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Loss of function of inner hair cells, primary auditory neurons and synapses between inner hair cells and neurons can all lead to a reduced flow of information in the auditory nerve and to less precise coding of the envelopes of sounds. The Envelope Regularity Discrimination (ERD) test was designed to provide a measure of the precision of envelope coding in the auditory system that, unlike amplitude-modulation (AM) detection, is not affected by the presence or absence of loudness recruitment. The ERD test requires discrimination of regular sinusoidal AM with modulation depth m from AM that is irregular in rate and depth but has the same mean m. The amount of irregularity, quantified by the irregularity index, II, is varied to determine the threshold value of II. A broadband threshold-equalizing noise is used to limit the range of characteristic frequencies of the neurons that respond to the signal. Loudness recruitment may magnify the perceived AM depth, but this effect will be similar for the two intervals of the forced-choice trial and should have little or no influence on performance of the task. To confirm that the baseline modulation depth had little effect, performance was measured for normal-hearing subjects using an AM rate of 8 Hz, a carrier frequency of 4 kHz and clearly-audible modulation depths ranging from 0.2 to 0.5. The carrier level was 80 dB SPL. This relatively high level was chosen since damage to synapses appears mainly to affect the operation of neurons with high thresholds and wide dynamic ranges. The effect of modulation depth was very small. However, marked individual differences were found. The individual differences were explored using a baseline modulation depth of 0.3. Although practice effects were observed, marked differences persisted with extended practice. The origin of the individual differences is being explored.
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