Growth Of Masking Research Articles

Accounting for nonmonotonic precursor duration effects with gain reduction in the temporal window model

The mechanisms of forward masking are not clearly understood. The temporal window model (TWM) proposes that masking occurs via a neural mechanism that integrates within a temporal window. The medial olivocochlear reflex (MOCR), a sound-evoked reflex that reduces cochlear amplifier gain, may also contribute to forward masking if the preceding sound reduces gain for the signal. Psychophysical evidence of gain reduction can be observed using a growth of masking (GOM) paradigm with an off-frequency forward masker and a precursor. The basilar membrane input/output (I/O) function is estimated from the GOM function, and the I/O function gain is reduced by the precursor. In this study, the effect of precursor duration on this gain reduction effect was examined for on- and off-frequency precursors. With on-frequency precursors, thresholds increased with increasing precursor duration, then decreased (rolled over) for longer durations. Thresholds with off-frequency precursors continued to increase with increasing precursor duration. These results are not consistent with solely neural masking, but may reflect gain reduction that selectively affects on-frequency stimuli. The TWM was modified to include history-dependent gain reduction to simulate the MOCR, called the temporal window model-gain reduction (TWM-GR). The TWM-GR predicted rollover and the differences with on- and off-frequency precursors whereas the TWM did not.

The time course of cochlear gain reduction measured using a more efficient psychophysical technique.

In a previous study it was shown that an on-frequency precursor intended to activate the medial olivocochlear reflex (MOCR) at the signal frequency reduces the gain estimated from growth-of-masking (GOM) functions. This is called the temporal effect (TE). In Expt. 1 a shorter method of measuring this change in gain is established. GOM functions were measured with an on- and off-frequency precursor presented before the masker and signal, and used to estimate Input/Output functions. The change in gain estimated in this way was very similar to that estimated from comparing two points measured with a single fixed masker level on the lower legs of the GOM functions. In Expt. 2, the TE was measured as a function of precursor duration and signal delay. For short precursor durations and short delays the TE increased (buildup) or remained constant as delay increased, then decreased. The TE also increased with precursor duration for the shortest delay. The results were fitted with a model based on the time course of the MOCR. The model fitted the data well, and predicted the buildup. This buildup is not consistent with exponential decay predicted by neural adaptation or persistence of excitation.

The relationship between quiet threshold and the forward-masking temporal effect.

Research in our laboratory has shown that the temporal effect (TE) in simultaneous masking is consistent with a decrease in gain, possibly mediated by the medial olivocochlear reflex (MOCR). The TE in simultaneous masking is a decrease in threshold signal-to-masker ratio for a signal at masker onset when a precursor is added. This work has been extended to a forward-masking paradigm. A growth of masking (GOM) function is measured with a short-duration off-frequency masker (which should not activate the MOCR) and a 4-kHz signal. Then the masker level is fixed at a point on the lower leg of the GOM function, and threshold is measured with a long-duration precursor which is intended to activate the MOCR. The estimated input-output function is compared for a precursor at and well below the signal frequency. The difference in thresholds is the TE, which is also a measure of the change in gain. In simultaneous masking, the size of the TE decreases with increasing quiet threshold. In the present study, this relationship was examined for the forward-masking TE. The TE was measured as a function of precursor level for listeners who had a range of quiet thresholds, including listeners with mild cochlear hearing impairment. [Research supported by a grant from NIH(NIDCD) R01 DC008327.]

Spatial and Temporal Effects of Interleaved Masking in Cochlear Implants

Modern cochlear implants utilize interleaved presentation of pulses on different electrodes to avoid physical interference among multiple current fields, yet neural interaction still exists. In the present study, masking was examined with four Nucleus24 users with the banded electrode array in an interleaved masking paradigm, where a probe stimulus was interleaved with a masker stimulus. Spatial and temporal aspects of masking were addressed by fixing the masker at the middle of the electrode array and changing the location of the probe and by testing various stimulation rates: 125, 500, 2,000, and 6,410 Hz. In addition, growth of masking (GOM) was assessed by changing the masker level in six steps. Results indicated that masking patterns were generally much wider, regardless of stimulation rate, than those in acoustic hearing. The amount of masking decreased from the peak at the rate of approximately 0.5 dB/mm even at the highest masker level. The pattern of GOM with the rates higher than 500 Hz was different from that observed in previous masking studies, characterized by markedly shallow growth at low masker levels or overall shallow growth. A facilitating effect of the masker (lowering the threshold) was suspected, except for the 125-Hz condition, due to the fibers that were subliminally excited, but not discharged, by the masker with local perturbations of membrane potentials, and were subsequently discharged easily by a lower level probe when the temporal gap between masker and probe was sufficiently short. These results suggest that both refractory characteristics of neurons and neural summation be considered in interleaved stimulation of pulses at high, but clinically relevant, stimulation rates. Overall, the present masking study might provide a basis for models in psychophysics and speech understanding in current cochlear implant systems utilizing high-rate interleaved stimulation.

Precursor effects on behavioral estimates of frequency selectivity and gain in forward masking

The experiments presented in this paper explore the hypothesis that cochlear gain is reduced, in a frequency-specific manner, over the course of a sound (called a "precursor") which was designed to activate the medial olivo-cochlear reflex (MOCR). Psychophysical tuning curves (PTCs) and off-frequency growth of masking (GOM) functions were measured with two precursors. The on-frequency precursor condition, which was hypothesized to activate the MOCR at the signal frequency, produced a PTC with a lower best frequency in all subjects consistent with less gain. This same condition produced a GOM function with less gain and an elevated compression breakpoint. The data were analyzed with two models. The gain-reduction model, which assumed a change in the basilar membrane input-output function, was superior at predicting the data relative to a model of additivity of masking.

The effect of a precursor on growth of forward masking

This study examined the effect of an on-frequency precursor on growth-of-masking (GOM) functions measured using an off-frequency masker. The signal was a 6-ms, 4-kHz tone. A GOM function was measured using a 40-ms, 2.8-kHz tone (the off-frequency masker). GOM functions were then measured with an on-frequency, fixed level precursor presented before the off-frequency masker. The precursor was 50 or 60 dB SPL, and 160 ms in duration. For the 60-dB SPL precursor, a 40-ms duration was also used. Two-line functions were fit to the GOM data to estimate the basilar membrane input-output function. The precursors reduced the gain of the input-output function, and this decrease was graded with precursor level. Both precursor durations had the same effect on gain. Changes in masking following a precursor were larger than would be predicted by additivity of masking. The observed decrease in gain may be consistent with activation of the medial olivocochlear reflex by the precursor.

Estimating the basilar-membrane input/output-function in normal-hearing and hearing-impaired listeners using forward masking

To partly characterize the function of cochlear processing in humans, the basilar membrane (BM) input-output (I/O) function can be estimated. In recent studies, forward masking has been used to estimate BM compression. If an on-frequency masker is processed compressively, while an off-frequency masker is transformed more linearly, the ratio between the slopes of growth of masking (GOM) functions provides an estimate of BM compression at the signal frequency. In this study, this paradigm is extended to also estimate the knee-point of the I/O-function between linear processing at low levels and compressive processing at medium levels. If a signal can be masked by a low-level on-frequency masker such that signal and masker fall in the linear region of the I/O-function, then a steeper GOM function is expected. The knee-point can then be estimated in the input level region where the GOM changes significantly. Data were collected from eight normal-hearing (NH) and five hearing-impaired (HI) listeners with mild to moderate sensorineural hearing loss. Both groups showed large inter-subject but low intra-subject variability. When the knee-point could be estimated for the HI listeners it was shifted towards higher input levels and compression was similar to that of NH listeners.

Reductions in gain from pre-masker stimulation at the signal frequency

Normal hearing individuals can detect a short duration signal at a lower signal-to-masker ratio when the signal is delayed from masker onset compared to near it. This is often referred to as the temporal effect. This effect is interesting because it may be related to changes in cochlear gain. It has been proposed that gain is high at masker onset and decreases, in a frequency specific manner, through the course of the masker. If this hypothesis was true, it is expected that frequency selectivity also decreases through the course of the masker. Using a forward masking paradigm, the current study examines the hypothesis that frequency selectivity decreases with pre-masker stimulation at the signal frequency. Growth of masking (GOM) functions and psychophysical tuning curves (PTC) were measured for the signal using a short masker preceded by a long duration precursor set at the signal frequency or well below the signal frequency. Preliminary results reveal a relatively broader PTC and a steeper GOM function when the precursor was at the signal frequency and are consistent with the hypothesis that preceding stimulation at the signal frequency reduces gain. [Work supported by the Hazelton Fund.]

The effects of hearing loss on growth-of-masking functions for sinusoidal and complex-tone maskers with differing phase spectra

Growth-of-masking (GOM) functions in forward masking (0-ms masker-signal delay) were measured for normally hearing (NH) and hearing-impaired (HI) listeners using as maskers complex tones (harmonics 1–40, 100-Hz fundamental frequency) with components starting in cosine or random phase, and on-frequency sinusoids. The signal was a 20-ms sinusoid, usually with a frequency of 1 or 2 kHz. It is argued that differences in the slopes of the GOM functions for the random- and cosine-phase maskers provide a measure of the strength of compression in the cochlea. For the NH listeners and some of the HI listeners, the slopes were significantly greater for the random- than for the cosine-phase maskers, and for these listeners the slopes for the complex-tone maskers were less than for the sinusoidal maskers. For the remaining HI listeners, the slopes of the GOM functions were similar for all masker types. It is argued that these listeners had almost complete loss of cochlear compression. The GOM functions for the sinusoidal maskers had slopes between 0.45 and 0.78 and were typically in the range 0.6–0.7. The finding of slopes below one for listeners in whom cochlear compression was probably absent is not consistent with linear-integrator models of forward masking.

The effect of a steep high-frequency hearing loss on growth-of-masking functions in simultaneous masking for fm < fs

In normal-hearing subjects, the slope of the growth-of-masking (GOM) function obtained in simultaneous masking when the masker frequency ( f m) is much less than the signal frequency ( f s) often changes from a value near 2.0 to a value near 1.0 at high levels. The purpose of the present study was to evaluate whether this change in slope reflects a basal shift in the peak of the signal’s basilar-membrane vibration pattern. To discourage the use of basally shifted peak excitation, GOM functions were obtained in seven subjects with a precipitously sloping high-frequency hearing loss. The signal was located at the normal-hearing edge of the loss, and the masker was located 3 equivalent rectangular bandwidths below f s. In addition, GOM functions for an f s of 2000 Hz were obtained in four subjects with normal hearing, either “in quiet” or in the presence of a restrictor tone with a frequency of 2400 or 2600 Hz and a level of 90 dB SPL. Overall, the results generally are not consistent with the change in slope at high levels being due to a basal shift in the peak of the signal’s basilar-membrane vibration pattern. Instead, the results are consistent with a decrease in compression at high input levels at the place corresponding to f s.

Comparing different estimates of cochlear compression in listeners with normal and impaired hearing.

A loss of cochlear compression may underlie many of the difficulties experienced by hearing-impaired listeners. Two behavioral forward-masking paradigms that have been used to estimate the magnitude of cochlear compression are growth of masking (GOM) and temporal masking (TM). The aim of this study was to determine whether these two measures produce within-subjects results that are consistent across a range of signal frequencies and, if so, to compare them in terms of reliability or efficiency. GOM and TM functions were measured in a group of five normal-hearing and five hearing-impaired listeners at signal frequencies of 1000, 2000, and 4000 Hz. Compression values were derived from the masking data and confidence intervals were constructed around these estimates. Both measures produced comparable estimates of compression, but both measures have distinct advantages and disadvantages, so that the more appropriate measure depends on factors such as the frequency region of interest and the degree of hearing loss. Because of the long testing times needed, neither measure is suitable for clinical use in its current form.

Growth of interleaved masking patterns for cochlear implant listeners at different stimulation rates

This study investigates the pattern of growth of masking (GOM) for interleaved masking with Nucleus cochlear implant users. For an interleaved masking paradigm, where the masker and probe overlap in a same time window, the masker may have contrasting effects: it may increase the threshold (as a masker normally does) or decrease it due to a neural summation effect, facilitating detection of the probe. Several stimulation rates and masker levels were tested to examine under what conditions what phenomenon would occur. The results indicated that, in most of the conditions, the amount of masking was positive, i.e., the facilitating effect was not consistently observed. However, the slope of the GOM appears to be dependent upon the stimulation rate: the higher the stimulation rate, the lower the slope, implying that the facilitating effect might be always present and make a bigger impact on overall masking as the stimulation rate becomes high. The amount of masking was also often nonzero (positive) even when the masker was below the threshold level. Overall, the present findings indicate that interleaved masking should be handled with care to understand cochlear implant users speech perception and improve speech coding, as it contains some nontraditional aspects of masking.