Effects of spectral contrast on perceptual compensation for spectral-envelope distortion

Features in a sound's spectral envelope are important for perceptual identification but they are likely to be accompanied by spurious features due to distortion by the transmission channel between source and listener. Previous experiments have demonstrated that there is perceptual compensation for this distortion, and the present experiments ask whether the compensation involves a separation of spurious and salient features. Listeners identified words containing a vowel test sound in an /aept/ to /ppt/ continuum, with a carrier phrase before each word. Effects of transmission channels were simulated by filtering the carrier and the /pt/ following the test sound. Filters were pairs with frequency responses that were the difference of the spectral envelopes from the end-point vowels. Contrasts were altered by multiplying decibel values of the carrier filter's frequency response or the test sound's spectral envelope by a positive number. This keeps features such as peaks at the same frequencies but changes the difference in level between peaks and valleys. When the contrasts of the carrier filters and test sound were the same, the continuum's phoneme boundary was shifted in a manner consistent with a perceptual compensation for the filters that affects the neighboring test sound. However, this shift decreased when the carrier-filter's contrast was less than that of the test sound, and increased slightly when the test-sound's contrast was less than the carrier-filter's contrast. Therefore, the amount of compensation increases with the amount of distortion, even when spectral features such as peaks are kept at the same frequencies. So compensation seems to occur before any perceptual extraction of these features.