Discrimination of amplitude-modulation phase spectrum

Listeners were asked to discriminate between two amplitude-modulation functions imposed on white noise and consisting of the sum of two sinusoids. The frequency ratio of the sinusoids constituting each function was 2 or 3. In one function, the sinusoids had a constant relative phase. In the other function, their phase relation was continuously and cyclically changing, at a slow rate. For all listeners, the two functions with a frequency ratio of 2 were easily discriminated. However, discrimination was impossible when the frequency ratio was 3. Simulations were performed using an envelope-detector model and various decision statistics. The max/min statistic predicted discrimination above chance level when the frequency ratio was 3. It seems, therefore, that listeners are unable to use this statistic. In contrast, the crest factor and skewness of the envelope accounted well for the discrimination data.