The detection of sinusoidal modulation is commonly used for assessing the auditory perception of temporal, spectral, and spectro-temporal acoustic features. For temporal (amplitude) modulation, the sinusoidal modulator usually is expressed on a linear amplitude scale. For spectral modulation, the sinusoidal modulator has been specified on a linear amplitude scale, consistent with temporal modulation, or on a logarithmic amplitude scale, with the notion of approximating a sinusoidal excitation pattern. The definition of modulation depth depends on the measurement points (i.e., midpoint to peak or peak to peak) and order of operations when expressing depth in dB. Such differences can make it difficult to compare results for similar tasks among investigations. Here we quantify differences among methods and provide a complete matrix for translating among methods. Spectral modulation detection was measured for 9 normal-hearing listeners in ten conditions (linear vs. logarithmic shape at 0.5, 1, 2, 4, and 8 cycles/octave). Peak-to-peak values of the modulation envelope were equalized, thus only modulation waveform shape differed. Noise carriers had a passband from 400 to 3200 Hz. Thresholds revealed statistically significant effects of both spectral shape and spectral modulation frequency. Several methods for expressing threshold modulation depth were compared to highlight differences among the methods.

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Frequency modulation (FM) detection at low modulation frequencies can be used as an index of temporal fine structure processing and is sensitive to changes in age- and hearing-related deficits associated with impaired speech intelligibility in noise. Monaural FM detection has been shown to improve with increased signal duration, yet it is unclear whether or not that increase depends on the actual duration or the number of modulation cycles. The dependence of dichotic FM thresholds on stimulus duration remains unexplored. Experiment I confirmed the hypotheses that the improvement in FM detection with duration is predicted by the independent-samples model in the monaural, but not dichotic condition. Experiment II established that monaural FM detection is dependent on the number of modulation cycles and not duration per se. The results of Experiment III supported the hypothesis that dichotic FM detection improved less with duration due to the onset-biased use of interaural time difference cues and highlight several unknowns related to binaural auditory processing. Despite nearly universal use of long-duration stimuli in prior FM detection experiments, the current results indicate that very brief stimulus durations may be used without compromising validity or measurement error.

Frequency modulation (FM) detection at low modulation frequencies is commonly used as an index of temporal fine-structure processing. The present study evaluated the rate of improvement in monaural and dichotic FM across a range of test parameters. In experiment I, dichotic and monaural FM detection was measured as a function of duration and modulator starting phase. Dichotic FM thresholds were lower than monaural FM thresholds and the modulator starting phase had no effect on detection. Experiment II measured monaural FM detection for signals that differed in modulation rate and duration such that the improvement with duration in seconds (carrier) or cycles (modulator) was compared. Monaural FM detection improved monotonically with the number of modulation cycles, suggesting that the modulator is extracted prior to detection. Experiment III measured dichotic FM detection for shorter signal durations to test the hypothesis that dichotic FM relies primarily on the signal onset. The rate of improvement decreased as duration increased, which is consistent with the use of primarily onset cues for the detection of dichotic FM. These results establish that improvement with duration occurs as a function of the modulation cycles at a rate consistent with the independent-samples model for monaural FM, but later cycles contribute less to detection in dichotic FM.

Listeners use the spatial location or change in spatial location of coherent acoustic cues to aid in auditory object formation. From stimulus-evoked onset responses in normal-hearing listeners using electroencephalography (EEG), we have previously shown measurable tuning to stimuli changing location in quiet, revealing a potential window into the cortical representations of auditory scene analysis. These earlier studies used non-fluctuating, spectrally narrow stimuli, so it was still unknown whether previous observations would translate to speech stimuli, and whether responses would be preserved for stimuli in the presence of background maskers. To examine the effects that selective auditory attention and interferers have on object formation, we measured cortical responses to speech changing location in the free field with and without background babble (+6 dB SNR) during both passive and active conditions. Active conditions required listeners to respond to the onset of the speech stream when it occurred at a new location, explicitly indicating ‘yes’ or ‘no’ to whether the stimulus occurred at a block-specific location either 30 degrees to the left or right of midline. In the aggregate, results show similar evoked responses to speech stimuli changing location in quiet compared to babble background. However, the effect of the two background environments diverges somewhat when considering the magnitude and direction of the location change and where the subject was attending. In quiet, attention to the right hemifield appeared to evoke a stronger response than attention to the left hemifield when speech shifted in the rightward direction. No such difference was found in babble conditions. Therefore, consistent with challenges associated with cocktail party listening, directed spatial attention could be compromised in the presence of stimulus noise and likely leads to poorer use of spatial cues in auditory streaming.