FREQUENCY SPECIFICITY OF ABR EVALUATED BY TUNING CURVES

Abstract

A tone on tone simultaneous masking paradigm was used to determine tuning curves of ABR both from the normal and hearing-impaired subjects. ABR tuning curves were constructed to define masker intensity that resulted in a 50% reduction in probe elicited wave V amplitude. The frequency specificity of each probe stimulus was evaluated by Q10, low cut-off slope, high cut-off slope and the maximum masker frequency calculated for the tuning curves.
The results were as follows ;
1) Q10, low cut-off slope and high cut-off slope increased gradually with the increase in rise time. However, prolongations of the rise time beyond 3 cycles of probe frequency yielded little improvement in Q10, low cut-off slope and high cut-off slope.
2) Q10, low cut-off slope and high cut-off slope for normal-hearing subjects increased gradu- ally with the increase in stimulus frequency or the decrease in stimulus pressure. Maximum masker frequency of the tuning curves was not always equal to the frequency of probe without the 2-kHz. For the 0. 5, 1kHz probe, the maximum masker frequency of the tuning curves showed higher values than the frequency of probe. For the 4kHz probe, the maximum masker frequency of the tuning curves showed lower values than the frequency of probe. The results indicate that the tone pip stimuli will allow to assess certain conditions of auditory function at different fre- quencies, and they show wider frequencies' spread in the cochlea area near stimulus frequencies.
3) For subject with abrupt high-frequency hearing loss (30dB/oct), a pronounced down-ward shift of maximum masker frequency, down-ward shift of high cut-off slope and up-ward shift of low cut-off slope were found when the probe was placed in the region of elevated threshold. For subject with low-frequency hearing loss (25dB/oct), a pronounced up-ward shift of maximum masker frequency, down-ward shift of low cut-off slope were found.
Maximum masker frequency, low and high cut-off slope of hearing-impaired subjects were not always equal to those of normal subjects for same probe. Especially the value of the maximum masker frequency shifted to the direction in which the most sensitive frequency was observed in audiogram. The threshold of ABR reflected the cochlea function of the most sensitive area near stimulus frequencies. Greatest predictive error was observed in steeply sloping audiograms.