Investigating possible mechanisms behind the effect of threshold fine structure on amplitude modulation perception

Detection thresholds for sinusoidal amplitude modulation at low levels are higher (worse) when the carrier of the signal falls in a region of high pure-tone sensitivity (a minimum of the fine structure of the threshold in quiet) than when it falls at a fine-structure maximum. This study explores possible mechanisms behind this phenomenon by measuring modulation detection thresholds as a function of modulation frequency (experiment 1) and of carrier level for tonal carriers (experiment 2) and for 32-Hz wide noise carriers (experiment 3). The carriers could either fall at a fine-structure minimum, a fine-structure maximum, or in a region without fine structure. Modulation frequencies varied between 8 Hz and one fine-structure cycle, and carrier levels varied between 7.5 and 37.5 dB sensation levels. A large part of the results can be explained by assuming a reduction in effective modulation depth by spontaneous otoacoustic emissions-or more generally cochlear resonances-that synchronize to the carrier at fine-structure minima. Beating between cochlear resonances and the stimulus ("monaural diplacusis") may hamper the detection task, but this cannot account for the whole effect.