SENSITIVITY TO CHANGES IN OVERALL LEVEL AND SPECTRAL SHAPE - AN EVALUATION OF A CHANNEL MODEL

Two experiments involving level and spectral shape discrimination whic h test an optimal channel model developed by Durlach et al. [J. Acoust . Soc. Am. 80, 63-72 (1986)] are described. The model specifies how th e auditory system compares and/or combines intensity information in di fferent frequency channels. In the first experiment, psychometric func tions were obtained for the discrimination of changes in level and dis crimination of changes in spectral shape for an eight-tone complex sou nd. A variety of different base spectral shapes were tested. In some c onditions, level randomization was introduced to reduce the reliabilit y of across-interval changes in level. Increasing the amount of level variation degraded performance for the level discrimination task but h ad no effect on the shape discrimination task. In all conditions, sens itivity to changes in spectral shape was superior to sensitivity to ch anges in level. Consequently, two models of central noise are evaluate d in an attempt to explain these results; one in which central noise a cts prior to the formation of the likelihood ratio and one in which ce ntral noise degrades the likelihood ratio. The former model is more su ccessful in accounting for the data. In a second experiment, the detec tability of a level increment to one component of a multitone complex was measured. The frequency content of the complex was varied by syste matically removing six components from a 23-component complex. Thresho lds were measured for increments at three different signal frequencies . A common trend in the data was that when there was a spectral gap di rectly above the signal frequency, thresholds were lowest. This result differs from the predictions of a simple channel model, and contrasts with results presented by Green and Berg [Q. J. Exp. Psychol. 43A, 44 9-458 (1991)]. (C) 1997 Acoustical Society of America.