Marion F. Cohen

Influence of place synchrony on detection of a sinusoid

A series of experiments was performed to study the ability of the ear to code the temporal envelope of a waveform as demonstrated by comodulation masking release (CMR). The stimulus for all experiments was composed of a tone-burst signal, a 100-Hz-wide masker band centered at the signal frequency, and a second 100-Hz-wide noise band of variable frequency, the cue band. The cue band had a temporal envelope which was either correlated with or independent of that of the masker. The signal was a 100-Hz tone burst for most experiments. For the monotic stimulus, the correlated cue band results in lowered signal detection thresholds over a range extending from around 2/3 oct below the signal frequency to 1/3 oct above that frequency. When measured dichotically, with the signal and masker band in one ear and the cue band in the opposite ear, that effective range is expanded but the detection threshold shifts are a bit smaller. The greatest CMR is observed when the stimulus is presented diotically. With regard to effects of level and frequency, our data show CMR increasing with increasing stimulus level for a cue band lower in frequency than the signal, but show little effect of level for a cue band higher in frequency. Similarly, CMR increases with increasing stimulus frequency when the cue band is lower in frequency, but shows little effect of frequency for a cue band higher in frequency.

Detection threshold microstructure and its effect on temporal integration data

Auditory detection thresholds were measured at several preselected frequencies using an adaptive 2IFC procedure. Results confirm the existence of shifts in detection threshold ranging from 2 to 14 dB with quite small changes in signal frequency. There does not appear to be a uniform pattern associated with the microstructure of the detection threshold curve. An additional experiment was performed to determine the effect of signal duration on detection threshold microstructure. Results indicate that the temporal integration function is considerably steeper for more sensitive frequencies (3.7 dB/doubling of duration), than for less sensitive frequencies (1.7 dB/doubling). This probably is not due to differences in processing as much as it is to the effect of the energy spread associated with decreasing signal duration.

The effect of cross‐spectrum correlation on the detectability of a noise band

An experiment was performed to study the interaction of two narrow-band noises having correlated temporal envelopes. The detection threshold of a 100-Hz-wide noise-band signal was measured at different center frequencies in the presence of a continuous 100-Hz-wide noise band having a center frequency of 1000 Hz. The two noise bands had either correlated or independent temporal envelopes. Measured signal detection thresholds are lower when the two noise bands are independent, but the magnitude of this difference is not a simple function of the frequency separation between the two noise bands.

Comodulation masking release over a three octave range

Comodulation masking release (CMR) for a variable frequency 50‐ms tone‐burst signal was measured in the usual manner, except that the overall level of the 100‐Hz‐wide masker band was varied from 18 to 78 dB SPL in 20‐dB steps. The 100‐Hz‐wide cue band, which remained centered at 1000 Hz, was presented at an overall level of 78 dB SPL. Presentation was either monotic or dichotic, with the cue band presented to the nonsignal ear only. Averaged results show monotic CMR values as high as 7 dB when the masker band/signal was centered at 8000 Hz, or 3 oct away from the cue band, and 11 dB when it was 2 oct away. CMR for all frequency separations was reduced when the two noise bands were equal in level. The dichotic presentation resulted in a CMR that was somewhat smaller, and less dependent on, the relative levels of the two noise bands.

Interaural time discrimination in noise

Interaural time jnds were measured for a 250‐Hz tone‐burst signal as a function of signal level and interaural phase relations of a background white noise. Results indicate that midline interaural time jnds for a sinusoid of constant SPL become larger as the background noise changes from diotic to statistically independent to interaurally phase reversed. That background noise which favors signal detection impedes interaural time discrimination.

Dynamic frequency change among stimulus components: Effects of coherence on detectability

Experiments were performed to determine the effect of coherent dynamic frequency change among stimulus components on detection of one of those components. Detectability of a frequency glide signal, centered at 660 Hz, was measured in the presence of two additional frequency glides centered at 220 and 440 Hz. Results show that the signal is most difficult to detect when it is changing coherently with the other stimulus components, and that detectability gradually improves as the frequency change among stimulus components becomes less coherent. A control experiment indicated that detectability of the signal is influenced somewhat by the average spectral distance between the signal and the other stimulus components. In order to separate the effects of dynamic frequency change from those of altering harmonic relationships, the experiment was repeated with stimulus components not harmonically related. Results are similar in pattern, though with somewhat smaller shifts. A final experiment was performed to determine if the observed shifts in detectability might be due to changes in the maximum spectral distance between signal and masker when their frequencies are changing incoherently. Results of this experiment indicate that detectability improves when frequency change of signal and masker is not coherent, even when maximum spectral distance does not change. These data indicate the likelihood that the auditory system is able to use coherent dynamic frequency change among stimulus components.

Comodulation masking release and the masking-level difference

An experiment was performed to determine if the mechanism that mediates comodulation masking release (CMR) is associated with that used to improve detection by the masking-level difference (MLD). The experiment consisted of first improving detectability of a masked diotic tone burst by adding a synchronous noise band at another frequency region (CMR), and then measuring an MLD in the usual manner, by inverting the tone-burst signal to one ear. Results indicate that a substantial MLD can be measured for a signal whose detectability has already been improved by CMR. However, that MLD (9 dB) is smaller than that measured in random noise (14 dB). Put another way, a small CMR (4 dB) can be produced even when the detectability of a stimulus has already been improved due to the MLD. These data are in general agreement with those of Hall et al. [J. Acoust. Soc. Am. 83, 1839-1845 (1988)] and Schooneveldt and Moore [J. Acoust. Soc. Am. 85, 262-272 (1989)].

Place synchrony and the masking‐level difference

Detection threshold of a masked signal can be lowered in two ways: First, by varying the interaural configuration of the signal and masker, the MLD, and second, by a noise masker which has the same envelope at different frequencies, place synchrony. This experiment was performed to determine if these two processes are independent. The signal was a tone burst. The masker, a continuous 100‐Hz‐wide noise band centered at the signal frequency, was created by modulating a tone with a 50‐Hz low‐pass noise. A second noise band, centered at a different frequency, had either the same envelope as the first noise or a different envelope. The MLD was measured for two conditions; when the two noise bands in the homophasic condition were synchronous, and when they were not. MLDs measured in the synchronous noise were substantially smaller than those in the independent noise or in the 700‐Hz noise alone. It is probable that the detection threshold shifts which are due to the MLD and those due to place synchrony are not ...

Detection and interaural time discrimination of a sinusoid masked by interaurally delayed white noise

Masking‐level differences and masked interaural time jnds were measured for a 560‐ and a 1000‐Hz tone‐burst signal. The masker, a diotic white noise, was interaurally time delayed by amounts ranging from no delay to one full period of the signal frequency. Results show that the size of the jnd varies systematically with the masking‐level difference. Those masker delays which most benefit detection cause poorer signal interaural time discrimination.

Effects of Stimulus Presentation Rate upon Intelligibility‐Test Scores

There is a need standardization of intelligibility testing as it is used for evaluating communications systems. The purpose of this experiment was to evaluate the effects of different rates of stimulus presentation upon intelligibility scores. Stimulus materials provided by the Fairbanks rhyme test and the diagnostic rhyme test were recorded several times with various time intervals between words. They were presented to listening crews under two different conditions: (1) bandpassed from 200 to 4000 cps; (2) processed by an 18‐channel vocoder. The data for each condition were analyzed to determine the effects of the various stimulus rates upon both the intelligibility scores and the standard error reliability of these scores. [The research reported in this paper was sponsored in part by the U. S. Air Force Cambridge Research Laboratories, Office of Aerospace Research.]