Characterizing frequency selectivity for envelope fluctuations

Three experimental paradigms were used to specify the auditory system's fre quency selectivity for amplitude modulation (AM). In the first experiment, masked-threshold patterns were obtained for signal-modulation frequencies o f 4, 16, 64, and 256 Hz in the presence of a half-octave-wide modulation ma sker, both applied to the same noise carrier with a bandwidth ranging from 1 to 4 kHz. In the second experiment, psychophysical tuning curves (PTCs) w ere obtained for signal-modulation frequencies of 16 and 64 Hz imposed on a noise carrier as in the first experiment. In the third experiment, masked thresholds for signal-modulation frequencies of 8, 16, 32, and 64 Hz were o btained according to the "classical" band-widening paradigm, where the band width of the modulation masker ranged from 1/8 to 4 octaves, geometrically centered on the signal frequency. The first two experiments allowed a direc t derivation of the shape of the modulation filters while the latter paradi gm only provided an indirect estimate of the filter bandwidth. Thresholds f rom the experiments were predicted on the basis of an envelope power-spectr um model (EPSM) which integrates the envelope power of the modulation maske r in the passband of a modulation filter tuned to the signal-modulation fre quency. The Q-value of second-order bandpass modulation filters was fitted to the masking patterns from the first experiment using a least-squares alg orithm. Q-values of about 1 for frequencies up to 64 Hz suggest an even wea ker selectivity for modulation than assumed in earlier studies. The same mo del also accounted reasonably well for the shape of the temporal modulation transfer function (TMTF) obtained for carrier bandwidths in the range from 1 to 6000 Hz. Peripheral filtering and effects of peripheral compression w ere also investigated using a multi-channel version of the model. Waveform compression did not influence the simulated results. Peripheral bandpass fi ltering only influenced thresholds for high modulation frequencies when sig nal information was strongly attenuated by the transfer function of the per ipheral filters. (C) 2000 Acoustical Society of America [S0001-4966(00)0350 9-8].