4aPP17. Effect of signal frequency uncertainty for random multi-burst maskers Rong Huang and Virginia M. Richards Department of Psychology, University of Pennsylvania Table I: Average (N=4) d’ values for joint changes in p and delay. Experiment III: Joint effect of changes in p and in target delay The (known) target frequency is 1000 Hz. d’ values are estimated for many p / target delay pairs p delay (ms) Results and Conclusions Thresholds for the detection of a coherent sequence of fixed- frequency tone pips are lower for known target frequencies vs. randomly chosen frequencies. (Figure 3) Thresholds are lowest for the 1000-Hz target, which is the central frequency in the current experiment (Figure 3). Except for the lowest frequency, very small target delay (1 ms or so) increases sensitivity from d’=0.5 to d’=1 (Figure 5). This result is notable because the delay is between (a) a single target tone pip & relatively few masker components (b) the maskers have randomly chosen frequencies across time Sensitivity relies on delay and p in an orderly way – these two parameters trade with one another (Table I). Citations: Kidd,G., Mason,C.R., and Dai, H (1995). Discriminating Coherence in Spectro-Temporal Patterns, J. Acoust. Soc. Am., 97. Acknowledgements: This work was supported by grant DC from the National Institutes of Health. The independent variable is the probability (p) that a tone pip falls in any time-by-frequency cell. As p increases, the task becomes more difficult (e.g., Kidd et al., 1995). Introduction: coherent target target frequency The detectability of a coherent target of sequential tone pips amongst randomly presented tone pips (masker) is estimated. This task may reflect listeners’ ability to segregate a single sound source from competing sources that are random in time and frequency. masker alone no target Time (ms) Frequency (Hz) target + masker Figure 1: Stimuli for two intervals of a trial masker alone small p Time (ms) masker alone large p Sound source segregation is determined by a variety of aspects of the environment, including harmonicity of the target (if complex), onset/offset of the target relative to competing organizations, relatively stable frequency trajectory over time, etc. In the present experiments the effects of (a) Target frequency uncertainty, (b) Relative onset/offset delay (target pips vs. masker pips) (c) joint effects of masker density (p) and onset/offset delay are studied. Figure 2: Increase in p leads to lower d’ Frequency (Hz) Experiment II: Target delay relative to masker Experiment I: Target Frequency Uncertainty Thresholds of p are measured for 5 known target frequencies and a Ran condition in which the target frequency is chosen at random. Four normal-hearing listeners participated. 0.5 The target tone pips are delayed relative to the masker tone pips, yielding both onset and offset delays. The delay required to increased d’ from 0.5 to 1 is estimated. Low MidLow MidMidHigh High Ran Threshold delay in ms (d’ = 1) Target Frequency (Hz) Figure 5: Threshold delay as a function of frequency and frequency uncertainty 240 target Time (ms) Target (delayed) + masker Figure 4: Example of target delay 0 Frequency (Hz) MidHigh Ran Low MidLow Mid High Threshold p (d’ = 1) Target Frequency (Hz) Figure 3: Threshold p as a function of frequency and frequency uncertainty