Amount Of Forward Masking Research Articles

Influence of centrifugal pathways on forward masking of ventral cochlear nucleus neurons.

When responses to one part of a sequence of auditory signals reduce the responses to a subsequent portion of the signal, "forward masking" results. Although forward masking occurs in the auditory nerve, that observed in the ventral cochlear nucleus (VCN) more closely resembles psychophysical forward masking. In contrast to the auditory nerve in which the amount of forward masking is proportional to the amount of excitation produced by the masker, most VCN neurons show a poor correlation between forward masking and excitation produced by the masker, indicating a more complex interaction between responses to adjacent signals. This study tested the hypothesis that one component of forward masking is produced by inputs from centrifugal neural connections to the VCN. The centrifugal pathways were interrupted with knife-cut lesions medial to the CN. Responses of single units obtained 60 minutes after the lesions were compared to those obtained before the lesions. In primarylike, sustained chopper and on units the lesions resulted in a reduction in forward masking and enhanced recovery. In contrast, lesions resulted in increased masking in primarylike-notch and low-intensity chopper units. The relationship between masker-elicited excitation and forward masking became more monotonic for transient choppers and on units, approaching that observed for auditory nerve fibers. These effects are probably the result of removal of both inhibitory and excitatory inputs, ultimately reflecting a balance of excitation and inhibition to each neural population in the VCN.

Influence of signal rise time on the amount of forward masking.

Previously, a discrepancy between the amount of forward masking in the auditory nerve and published behavioral data was reported by these laboratories [E. M. Relkin and C. W. Turner, J. Acoust. Soc. Am. 84, 584–591 (1988)]; in that physiological experiment, however, a 2-ms signal rise time was employed, while the previous behavioral data used longer rise times. In the present behavioral study, both 2-ms and 10-ms signal rise-fall times were employed. The results show that for 10-ms ramps, the maximum amount of masking is large (at least 30 dB), in agreement with previous data. In contrast, for 2-ms ramps, the maximum amount of masking is smaller (15 dB). This is similar to the amount of masking in the auditory nerve reported in Relkin and Turner (1988), and indicates little or no discrepancy between behavioral and auditory nerve forward masking. This difference is observed for wide- and narrow-band maskers, suggesting that the results are not simply explained by the short-term spectra of the ramps. [Work supported by NINCDS.]

Factors affecting thresholds for sinusoidal signals in narrow-band maskers with fluctuating envelopes.

When a signal is higher in frequency than a narrow-band masker, thresholds are lower when the masker envelope fluctuates than when it is constant. This article investigates the cues used to achieve the lower thresholds, and the factors that influence the amount of threshold reduction. In experiment I the masker was either a sinusoid (constant envelope) or a pair of equal-amplitude sinusoids (fluctuating envelope) centered at the same frequency as the single sinusoid (250, 1000, 3000, or 5275 Hz). The signal frequency was 1.8 times the masker frequency. At all center frequencies, thresholds were lower for the two-tone masker than for the sinusoidal masker, but the effect was smaller at the highest and lowest frequencies. The reduced effect at high frequencies is attributed to the loss of a cue related to phase locking in the auditory nerve. The reduced effect at low frequencies can be partly explained by reduced slopes of the growth-of-masking functions. In experiment II the masker was a sinusoid amplitude modulated at an 8-Hz rate. Masker and signal frequencies were the same as for the first experiment. Randomizing the modulation depth between the two halves of a forced-choice trial had no effect on thresholds, indicating that changes in modulation depth are not used as a cue for signal detection. Thresholds in the modulated masker were higher than would be predicted if they were determined only by the masker level at minima in the envelope, and the threshold reduction produced by modulating the master envelope was less at 250 Hz than at higher frequencies. Experiments III and IV reveal two factors that contribute to the reduced release from masking at low frequencies: The rate of increase of masked threshold with decreasing duration is greater at 250 Hz than at 1000 Hz; the amount of forward masking, relative to simultaneous masking, is greater at 250 Hz than at 1000 Hz. The results are discussed in terms of the relative importance of across-channel cues and within-channel cues.

Forward masking as a function of masker duration

The amount of forward masking was determined for maskers that varied from 10 to 320 ms in duration. All maskers and signals were 1000‐Hz sinusoids with 5‐ms cosine‐squared ramps. A 10‐ms signal followed the masker by 4, 8, 16, and 32 ms. Four levels of the masker were used for each combination of masker duration and masker‐signal inverval (Δt). Because forward masking increases linearly with the level of the masker in all conditions, it is possible to estimate the amount of masking that would be obtained for any level within the range used. Comparisons for equal‐energy maskers are of particular interest. Longer duration maskers produce more masking than shorter duration maskers of equal energy, despite the fact that the energy in the longer duration maskers is separated by a longer period of time from the signal. This result is in agreement with data recently reported by Penner [J. Acoust. Soc. Am. 66, 1719–1724 0979)] for noise maskers and click signals. The effect is greater for smaller values of Δt and higher masker levels. [Work supported by NIH.]

Frequency selectivity of the ear in forward masking.

In a stimulus paradigm in which the masker terminates prior to the signal presentation (forward masking), threshold shifts as a function signal frequency closely resemble those observed for simultaneous masking. The purpose of the present study was to determine if a “critical band” could be identified for forward masking. Two-component masker (primary) stimuli centered in the regions of 800 and 3200 Hz were presented to 10 listeners. The amount of forward masking was measured as a function of signal frequency, while the frequency separation of the sinusoids comprising the primary stimulus was varied systematically. Previously reported simultaneous-masking data suggested that a critical bandwidth could be defined as the frequency separation of masker components coincident with a change in the shape of the function relating amount of masking to signal frequency. For the conditions employed in the present study, no changes in this function were observed even at wide separation of the primary components.