Detection of mixed modulation using correlated and uncorrelated noise modulators.

This article is concerned with the mechanisms underlying the detection of amplitude modulation (AM), frequency modulation (FM), and mixed modulation (MM), i.e., simultaneously occurring AM and FM. In a previous study [B. C. J. Moore and A. Sek, J. Acoust. Soc. Am. 92, 3119-3131 (1992)], psychometric functions were measured for the detection of AM alone and FM alone, using a 10-Hz sinusoidal modulator and a 1-kHz carrier frequency. Detectability was then measured for combined AM and FM, with modulation depths selected so that each type of modulation would be equally detectable if presented alone. The detectability of the MM was better than would be predicted if the two types of modulation were coded completely independently. This study examined the possibility that the good detectability of MM was caused by the fact that the AM and the FM were correlated, so that each was predictable from the other. The design was similar to that of our earlier study, but the 10-Hz sinusoidal modulator was replaced by a narrow-band noise modulator. In the MM conditions, the modulators for AM and FM were either strongly positively correlated or essentially uncorrelated. In experiment 1, the waveforms of the noise modulators were fixed throughout the experiment (frozen noise). In experiment 2, the waveforms of the noise modulators were chosen independently for each trial. In both experiments, for both correlated and uncorrelated modulators, the detectability of the MM was better than would be predicted if the two types of modulation were coded completely independently.(ABSTRACT TRUNCATED AT 250 WORDS)