Detection of modulated tones in modulated noise by non-human primates

Abstract

In natural environments, many sounds are amplitude-modulated. Amplitude modulation is thought to be a signal that aids auditory object formation. A previous study of the detection of signals in noise found that when tones or noise were amplitude-modulated, the noise was a less effective masker, and detection thresholds for tones in noise were lowered. These results suggest that the detection of modulated signals in modulated noise would be enhanced. This paper describes the results of experiments investigating how detection is modified when both signal and noise were amplitude-modulated. Two monkeys (Macaca mulatta) were trained to detect amplitude-modulated tones in continuous, amplitude-modulated broadband noise. When the phase difference of otherwise similarly amplitude-modulated tones and noise were varied, detection thresholds were highest when the modulations were in phase and lowest when the modulations were anti-phase. When the depth of the modulation of tones or noise was varied, detection thresholds decreased if the modulations were anti-phase. When the modulations were in phase, increasing the depth of tone modulation caused an increase in tone detection thresholds, but increasing depth of noise modulations did not affect tone detection thresholds. Changing the modulation frequency of tone or noise caused changes in threshold that saturated at modulation frequencies higher than 20 Hz; thresholds decreased when the tone and noise modulations were in phase and decreased when they were anti-phase. The relationship between reaction times and tone level were not modified by manipulations to the nature of temporal variations in the signal or noise. The changes in behavioral threshold were consistent with a model where the brain subtracted noise from signal. These results suggest that the parameters of the modulation of signals and maskers heavily influence detection in very predictable ways. These results are consistent with some results in humans and avians and form the baseline for neurophysiological studies of mechanisms of detection in noise.

FIG. 1

Thresholds to tones alone and to tones in noise. Threshold to a 200-ms tone is plotted against the tone frequency for monkeys C (red circles and lines) and D (blue diamonds and lines). Thresholds are shown when tones were presented alone (large symbols, solid lines) and when tones were presented embedded in continuous broadband noise at a 55-dB overall level.

FIG. 2

The effect of changing the phase difference between the amplitude modulation of the signal and noise waveform during a detection task. A Hit rate (probability of lever release) vs. tone level during detection of a 12.8-kHz tone embedded in broadband noise. Tone and noise were both amplitude-modulated at 10 Hz and a depth of 1. Noise level was 55-dB overall level. Performance during phase differences of 0 ° (black), 90 ° (green), 180 ° (blue), and 270 ° (red) are shown. Dashed horizontal lines represent false alarm rate (FA) during the blocks of the phase differences shown and are color-coded. B Behavioral accuracy (probability correct, see the “METHODS” section for calculation) vs. tone level for the exemplar conditions shown in A. The symbols are color-coded as in A. Weibull cumulative distribution function (cdf) fits are shown and are color-coded by phase. The horizontal line shows p(c) = 0.76; the vertical dashed lines show the behavioral thresholds under the phase difference conditions shown. C Reaction time vs. sound level for during the detection of the amplitude-modulated tone. The reaction times are color-coded based on the phase difference between tone and noise modulation as in A and B. The reaction time vs. level relationship was captured by a linear fit (shown color-coded).

FIG. 3

Behavioral performance as a result of varying phase difference between tone and noise modulations. A Threshold as a function of modulation phase difference for the exemplar frequency shown in Figure 1. The circles represent thresholds at the various modulation phase differences, and the dashed red line represents the best fit (sinusoid) to the threshold variations. B Threshold as a function of modulation phase difference for multiple tone frequencies tested. The individual frequencies are color-coded. Fits to individual f _c values are not shown. The dashed line is the best fit to the entire data. C Trends of reaction time as a result of modulation phase difference. The lines connect median thresholds at specific sound levels. Different colors show different sound levels. D Similar to C, but levels are considered relative to threshold.

FIG. 4

The effect of varying depth of modulation of tones (A–C) or noise (D–F) on the detection of modulated tones in modulated noise. Format is similar to Figure 2. A Hit rate vs. tone level during detection of a 8-kHz tone in broadband noise at a 55-dB overall level for two depths of tone modulation: 0.25 (green) and 1.0 (red). Tone and noise were amplitude-modulated at 10 Hz, and the modulations had a phase difference of 0 °. The depth of noise modulation was held at 1. Dashed lines show false alarm rates. B Probability correct vs. tone level for the two depths of tone modulation in A. The psychometric functions (circles) were fit with a Weibull cdf (solid lines). The horizontal line represents the threshold criterion (p(c) = 0.76), and the vertical lines represent threshold under the two conditions. C Reaction time vs. tone level during detection at the two depths of tone modulation. The reaction times (circles) relation to sound level was captured by a linear fit (solid lines). D–F Same as A–C, but hit rates (D), probability correct and thresholds (F) and reaction times (F) vs. tone level when the depth of noise modulation was manipulated. Tone frequency was 25.6 kHz, tone and noise modulation frequencies were set at 10 Hz, and noise level was 55 dB. The depth of tone modulation was held at 1.0, and the modulations had a phase difference of 180 °.

FIG. 5

The effects of varying depths of tone and noise modulations. A Threshold as a function of change in the depth of tone modulation. Thresholds are shown for three different tone frequencies (different colors) at various depths of tone modulation. The relationship was best captured by a linear fit (solid lines). The tone and noise modulations were in phase during these blocks. B Similar to A, but for these blocks, the tone and noise modulations were anti-phase (phase difference = 180 °). C Threshold as a function of depth of noise modulation. Format is the same as in A and B. For these blocks, the tone and noise modulations were in phase. D Similar to C, but for these blocks, tone and noise modulations were anti-phase.

FIG. 6

The effects of varying modulation frequency of tones (A–C) or noise (D–F) on detection of modulated tone in modulated noise. Format of the figures are same as in Figure 4. A–C Hit rate vs. tone level (A), psychometric functions, Weibull cdf fits and detection thresholds (B), and reaction times vs. tone level (C) during detection of modulated tone in modulated noise. Tone frequency was 3.2 kHz; noise level was 55 dB, noise modulation frequency was 10 Hz, tone and noise modulation depths were 1.0 each, and the modulations were in phase at tone onset. Data is shown for three tone modulation frequencies—10 Hz (blue), 20 Hz (green), and 40 Hz (red). D–F Similar to A–C, but as noise modulation frequencies were changed. Tone frequency was 25.6 kHz, noise level 55 dB, frequency of tone modulation was10 Hz, depth of modulation of tone and noise 1.0, and the tone and noise modulations were in phase at tone onset.

FIG. 7

The effects of varying the frequency of tone or noise modulations. A Threshold as a function of tone modulation frequency. Thresholds for detection of tones of varying carrier frequencies (fc, different colors and symbols; legend with panel B) in modulated noise, when tone modulation frequencies were changed. The tone and noise modulation frequencies were in phase during these blocks. B Similar to A, but shows detection thresholds for tones having same fc values as in A (different colors) in modulated noise when the noise modulation frequencies were varied. C Threshold as a function of tone modulation frequency when the tone and noise modulations were in phase (red) or anti-phase (blue) at tone onset for two subjects (solid and dashed lines, respectively). The tone frequency was 3.2 kHz. D Similar to C, but for variations in frequency of noise modulation.

FIG. 8

Results of a simple energy difference model that predicts variation in behavioral threshold as a result of the parameter manipulations for the studies presented here. Parameters match those used in the experiments. A Effect of varying phase difference between tone and noise modulations. Circles show thresholds, solid line represents best fit to the data. B Effect of varying frequency of noise amplitude modulation. Thresholds are shown when tone and noise modulation were in phase (red triangles) and anti-phase (blue diamonds) at tone onset. Dashed and dotted lines show best fit to the symbols (exponential functions). C Similar to B, but for variations in tone amplitude modulation frequency. Inset. The results of behavioral experiments in two monkeys testing the effects of frequency of tone modulation between 10 and 20 Hz shows an undershoot in threshold to match model predictions when tone and noise modulations were in phase at tone onset. D Effects of varying depth of noise modulation. Format is similar to C. E Similar to D, but parameter varied was depth of tone modulation.