Presence Of Masker Research Articles

Auditory brainstem responses: masking related changes in wave VI.

Auditory brainstem responses (ABRs) were recorded in a group of nine normally-hearing subjects. Wide-band clicks were delivered at 70, 80 and 90 dB SL, both with and without 50 dB SL of contralateral masking. The amplitude of wave VI was significantly reduced in the presence of masking for the 90 dB SL stimulus but there was no effect at the lower stimulus levels. A possible explanation for this is that the observed response, to a 90 dB SL stimulus without masking, is a summation of the ipsilateral wave VI and a wave V elicited from the contralateral ear. Supporting evidence is provided and the implications of this result are discussed.

A comparison of forward masking for the monoaural and binaural hearing systems

Forward masking functions were obtained using a 500‐Hz tonal musket of 100‐ms duration, followed by a 20‐ms sinusoidal test probe at the same frequency. A continuous, broadband noise was also present during the experiment. Noise levels were chosen so that for long ISI's, masking functions for both the monaural and binaural system conditions would asymptote at a common level of signal detectability. In the monaural system condition, the masker, signal, and noise were all presented to one ear alone, while for the binaural system conditions, the signal was antiphasic, the noise homophasic, and the tonal masker was either homophasic or antiphasic. Results showed the mortaural system condition to exhibit the fastest recovery from masking. For the binaural system conditions, the homophasic tonal masker was more effective than the antiphasic masker when masker and signal were temporally nonoverlapping, but the reverse was found for simultaneous presentation of signal and masker. These data are interpreted as demonstrating that the binaural system has a longer integration time than does the monaural system. [Work supported by the Neurobehavioral Sciences Program, University of Iowa.]

Detectability of a pulsed tone in the presence of a masker with time-varying interaural correlation

Detectability of a filtered probe tone (250, 500, or 1000 Hz) was measured in the presence of a narrow-band Gaussian masker centered at the signal frequency. The signal was interaurally phase-reversed (Spi), and the masker's interaural correlation varied sinusoidally between +1.00 (NO) and -1.00 (Npi) at a varaible rate (fm = 0--4 Hz). The signal was presented at various points on the masker's modulation cycle. For 0-Hz modulation (fixed interaural correlation) signal threshold decreased monotonically as the masker's interaural correlation was changed from -1.00 to +1.00 (by a total of about 20, 16, and 8 dB, respectively, for 250-, 500-, and 1000-Hz signals). For fm greater than 0 the function relating signal threshold to the masker's interaural correlation at the moment of signal presentation became progressively flatter with increasing fm for all signal frequencies. For fm = 4 Hz the function was flat; there was no measurable effect of masker interaural correlation on signal detectability. Estimates of minimum binaural integration time based on these data ranged from 44--243 ms, supporting previous studies which have noted the binaural system's relative insensitivity to dynamic stimulation. Additionally, the estimated time constants were approximately twice as large at 250 Hz as at 500 Hz, indicating observers could follow binaural fluctuations better at 500 Hz. The time-constant estimates at 1000 Hz were not suggiciently reliable to permit comparisons with the lower-frequency data.

Difference Limen for Frequency in the Presence of Masking

The monaural differential threshold for frequency at 1000 cps in the presence of wide-band masking was investigated in 26 listeners, using the method of constant stimulus differences. Enough masking was introduced into the same ear as the 1000-cps tone to raise the absolute threshold of the sinusoid 10 or 45 dB. Four SL's relative to masked threshold were investigated (5, 15, 25, and 35 dB SL). The DL was also obtained under quiet conditions at the four SL's listed above. The results indicate that the introduction of masking simultaneously with the sinusoid simply translated the function relating the DL to SL along an intensity dimension. That is, when SL is referred to either absolute threshold under conditions of quiet, or to masked threshold under conditions of masking, the DL functions are identical. Loudness and over-all SPL were found not to be of particular importance per se.