Distortion of the middle latency auditory response produced by analog filtering.

The effect of analog filtering on the middle latency auditory response (MLAR) was investigated. Substantial waveform distortions and latency shifts were found when using similar filter settings as reported in the literature. The waveform changes amounted to complete polarity reversal of the Na-Pa-Nb pattern due to the high-pass cut-off with a steep slope characteristic. The analog filters were simulated by recursive digital filters and compared with symmetrical digital filters having the same cut-off frequency and slope. MLAR waveform and latencies were scarcely affected by the digital zero-phase shift filters. In contrast, the oscillations in the step function response of the analog filter elucidated how early activity of the MLAR is folded onto later components, leading to a much larger late activity than present physiologically. Since physiological interpretations of the MLAR ought to rely on unbiased recordings, minimal usage of analog filtering is recommended.