The Aural Discrimination of Simultaneous Changes in Amplitude and Frequency

Abstract

The present research had two aims. First, to investigate the ability of human observers to discriminate simultaneous changes in amplitude and frequency. Second, to find out whether this ability was a function of the interaural noise correlation. Two experiments were run using a two-alternative, forced-choice procedure with the tonal signals presented binaurally in phase. There were five possible increments in amplitude, and five in frequency. In the first experiment there were four separately run conditions. There was an amplitude discrimination task (frequency held constant), and a frequency discrimination task (amplitude held constant). Signals where only one of the signal cues was varied were called single-cue signals. The other two conditions used different subsets of combined-cue signals, where amplitude and frequency were simultaneously incremented. Observers always knew which signal condition was being used. Signals were presented in a continuous noise masker where the interaural noise correlation was one of +1.00 (NOSO), 0.00 (NuSO), or -1.00 (NΠSO). Each interaural condition was used with each of the four signal conditions giving 12 experimental conditions in all. The results showed that once the masking level differences created by the NΠSO and NuSO conditions were compensated for, there were no other effects of varying the interaural noise correlation on the discrimination of simultaneous changes in amplitude and frequency. The results also showed that performance levels for the combined-cue signals were frequently lower than those obtained for the best comparable single-cue signals. One factor associated with performance level was the size of the absolute difference between the discrimination probabilities of the two signal cues. The larger this difference, the less likely was there to be any gain in performance. The results of the first experiment were surprising in that performance decrements occurred for so many combined-cue signals. A second experiment was run partially to verify this result, and also to ascertain whether the type of signal ensemble used in the first study had affected the outcome. All 25 possible combined-cue signals were presented randomly over a 10,000 trial sequence, with the continuous noise masker in phase. A comparison of the results from the two experiments showed that signal subset size had no effect on performance levels, and that decrements in performance again occurred for many of the combined-cue signals. The data from both experiments provided an opportunity to test the predictions of the integration model and the decision threshold model, two decision models that have been used to predict performance for the discrimination of simultaneous changes in amplitude and frequency. Reasons for the failure of both models to predict the present results were discussed. The overall pattern of results was discussed in terms of a noisy decision process. The size of the absolute difference between the discrimination probabilities of the two signal cues, and the asymmetry in these discrimination probabilities in favour of the amplitude cue were identified as two major factors contributing to the decision noise.