Effect of high-pass filter on auditory brain stem responses to tone pips.

The effect of high-pass filters on the vertex-positive brain stem response to tone pips at 500, 1000, 2000 and 4000 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 2000 Hz were only 0.2, 2.9 and 5.4 dB respectively. The responses at 4000 Hz were similar to those at 2000 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 2000 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.