Middle latency response to a 500-Hz tone pip in normal-hearing and in hearing-impaired subjects

Measurement of middle latency responses (MLR) appears to be one of the most useful ways of determining low-frequency auditory threshold. It is known that analog filtering can drastically affect MLR and since most laboratories use different recording characteristics, we carried out these measurements with a number of filter configurations. Ten subjects with normal hearing sensitivity were tested. The stimuli were 500-Hz tone pips (alternating in polarity) with a 4-ms rise-fall time and a plateau of 2 ms, presented at the rate of 9.3/s. The MLR were recorded with filter settings of 10-100, 10-250, 10-1,500, 10-3,000, 30-100 and 30-250 Hz at 12 dB per octave roll-off with a time base of 100 ms. MLR threshold was found between 8.0 and 11.5 dB nHL. The Wilcoxon statistical test showed that mean MLR thresholds did not differ significantly at the various bandpass configurations. An additional objective of this study was to compare hearing sensitivity based on MLR threshold to tone pips at 500 Hz and behavioural results obtained by conventional pure-tone audiometry. Although the correlation coefficient between the behavioural and electrophysiological measurements was statistically significant (r = 0.85), further studies are required in order to determine the real magnitude of the predicted errors obtained, before this electrophysiological measure can be applied clinically.