latency response (MLR) were recorded in 22 adult subjects with normal hearing and the possibility of their clinical application in objective audiometry was studied. Tone pips at 250, 500, 1000, 2000 and 4000 Hz with 2-cycle rise-decay time and one-cycle peak were delivered to the subjects with the intensity of 0, 10, 20 and 30 dB above the threshold. It has been known that BSR is less sensetive to low frequency stimuli below 500 Hz. Most investigators applied a high-pass filter with cut-off frequencies of 150-500 Hz to the recording system. The high-pass filter reduces, or completely eliminates the low frequency components of the BSR and makes the judgemect of the waves rather difficult. For the purpose of investigating the frequency characteristics, a high-pass filter was not used in the present study. Our results on the mean latency suggested that the BSR was evoked through the pertinent regions on the basilar membrane as a response to 500 Hz tone pips as sensitively as to the stimuli of higher frequencies. In addition, it was clearly demonstrated that no significant difference was found in the response detectability of Po, Na and Pa between four different frequencies in the range from 500 to 4000 Hz and the threshold for these components was 10 to 20 dBSL. For 250 Hz tone pips, definite responses were obtained, with the threshold ranging from 20 to 30 dBSL. The question whether these responses are definitely concerned with the constant 250 Hz tone pip stimulation requires further investigations. These results indicate that BSR (Po) and MLR (Na, Pa) would be clinically useful as a reliable response index for objective threshold audiometry.