Moore B C, Alcántara J I
Department of Experimental Psychology, University of Cambridge, United Kingdom.
J Acoust Soc Am. 1996 Apr;99(4 Pt 1):2332-43. doi: 10.1121/1.415420.
This study investigated the extent to which flat-spectrum harmonic complexes could be identified as one of six vowels when three pairs of successive harmonics, located at the first, second, and third formant frequency values, were amplitude modulated. In experiment 1, the amplitude modulation (AM) rate was at or close to 10 Hz. In condition 1, all components were added in cosine phase, and the 10-Hz AM was in phase for all "formants." Performance improved monotonically with increasing modulation index, m. In condition 2, m was fixed at 0.5 and the level of each background harmonic was varied randomly (roved) from stimulus to stimulus. Even a rove range of only +/- 2 dB reduced scores considerably. Condition 3 was like condition 1, but with components added in random phase. Performance was very poor for all modulation indices. This suggests that subjects were unable to use momentary differences in level between formant and background harmonics, and supports the idea that, for cosine-phase stimuli, they were using information from the low-amplitude portions ("valleys") of the cochlea-filtered waveforms. In further conditions, the components were added in cosine phase and the AM had a different phase and/or different rate (10, 16, and 24 Hz) on the different formants. Scores were very similar to those obtained when the AM was identical for all formants. In experiment 2, the AM rate was at or close to 2 Hz. When all formants were modulated in phase at 2 Hz, very good performance was found for components added in cosine phase, and performance was essentially unaffected by making the AM different in rate and/or phase across formants. When the components were added in random phase, performance was well above chance when the formants were modulated in-phase at 2-Hz, but worsened markedly when the modulation differed in rate and/or phase across formants. Randomizing the level of each background harmonic caused performance to deteriorate and to become similar for cosine-phase and random-phase stimuli. Performance deteriorated further when the AM differed in phase across formants. The results suggest that, for a 2-Hz modulation rate, and when information from the valleys is not available, performance depends on momentary increases in level of the formant harmonics relative to the background.
本研究调查了当位于第一、第二和第三共振峰频率值处的三对连续谐波进行幅度调制时，平坦频谱谐波复合体在多大程度上能够被识别为六个元音之一。在实验1中，幅度调制（AM）速率为10赫兹或接近10赫兹。在条件1中，所有分量以余弦相位相加，并且10赫兹的AM对于所有“共振峰”同相。随着调制指数m的增加，性能单调提高。在条件2中，m固定为0.5，并且每个背景谐波的电平在不同刺激之间随机变化（游动）。即使仅±2分贝的游动范围也会使分数大幅降低。条件3与条件1类似，但分量以随机相位相加。对于所有调制指数，性能都非常差。这表明受试者无法利用共振峰谐波与背景谐波之间电平的瞬时差异，并支持这样一种观点，即对于余弦相位刺激，他们使用的是来自耳蜗滤波波形低幅度部分（“波谷”）的信息。在进一步的条件下，分量以余弦相位相加，并且AM在不同共振峰上具有不同的相位和/或不同的速率（10、16和24赫兹）。分数与所有共振峰的AM相同时获得的分数非常相似。在实验2中，AM速率为2赫兹或接近2赫兹。当所有共振峰以2赫兹同相调制时，对于以余弦相位相加的分量发现了非常好的性能，并且通过使不同共振峰之间的AM在速率和/或相位上不同，性能基本不受影响。当分量以随机相位相加时，当共振峰以2赫兹同相调制时，性能远高于随机水平，但当不同共振峰之间的调制在速率和/或相位上不同时，性能明显恶化。使每个背景谐波的电平随机化导致性能下降，并且对于余弦相位和随机相位刺激变得相似。当不同共振峰之间的AM在相位上不同时，性能进一步恶化。结果表明，对于2赫兹的调制速率，并且当无法获得来自波谷的信息时，性能取决于共振峰谐波相对于背景电平的瞬时增加。
