Effect of High-Pass Filter on Auditory Brain Stem Responses to Tone Pips

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Abstract
The effect of high-pass filters on the vertex-positive brain stem response to tone pips at 500, 1 000, 2 000 and 4 000 Hz was investigated in normal adults. The tape-recorded responses were averaged through high pass filters with 3 dB attenuation at 50, 100 and 200 Hz and with a slope of 6 dB/oct, and their amplitudes were compared with those of the averaged responses without filtering. These 3 filters reduced the amplitudes of the responses to 500-Hz tone pips by 3.3, 7.1 and 10.6 dB respectively. On the other hand, the mean attenuations caused by the same series of filters on the response amplitudes at 2 000 Hz were only 0.2, 2.9 and 5.4 dB respectively. The responses at 4 000 Hz were similar to those at 2 000 Hz. This distinct difference in the effect of the high-pass filters suggests that the dominant frequency component of the brain stem response at 500 Hz is significantly lower than that at 2 000 Hz and above. The results indicate that the use of a high-pass filter with cutoff frequency over 50 Hz is not recommended for recording the slow positive deflection of the brain stem response to low frequency tones.

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