Psychophysical recovery from pulse-train forward masking in electric hearing

Psychophysical pulse-train forward-masking (PTFM) recovery functions were measured in fifteen subjects with the Nucleus mini-22 cochlear implant and six subjects with the Clarion cochlear implant. Masker and probe stimuli were 500-Hz trains of 200- or 77-micros/phase biphasic current pulses. Electrode configurations were bipolar for Nucleus subjects and monopolar for Clarion subjects. Masker duration was 320 ms. Probe duration was either 10 ms or 30 ms. Recovery functions were measured for a high-level masker on a middle electrode in all 21 subjects, on apical and basal electrodes in 7 of the Nucleus and 3 of the Clarion subjects, and for multiple masker levels on the middle electrode in 8 Nucleus subjects and 6 Clarion subjects. Recovery functions were described by an exponential process in which threshold shift (in microA) decreased exponentially with increasing time delay between the offset of the masker pulse train and the offset of the probe pulse train. All but 3 of the 21 subjects demonstrated recovery time constants on a middle electrode that were less than 95 ms. The mean time constant for these 18 subjects was 54 ms (s.d. 17 ms). Three other subjects tested on three electrodes exhibited time constants larger than 95 ms from an apical electrode only. Growth-of-masking slopes depended upon time delay, as expected from an exponential recovery process, i.e., progressively shallower slopes were observed at time delays of 10 ms and 50 ms. Recovery of threshold shift (in microA) for PTFM in electrical hearing behaves inthe same way as recovery of threshold shift (in dB) for pure-tone forward masking in acoustic hearing. This supports the concept that linear microamps are the electrical equivalent of acoustic decibels. Recovery from PTFM was not related to speech recognition in a simple manner. Three subjects with prolonged PTFM recovery demonstrated poor speech scores. The remaining subjects with apparently normal PTFM recovery demonstrated speech scores ranging from poor to excellent. Findings suggest that normal PTFM recovery is only one of several factors associated with good speech recognition in cochlear-implant listeners. Comparisons of recovery curves for 10- and 30-ms probe durations in two subjects showed little or no temporal integration at time delays less than 95 ms where recovery functions have steep slopes. The same subjects exhibited large amounts of temporal integration at longer time delays where recovery slopes are more gradual. This suggests that probe detection depends primarily on detection of the final pulses in the probe stimulus and supports the use of offset-to-offset time delays for characterizing PTFM recovery in electric hearing.     

Gap detection and masking in hearing-impaired and normal-hearing subjects

Subjects with cochlear impairments often show reduced temporal resolution as measured in gap-detection tasks. The primary goals of these experiments were: to assess the extent to which the enlarged gap thresholds can be explained by elevations in absolute threshold; and to determine whether the large gap thresholds can be explained by the same processes that lead to a slower-than-normal recovery from forward masking. In experiment I gap thresholds were measured for nine unilaterally and eight bilaterally impaired subjects, using bandlimited noise stimuli centered at 0.5, 1.0, and 2.0 kHz. Gap thresholds were usually larger for the impaired ears, even when the comparisons were made at equal sensation levels (SLs). Gap thresholds tended to increase with increasing absolute threshold, but the scatter of gap thresholds was large for a given degree of hearing loss. In experiment II threshold was measured as a function of the delay between the onset of a 210-ms masker and the onset of a 10-ms signal in both simultaneous- and forward-masking conditions. The signal frequency was equal to the center frequency of the bandlimited noise masker, which was 0.5, 1.0, or 2.0 kHz. Five subjects with unilateral cochlear impairments, two subjects with bilateral impairments, and two normal subjects were tested. The rate of recovery from forward masking, particularly the initial rate, was usually slower for the impaired ears, even when the maskers were presented at equal SLs. Large gap thresholds tended to be associated with slow rates of recovery from forward masking.     

A computer model of the auditory-nerve response to forward-masking stimuli

A computer model of the auditory periphery is used to study the involvement of auditory-nerve (AN) adaptation in forward-masking effects. An existing model is shown to simulate published AN recovery functions both qualitatively and quantitatively after appropriate parameter adjustments. It also simulates published data showing only small threshold shifts when a psychophysical forward-masking paradigm is applied to AN responses. The model is extended to simulate a simple but physiologically plausible mechanism for making threshold decisions based on coincidental firing of a number of AN fibers. When this is used, much larger threshold shifts are observed of a size consistent with published psychophysical observations. The problem of how stimulus-driven firing can be distinguished from spontaneous activity near threshold is also addressed by the same decision mechanism. Overall, the modeling results suggest that poststimulatory reductions in AN activity can make a substantial contribution to the raised thresholds observed in many psychophysical studies of forward masking.     

Forward masking of amplitude modulation: basic characteristics

In this study we demonstrate an effect for amplitude modulation (AM) that is analogous to forward making of audio frequencies, i.e., the modulation threshold for detection of AM (signal) is raised by preceding AM (masker). In the study we focused on the basic characteristics of the forward-masking effect. Functions representing recovery from AM forward masking measured with a 150- ms 40- Hz masker AM and a 50- ms signal AM of the same rate imposed on the same broadband-noise carrier, showed an exponential decay of forward masking with increasing delay from masker offset. Thresholds remained elevated by more than 2 dB over an interval of at least 150 ms following the masker. Masked-threshold patterns, measured with a fixed signal rate (20, 40, and 80 Hz) and a variable masker rate, showed tuning of the AM forward-masking effect. The tuning was approximately constant across signal modulation rates used and consistent with the idea of modulation-rate selective channels. Combining two equally effective forward maskers of different frequencies did not lead to an increase in forward masking relative to that produced by either component alone. Overall, the results are consistent with modulation-rate selective neural channels that adapt and recover from the adaptation relatively quickly.     

Temporal mechanisms underlying recovery from forward masking in multielectrode-implant listeners

This paper describes a detailed study of recovery from forward masking in six users of the Nucleus-22 cochlear implant with a range of performance in speech-recognition tests. Recovery from a 300-ms-long pulse train presented at 1000 pps was found to be fastest in the poorer performers. The shape of the recovery function was found to be most strongly influenced by masker duration, suggesting that temporal integration plays a prominent role in recovery from forward masking. The recovery functions are reasonably well described by a sum of two exponentially decaying processes. Their relative weights depend on the amount of temporal integration occurring during the masker, and show strong intersubject variability. Nonmonotonicities sometimes observed in the recovery functions may be accounted for by considering the influence of neural adaptation.     

Temporal effects in masking and their influence on psychophysical tuning curves

Psychophysical tuning curves (PTCs) were obtained in simultaneous and forward masking for a 20-ms, 1000-Hz signal presented at 10 dB SL. The signal was presented at the beginning of, at the temporal center of, at the end of, or immediately following a 400-ms masker. The first experiment was done in quiet; the second experiment was done in the presence of two bands of noise on either side of 1000 Hz. The results were similar in quiet and in noise. In simultaneous masking, the PTCs were broadest for the signal at masker onset, and generally sharpest for the signal at temporal center; the differences were largest on the high-frequency side. In most cases, there was virtually no difference in Q10 between the forward-masking PTC and the simultaneous-masking PTC with the signal temporally centered, although the high-frequency slope was always steeper in forward masking. These results indicate that, at least for brief signals, frequency selectivity measured with simultaneous-masking PTCs and the degree of sharpening revealed in forward-masking PTCs depend upon the temporal position of the signal within the simultaneous masker.     

Comparisons of frequency selectivity in simultaneous and forward masking for subjects with unilateral cochlear impairments

Two experiments are described in which frequency selectivity was estimated, in simultaneous and forward masking, for each ear of subjects with moderate (25-60 dB HL) unilateral cochlear hearing losses. In both experiments, the signal level was fixed for a given ear and type of masking (simultaneous or forward), and the masker level was varied to determine threshold, using an adaptive, two-alternative forced-choice procedure. In experiment I, the masker was a noise with a spectral notch centered at the signal frequency (either 1.0 or 1.5 kHz); threshold was determined as a function of notch width. Signal levels were chosen so that the noise level required at threshold for a notch width of zero was similar for the normal and impaired ear of each subject in both simultaneous and forward masking. The function relating threshold to notch width had a steeper slope for the normal ear than for the impaired ear of each subject. For the normal ears, these functions were steeper in forward masking than in simultaneous masking. This difference was interpreted as resulting from suppression. For the impaired ears, significant differences in the same direction were observed for three of the five subjects, but the differences were smaller. In experiment II, psychophysical tuning curves (PTCs) were determined in the presence of a fixed notched noise centered at the signal frequency (1.0 kHz). For the normal ears, the PTCs were sharper in forward masking than in simultaneous masking. For the impaired ears, the PTCs were similar in simultaneous and forward masking, but those in forward masking tended to be sharper at masker frequencies far removed from the signal frequency. Overall, the results suggest that suppression is reduced, but not completely absent in cases of moderate cochlear hearing loss.     

Psychophysical tuning curves measured in simultaneous and forward masking

The level of a masker necessary to mask a probe fixed in frequency and level was determined as a function of masker frequency using a two-interval forced-choice technique. Both simultaneous- and forward- masking techniques were used. Parameters investigated include the level of the probe tone and the frequency of the probe tone. The general form of the psychophysical tuning curves obtained in this way is quite similar to that of single-neurone tuning curves, when low-level probe tones are used. However, the curves obtained to forward masking generally show sharper tips and steeper slopes than those found in simultaneous masking, and they are also generally sharper than neurophysiological tuning curves. For frequencies of the masker close to that of the probe a simultaneous masker was sometimes less effective than a forward masker. The results are discussed in relation to possible lateral suppression effects in simultaneous masking, and in relation to the observer's use of pitch cues in forward masking. It is concluded that neither the simultaneous-masking curves nor the forward-masking curves are likely to give an accurate representation of human neural tuning curves.     

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.     

Psychophysical recovery from single-pulse forward masking in electric hearing.

Psychophysical single-pulse forward-masking (SPFM) recovery functions were measured for three electrodes in each of eight subjects with the nucleus mini-22 cochlear implant. Masker and probe stimuli were single 200-micros/phase biphasic current pulses. Recovery functions were measured at several masker levels spanning the electric dynamic range of electrodes chosen from the apical, middle, and basal regions of each subject's electrode array. Recovery functions were described by an exponential process in which threshold shift (in microA) decreased exponentially with increasing time delay between the masker and probe pulses. Two recovery processes were observed: An initial, rapid-recovery process with an average time constant of 5.5 ms was complete by about 10 ms. A second, slow-recovery process involved less masking than the rapid-recovery process but encompassed much longer time delays, sometimes as long as several hundred milliseconds. Growth-of-masking slopes for the rapid process depended upon time delay, as expected in an exponential recovery process. Unity slopes were observed at a time delay of 0 ms, whereas progressively shallower slopes were observed at time delays of 2 ms and 5 ms. Many recovery functions demonstrated nonmonotonicities or "facilitation" at very short masker-probe delays (1-2 ms). Such nonmonotonicities were usually most pronounced at low masker levels. Time constants for the rapid-recovery process did not vary systematically with masker level or with electrode location along the implanted array. Most subjects demonstrated rapid-recovery time constants less than 7 ms; however, the subject with the longest duration of deafness prior to implantation exhibited clearly prolonged time constants (9-24 ms). Time constants obtained on basal electrodes were inversely related to word recognition scores.     

Temporal masking functions for listeners with real and simulated hearing loss

A functional simulation of hearing loss was evaluated in its ability to reproduce the temporal masking functions for eight listeners with mild to severe sensorineural hearing loss. Each audiometric loss was simulated in a group of age-matched normal-hearing listeners through a combination of spectrally-shaped masking noise and multi-band expansion. Temporal-masking functions were obtained in both groups of listeners using a forward-masking paradigm in which the level of a 110-ms masker required to just mask a 10-ms fixed-level probe (5-10 dB SL) was measured as a function of the time delay between the masker offset and probe onset. At each of four probe frequencies (500, 1000, 2000, and 4000 Hz), temporal-masking functions were obtained using maskers that were 0.55, 1.0, and 1.15 times the probe frequency. The slopes and y-intercepts of the masking functions were not significantly different for listeners with real and simulated hearing loss. The y-intercepts were positively correlated with level of hearing loss while the slopes were negatively correlated. The ratio of the slopes obtained with the low-frequency maskers relative to the on-frequency maskers was similar for both groups of listeners and indicated a smaller compressive effect than that observed in normal-hearing listeners.     

Ipsilateral masking between acoustic and electric stimulations

Residual acoustic hearing can be preserved in the same ear following cochlear implantation with minimally traumatic surgical techniques and short-electrode arrays. The combined electric-acoustic stimulation significantly improves cochlear implant performance, particularly speech recognition in noise. The present study measures simultaneous masking by electric pulses on acoustic pure tones, or vice versa, to investigate electric-acoustic interactions and their underlying psychophysical mechanisms. Six subjects, with acoustic hearing preserved at low frequencies in their implanted ear, participated in the study. One subject had a fully inserted 24 mm Nucleus Freedom array and five subjects had Iowa/Nucleus hybrid implants that were only 10 mm in length. Electric masking data of the long-electrode subject showed that stimulation from the most apical electrodes produced threshold elevations over 10 dB for 500, 625, and 750 Hz probe tones, but no elevation for 125 and 250 Hz tones. On the contrary, electric stimulation did not produce any electric masking in the short-electrode subjects. In the acoustic masking experiment, 125-750 Hz pure tones were used to acoustically mask electric stimulation. The acoustic masking results showed that, independent of pure tone frequency, both long- and short-electrode subjects showed threshold elevations at apical and basal electrodes. The present results can be interpreted in terms of underlying physiological mechanisms related to either place-dependent peripheral masking or place-independent central masking.     

Forward masking as a method of measuring place specificity of neural excitation in cochlear implants: a review of methods and interpretation

This paper reviews the psychophysical forward masking methods that have been used to investigate place specificity in cochlear implantees. These experiments are relevant for investigating whether the individual variability in outcomes for people using the same device can be explained by individual variations in frequency resolution or whether place specificity is affected by different modes of stimulation (such as bipolar, monopolar or tripolar) in the same person. Unfortunately, there has been no consensus about the methods used to derive electrical forward masking functions, or in the way that they are interpreted in relation to place specificity. Here, the different methods are critically examined to provide insight into the optimal methods that should be used to measure and interpret spatial forward masking functions in electric hearing. It is shown that, in order to separate the temporal effects of masking decay from the place-specificity information, different analyses of the functions are needed depending on whether a fixed-probe or fixed-masker method is employed. The effects of unit of measurement on specificity measures and the effects of subject listening strategy on the forward masked functions are also discussed.     

The mid-level hump at 2 kHz

Shortening the duration of a Gaussian-shaped 2-kHz tone-pip causes the intensity-difference limen (DL) to depart from the "near-miss to Weber's law" and swell into a mid-level hump [Nizami et al., J. Acoust. Soc. Am. 110, 2505-2515 (2001)]. For some subjects the size of this hump approaches or exceeds the size reported for longer tones under forward masking, suggesting that forward masking might make little difference to the DL for very brief probes. To test this hypothesis, DLs were determined over 30 to 90 dB SPL for a brief Gaussian-shaped 2-kHz tone-pip. DLs were obtained first without forward masking, then with the pip placed 10 or 100 ms after a 200-ms 2-kHz tone of 50 dB SPL, or 100 ms after a 200-ms 2-kHz tone of 70 dB SPL. DLs inflated significantly under all forward-masking conditions. DLs also enlarged under an 80 dB SPL forward masker at pip delays of 4, 10, 40, and 100 ms. The peaks of the humps obtained under forward masking clustered around a sensation level (SL) that was significantly lower than the average SL for the peaks of the humps obtained without forward masking. Overall, the results do not support the neuronal-recovery-rate model of Zeng et al. [Hear. Res. 55, 223-230 (1991)], but are not incompatible with the Carlyon and Beveridge hypothesis [J. Acoust. Soc. Am. 93, 2886-2895 (1993)] that nonsimultaneous maskers corrupt the memory trace evoked by the probe.     

Psychophysical estimates of nonlinear cochlear processing in younger and older listeners

The primary goal of this project was to compare the performance of younger and older listeners on a number of psychophysical measures thought to be influenced by nonlinear cochlear processing. Younger (mean of 25.6 years) and older (mean of 63.8 years) listeners with normal hearing were matched (within 5 dB) according to their quiet thresholds at the two test frequencies of 1200 and 2400 Hz. They were similarly matched at the adjacent octave frequencies of 600 and 4800 Hz (within 5 dB at one and 9 dB at the other). Performance was compared on measures of auditory filter shape, psychophysical suppression, and growth of forward masking. There was no difference between the two age groups on these psychophysical estimates reflecting nonlinear processing, suggesting that aging per se does not affect the cochlear nonlinearity, at least for the ages sampled here. The results did, however, consistently demonstrate an age-related increase in the susceptibility to forward masking.     

Psychophysical assessment of the level-dependent representation of high-frequency spectral notches in the peripheral auditory system

To discriminate between broadband noises with and without a high-frequency spectral notch is more difficult at 70-80 dB sound pressure level than at lower or higher levels [Alves-Pinto, A. and Lopez-Poveda, E. A. (2005). "Detection of high-frequency spectral notches as a function of level," J. Acoust. Soc. Am. 118, 2458-2469]. One possible explanation is that the notch is less clearly represented internally at 70-80 dB SPL than at any other level. To test this hypothesis, forward-masking patterns were measured for flat-spectrum and notched noise maskers for masker levels of 50, 70, 80, and 90 dB SPL. Masking patterns were measured in two conditions: (1) fixing the masker-probe time interval at 2 ms and (2) varying the interval to achieve similar masked thresholds for different masker levels. The depth of the spectral notch remained approximately constant in the fixed-interval masking patterns and gradually decreased with increasing masker level in the variable-interval masking patterns. This difference probably reflects the effects of peripheral compression. These results are inconsistent with the nonmonotonic level-dependent performance in spectral discrimination. Assuming that a forward-masking pattern is a reasonable psychoacoustical correlate of the auditory-nerve rate-profile representation of the stimulus spectrum, these results undermine the common view that high-frequency spectral notches must be encoded in the rate-profile of auditory-nerve fibers.     

The effects of cochlear hypothermia on compound action potential tuning

The effects of lowered cochlear temperature on eighth-nerve tuning were assessed by using forward masking of whole nerve action potential (AP) responses to generate AP tuning curves (APTCs) at cochlear temperatures ranging from 38.5 degrees to 30 degrees C for probe frequencies from 8 to 36 kHz. The data indicate that subnormal cochlear temperatures result in: broadened APTCs for probe frequencies above 10 kHz which are interpreted as resulting from reduced hair-cell frequency selectivity, lowered or more sensitive APTC tips where tone-burst thresholds are unchanged, and raised or less sensitive tips where thresholds to tone bursts were elevated. Increased tip sensitivity is explained in terms of enhanced eighth-nerve adaptation which occurred during hypothermia. Experiments directly addressing adaptation were performed, in which the masker-probe interval (delta t) was systematically lengthened. The normalized AP decrement versus delta t functions indicate an enhancement of both the amount and duration of adaptation during hypothermia. Functions relating the growth of response to the masker (AP decrement versus masker intensity functions) were reduced at low temperatures.     

The effect of broadband noise on the human brain-stem auditory evoked response. IV. Additivity of forward-masking and rate-induced wave V latency shifts

The additivity of forward masking and repetitive stimulation effects on wave V of the brain-stem auditory evoked response (BAER) was investigated. The effects of repetitive stimulation were evaluated for a stimulus train (called the adaptation series), with a 12.5-ms within-train interclick interval. The forward masker was a 100-ms, 80-dB SPL broadband noise with forward-masker intervals ranging from 12.5-87.5 ms. Forward masking and repetitive stimulation increased the latency of wave V of the BAER. The combined forward masking/adaptation series produced less wave V latency shift than the summed individual effects. Forward masking reduced wave V amplitude at brief forward masker intervals, while repetitive stimulation did not affect wave V amplitude. Wave V amplitude was decreased for the combined forward masking/adaptation series, and the time course of amplitude recovery of the combination was prolonged compared to the forward masking alone condition. The nonadditivity of forward masking and rate effects on wave V latency is similar to that found for repetitive stimulation and simultaneous masking [Burkard and Hecox, J. Acoust. Soc. Am. 74, 1204-1213 (1983)]. These findings are consistent with the position that forward masking and rate effects on wave V latency are produced by overlapping mechanisms.     

The effect of channel interactions on speech recognition in cochlear implant subjects: predictions from an acoustic model

Acoustic models that produce speech signals with information content similar to that provided to cochlear implant users provide a mechanism by which to investigate the effect of various implant-specific processing or hardware parameters independent of other complicating factors. This study compares speech recognition of normal-hearing subjects listening through normal and impaired acoustic models of cochlear implant speech processors. The channel interactions that were simulated to impair the model were based on psychophysical data measured from cochlear implant subjects and include pitch reversals, indiscriminable electrodes, and forward masking effects. In general, spectral interactions degraded speech recognition more than temporal interactions. These effects were frequency dependent with spectral interactions that affect lower-frequency information causing the greatest decrease in speech recognition, and interactions that affect higher-frequency information having the least impact. The results of this study indicate that channel interactions, quantified psychophysically, affect speech recognition to different degrees. Investigation of the effects that channel interactions have on speech recognition may guide future research whose goal is compensating for psychophysically measured channel interactions in cochlear implant subjects.     

Contributions of suppression and excitation to simultaneous masking: effects of signal frequency and masker-signal frequency relation

This study investigated the contributions of suppression and excitation to simultaneous masking for a range of masker frequencies both below and above three different signal frequencies (750, 2000, and 4850 Hz). A two-stage experiment was employed. In stage I, the level of each off-frequency simultaneous masker necessary to mask a signal at 10 or 30 dB sensation level was determined. In stage II, three different forward-masking conditions were tested: (1) an on-frequency condition, in which the signals in stage I were used to mask probes of the same frequency; (2) an off-frequency condition, in which the off-frequency maskers (at the levels determined in stage I) were used to mask the probes; and (3) a combined condition, in which the on- and off-frequency maskers were combined to mask the probes. If the off-frequency maskers simultaneously masked the signal via spread of excitation in stage I, then the off-frequency and combined maskers should produce considerable forward masking in stage II. If, on the other hand, they masked via suppression, they should produce little or no forward masking. The contribution of suppression was found to increase with increasing signal frequency; it was absent at 750 Hz, but dominant at 4850 Hz. These results have implications for excitation pattern analyses and are consistent with stronger nonlinear processing at high rather than at low frequencies.