Biomedical subjects

A J Oxenham

Publications and source records attributed to A J Oxenham.

At least 19 recordsLinked to original sources

PubMed2001

Forward masking: adaptation or integration?

The aim of this study was to attempt to distinguish between neural adaptation and persistence (or temporal integration) as possible explanations of forward masking. Thresholds were measured for a sinusoidal signal as a function of signal duration for conditions where the delay between the masker offset and the signal offset (the offset-offset interval) was fixed. The masker was a 200-ms broadband noise, presented at a spectrum level of 40 dB (re: 20 microPa), and the signal was a 4-kHz sinusoid, gated with 2-ms ramps. The offset-offset interval was fixed at various durations between 4 and 102 ms and signal thresholds were measured for a range of signal durations at each interval. A substantial decrease in thresholds was observed with increasing duration for signal durations up to about 20 ms. At short offset-offset intervals, the amount of temporal integration exceeded that normally found in quiet. The results were simulated using models of temporal integration (the temporal-window model) and adaptation. For both models, the inclusion of a peripheral nonlinearity, similar to that observed physiologically in studies of the basilar membrane, was essential in producing a good fit to the data. Both models were about equally successful in accounting for the present data. However, the temporal-window model provided a somewhat better account of similar data from a simultaneous-masking experiment, using the same parameters. This suggests that the linear, time-invariant properties of the temporal-window approach are appropriate for modeling forward masking. Overall the results confirm that forward masking can be described in terms of peripheral nonlinearity followed by linear temporal integration at higher levels in the auditory system. However, the difference in predictions between the adaptation and integration models is relatively small, meaning that influence of adaptation cannot be ruled out.

Adaptation, Physiological↗

PubMed2001

Modulation detection interference: effects of concurrent and sequential streaming.

The presence of amplitude fluctuations in one frequency region can interfere with our ability to detect similar fluctuations in another (remote) frequency region. This effect is known as modulation detection interference (MDI). Gating the interfering and target sounds asynchronously is known to lead to a reduction in MDI, presumably because the two sounds become perceptually segregated. The first experiment examined the relative effects of carrier and modulator gating asynchrony in producing a release from MDI. The target carrier was a 900-ms, 4.3-kHz sinusoid, modulated in amplitude by a 500-ms, 16-Hz sinusoid, with 200-ms unmodulated fringes preceding and following the modulation. The interferer (masker) was a 1-kHz sinusoid, modulated by a narrowband noise with a 16-Hz bandwidth, centered around 16 Hz. Extending the masker carrier for 200 ms before and after the signal carrier reduced MDI, regardless of whether the target and masker modulators were gated synchronously or were gated with onset and offset asynchronies of 200 ms. Similarly, when the carriers were gated synchronously, asynchronous gating of the modulators did not produce a release from MDI. The second experiment measured MDI with a synchronous target and masker and investigated the effect of adding a series of precursor tones, which were designed to promote the forming of a perceptual stream with the masker, thereby leaving the target perceptually isolated. Four modulated or unmodulated precursor tones presented at the masker frequency were sufficient to completely eliminate MDI. The results support the idea that MDI is due to a perceptual grouping of the masker and target, and show that conditions promoting sufficient perceptual segregation of the masker and target can lead to a total elimination of MDI.

Adult↗

PubMed2001

Reconciling frequency selectivity and phase effects in masking.

The effects of auditory frequency selectivity and phase response on masking were studied using harmonic tone complex maskers with a 100-Hz fundamental frequency. Positive and negative Schroeder-phase complexes (m+ and m-), were used as maskers and the signal was a long-duration sinusoid. In the first experiment, thresholds for signal frequencies of 1 and 4 kHz were measured as a function of masker bandwidth and number of components. A large difference in thresholds between the m+ and m- complexes was found only when masker components were presented ipsilateral to the signal over a frequency range wider than the traditional critical band, regardless of the absolute number of components. In the second experiment, frequency selectivity was measured in harmonic tone complexes with fixed or random phases as well as in noise, using a variant of the notched-noise method with a fixed masker level. The data showed that frequency selectivity is not affected by masker type, indicating that the wide listening bandwidth suggested by the first experiment cannot be ascribed to broader effective filters in complex-tone maskers than in noise maskers. The third experiment employed a novel method of measuring frequency selectivity, which has the advantage that the overall level at the input and the output of the auditory filter remains roughly constant across all conditions. The auditory filter bandwidth measured using this method was wider than that measured in the second experiment, but may still be an underestimate, due to the effects of off-frequency listening. The data were modeled using a single-channel model with various initial filters. The main findings from the simulations were: (1) the magnitude response of the Gammatone filter is too narrow to account for the phase effects observed in the data; (2) none of the other filters currently used in auditory models can account for both frequency selectivity and phase effects in masking; (3) the Gammachirp filter can be made to provide a good account of the data by altering its phase response. The final conclusion suggests that masker phase effects can be accounted for with a single-channel model, while still remaining consistent with measures of frequency selectivity: effects that appear to involve broadband processing do not necessarily require across-channel mechanisms.

Acoustic Stimulation↗

PubMed2001

Towards a measure of auditory-filter phase response.

This study investigates how the phase curvature of the auditory filters varies with center frequency (CF) and level. Harmonic tone complex maskers were used, with component phases adjusted using a variant of an equation proposed by Schroeder [IEEE Trans. Inf. Theory 16, 85-89 (1970)]. In experiment 1, the phase curvature of the masker was varied systematically and sinusoidal signal thresholds were measured at frequencies from 125 to 8000 Hz. At all signal frequencies, threshold differences of 20 dB or more were observed between the most effective and least effective masker phase curvature. In experiment 2, the effect of overall masker level on masker phase effects was studied using signal frequencies of 250, 1000, and 4000 Hz. The results were used to estimate the phase curvature of the auditory filters. The estimated relative phase curvature decreases dramatically with decreasing CF below 1000 Hz. At frequencies above 1000 Hz, relative auditory-filter phase curvature increases only slowly with increasing CF, or may remain constant. The phase curvature of the auditory filters seems to be broadly independent of overall level. Most aspects of the data are in qualitative agreement with peripheral physiological findings from other mammals, which suggests that the phase responses observed here are of peripheral origin. However, in contrast to the data reported in a cat auditory-nerve study [Carney et al., J. Acoust. Soc. Am. 105, 2384-2391 (1999)], no reversal in the sign of the phase curvature was observed at very low frequencies. Overall, the results provide a framework for mapping out the phase curvature of the auditory filters and provide constraints on future models of peripheral filtering in the human auditory system.

Adult↗

PubMed2000

Effects of masker frequency and duration in forward masking: further evidence for the influence of peripheral nonlinearity.

Forward masking has often been thought of in terms of neural adaptation, with nonlinearities in the growth and decay of forward masking being accounted for by the nonlinearities inherent in adaptation. In contrast, this study presents further evidence for the hypothesis that forward masking can be described as a linear process, once peripheral, mechanical nonlinearities are taken into account. The first experiment compares the growth of masking for on- and off-frequency maskers. Signal thresholds were measured as a function of masker level for three masker-signal intervals of 0, 10, and 30 ms. The brief 4-kHz sinusoidal signal was masked by a 200-ms sinusoidal forward masker which had a frequency of either 2.4 kHz (off-frequency) or 4 kHz (on-frequency). As in previous studies, for the on-frequency condition, the slope of the function relating signal threshold to masker level became shallower as the delay between the masker and signal was increased. In contrast, the slopes for the off-frequency condition were independent of masker-signal delay and had a value of around unity, indicating linear growth of masking for all masker-signal delays. In the second experiment, a broadband Gaussian noise forward masker was used to mask a brief 6-kHz sinusoidal signal. The spectrum level of the masker was either 0 or 40 dB (re: 20 microPa). The gap between the masker and signal was either 0 or 20 ms. Signal thresholds were measured for masker durations from 5 to 200 ms. The effect of masker duration was found to depend more on signal level than on gap duration or masker level. Overall, the results support the idea that forward masking can be modeled as a linear process, preceded by a static nonlinearity resembling that found on the basilar membrane.

Auditory Threshold↗

PubMed2000

Basilar-membrane nonlinearity estimated by pulsation threshold.

The pulsation threshold technique was used to estimate the basilar-membrane (BM) response to a tone at characteristic frequency (CF). A pure-tone signal was alternated with a pure-tone masker. The frequency of the masker was 0.6 times that of the signal. For signal levels from around 20 dB above absolute threshold to 85 dB SPL, the masker level was varied to find the level at which a transition occurred between the signal being perceived as "pulsed" or "continuous" (the pulsation threshold). The transition is assumed to occur when the masker excitation is somewhat greater than the signal excitation at the place on the BM tuned to the signal. If it is assumed further that the response at this place to the lower-frequency masker is linear, then the shape of the masking function provides an estimate of the BM response to the signal. Signal frequencies of 0.25, 0.5, 1, 2, 4, and 8 kHz were tested. The mean slopes of the masking functions for signal levels between 50 and 80 dB SPL were 0.76, 0.50, 0.34, 0.32, 0.35, and 0.41, respectively. The results suggest that compression on the BM increases between CFs of 0.25 and 1 kHz and is roughly constant for frequencies of 1 kHz and above. Despite requiring a subjective criterion, the pulsation threshold measurements had a reasonably low variability. However, the estimated compression was less than in an earlier study using forward masking. The smaller amount of compression observed here may be due to the effects of off-frequency listening.

Adult↗

PubMed2000

Level discrimination of sinusoids as a function of duration and level for fixed-level, roving-level, and across-frequency conditions.

The ability of listeners to detect level differences between two sinusoidal stimuli in a two-interval forced-choice procedure was measured as a function of duration and level in three conditions: (1) the pedestal was fixed in level and the stimuli in the two intervals had the same frequency of either 1 or 2 kHz (fixed-level condition); (2) the pedestal was roved in level over a 20-dB range from trial to trial, but the stimuli still had the same frequency of either 1 or 2 kHz (roving-level condition); and (3) the pedestal was roved in level over a 20-dB range and the two stimuli differed in frequency, such that one was around 1 kHz while the other was around 2 kHz (across-frequency condition). In the fixed-level conditions, difference limens decreased (improved) with both increasing duration and level, as found in previous studies. In the roving-level conditions, difference limens increased and the dependence on duration and level decreased. Difference limens in the across-frequency conditions were generally highest and showed very little dependence on either stimulus duration or level. The results may be understood in terms of different internal noise components with additive variances: In the fixed-level conditions, sensation noise, which is dependent on stimulus attributes such as duration and level, is dominant. In more difficult conditions, where trace-memory and/or across-channel comparisons are required, a more central, stimulus-independent noise dominates.

Adult↗

PubMed2000

Influence of spatial and temporal coding on auditory gap detection.

This study investigated the effect on gap detection of perceptual channels, hypothesized to be tuned to spatial location or fundamental frequency (f0). Thresholds were measured for the detection of a silent temporal gap between two markers. In the first experiment, the markers were broadband noise, presented either binaurally or monaurally. In the binaural conditions, the markers were either diotic, or had a 640-micros interaural time difference (ITD) or a 12-dB interaural level difference (ILD). Reversing the ITD across the two markers had no effect on gap detection relative to the diotic condition. Reversing the ILD across the two markers produced a marked deterioration in performance. However, the same deterioration was observed in the monaural conditions when a 12-dB level difference was introduced between the two markers. The results provide no evidence for the role of spatially tuned neural channels in gap detection. In the second experiment, the markers were harmonic tone complexes, filtered to contain only high, unresolved harmonics. Using complexes with a fixed spectral envelope, where the f0 (of 140 or 350 Hz) was different for the two markers, produced a deterioration in performance, relative to conditions where the f0 remained the same. A larger deterioration was observed when the two markers occupied different spectral regions but had the same f0. This supports the idea that peripheral coding is dominant in determining gap-detection thresholds when the two markers differ along any physical dimension. Higher-order neural coding mechanisms of f0 and spatial location seem to play a smaller role and no role, respectively.

Acoustic Stimulation↗

PubMed1999

Sequential stream segregation in the absence of spectral cues.

This paper investigates the cues used by the auditory system in the perceptual organization of sequential sounds. In particular, the ability to organize sounds in the absence of spectral cues is studied. In the first experiment listeners were presented with a tone sequence ABA ABA ..., where the fundamental frequency (f0) of tone A was fixed at 100 Hz and the f0 difference between tones A and B varied across trials between 1 and 11 semitones. Three spectral conditions were tested: pure tones, harmonic complexes filtered with a bandpass region between 500 and 2000 Hz, and harmonic complexes filtered with a bandpass region chosen so that only harmonics above the tenth would be passed by the filter, thus severely limiting spectral information. Listeners generally reported that they could segregate tones A and B into two separate perceptual streams when the f0 interval exceeded about four semitones. This was true for all conditions. The second experiment showed that most listeners were better able to recognize a short atonal melody interleaved with random distracting tones when the distracting tones were in an f0 region 11 semitones higher than the melody than when the distracting tones were in the same f0 region. The results were similar for both pure tones and complex tones comprising only high, unresolved harmonics. The results from both experiments show that spectral separation is not a necessary condition for perceptual stream segregation. This suggests that models of stream segregation that are based solely on spectral properties may require some revision.

Adult↗

PubMed1999

The role of spectral and periodicity cues in auditory stream segregation, measured using a temporal discrimination task.

In a previous paper, it was shown that sequential stream segregation could be based on both spectral information and periodicity information, if listeners were encouraged to hear segregation [Vliegen and Oxenham, J. Acoust. Soc. Am. 105, 339-346 (1999)]. The present paper investigates whether segregation based on periodicity information alone also occurs when the task requires integration. This addresses the question: Is segregation based on periodicity automatic and obligatory? A temporal discrimination task was used, as there is evidence that it is difficult to compare the timing of auditory events that are perceived as being in different perceptual streams. An ABA ABA ABA... sequence was used, in which tone B could be either exactly at the temporal midpoint between two successive tones A or slightly delayed. The tones A and B were of three types: (1) both pure tones; (2) both complex tones filtered through a fixed passband so as to contain only harmonics higher than the 10th, thereby eliminating detectable spectral differences, where only the fundamental frequency (f0) was varied between tones A and B; and (3) both complex tones with the same f0, but where the center frequency of the spectral passband varied between tones. Tone A had a fixed frequency of 300 Hz (when A and B were pure tones) or a fundamental frequency (f0) of 100 Hz (when A and B were complex tones). Five different intervals, ranging from 1 to 18 semitones, were used. The results for all three conditions showed that shift thresholds increased with increasing interval between tones A and B, but the effect was largest for the conditions where A and B differed in spectrum (i.e., the pure-tone and the variable-center-frequency conditions). The results suggest that spectral information is dominant in inducing (involuntary) segregation, but periodicity information can also play a role.

Adolescent↗

PubMed1999

Inter-relationship between different psychoacoustic measures assumed to be related to the cochlear active mechanism.

The active mechanism in the cochlea is thought to depend on the integrity of the outer hair cells (OHCs). Cochlear hearing loss is usually associated with damage to both inner hair cells (IHCs) and OHCs, with the latter resulting in a reduction in or complete loss of the function of the active mechanism. It is believed that the active mechanism contributes to the sharpness of tuning on the basilar membrane (BM) and is also responsible for compressive input-output functions on the BM. Hence, one would expect a close relationship between measures of sharpness of tuning and measures of compression. This idea was tested by comparing three different measures of the status of the active mechanism, at center frequencies of 2, 4, and 6 kHz, using subjects with normal hearing, with unilateral or highly asymmetric cochlear hearing loss, and with bilateral loss. The first measure, HLOHC, was an indirect measure of the amount of the hearing loss attributable to OHC damage; this was based on loudness matches between the two ears of subjects with unilateral hearing loss and was derived using a loudness model. The second measure was the equivalent rectangular bandwidth (ERB) of the auditory filter, which was estimated using the notched-noise method. The third measure was based on the slopes of growth-of-masking functions obtained in forward masking. The ratio of slopes for a masker centered well below the signal frequency and a masker centered at the signal frequency gives a measure of BM compression at the place corresponding to the signal frequency; a ratio close to 1 indicates little or no compression, while ratios less than 1 indicate that compression is occurring at the signal place. Generally, the results showed the expected pattern. The ERB tended to increase with increasing HLOHC. The ratio of the forward-masking slopes increased from about 0.3 to about 1 as HLOHC increased from 0 to 55 dB. The ratio of the slopes was highly correlated with the ERB (r = 0.92), indicating that the sharpness of the auditory filter decreases as the compression on the BM decreases.

Adult↗

PubMed1998

Psychoacoustic consequences of compression in the peripheral auditory system.

Input-output functions on the basilar membrane of the cochlea show a strong compressive nonlinearity at midrange levels for frequencies close to the characteristic frequency of a given place. This article shows how many different phenomena can be explained as consequences of this nonlinearity, including the "excess" masking produced when 2 nonsimultaneous maskers are combined, the nonlinear growth of forward masking with masker level, the influence of component phase on the effectiveness of complex forward maskers, changes in the ability to detect increments and decrements with level, temporal integration, and the influence of component phase and level on the perception of vowellike sounds. Cochlear hearing loss causes basilar-membrane responses to become more linear. This can account for loudness recruitment, linear additivity of nonsimultaneous masking, linear growth of forward masking, reduced temporal resolution for sounds with fluctuating envelopes, and reduced temporal integration.

Auditory Perception↗

PubMed1998

Temporal integration at 6 kHz as a function of masker bandwidth.

Thresholds were measured for a 6-kHz sinusoidal signal presented within a 500-ms masker. The masker was either a bandpass Gaussian noise of varying bandwidth, or a sinusoid of the same frequency as the signal. The spectrum level of the noise masker was kept constant at 20 dB SPL, and the level of the sinusoidal masker was 40 dB SPL. Thresholds for signal durations between 2 and 300 ms were measured for masker bandwidths ranging from 60 to 12,000 Hz. The masker was spectrally centered around 6 kHz. For masker bandwidths less than 600 Hz, the slope of the temporal integration function decreased with decreasing masker bandwidth. The results are not consistent with current models of temporal integration or temporal resolution. It is suggested that the results at narrow bandwidths can be understood in terms of changes in the power spectrum of the stimulus envelope or modulation spectrum. According to this view, the onset and offset ramps of the signal introduce detectable high-frequency components into the modulation spectrum, which provide a salient cue in narrowband maskers. For broadband maskers, these high-frequency components are masked by the inherent rapid fluctuations in the masker envelope. Additionally, for signal durations between 7 and 80 ms, signal thresholds decreased by up to 5 dB as the masker bandwidth increased from 1200 to 12,000 Hz. The mechanisms underlying this effect are not yet fully understood.

Auditory Perception↗

PubMed1998

Basilar-membrane nonlinearity and the growth of forward masking.

Forward masking growth functions were measured for pure-tone maskers and signals at 2 and 6 kHz as a function of the silent interval between the masker and signal. The inclusion of conditions involving short signals and short masker-signal intervals ensured that a wide range of signal thresholds were recorded. A consistent pattern was seen across all the results. When the signal level was below about 35 dB SPL the growth of masking was shallow, so that signal threshold increased at a much slower rate than masker level. When the signal level exceeded this value, the masking function steepened, approaching unity (linear growth) at the highest masker and signal levels. The results are inconsistent with an explanation for forward-masking growth in terms of saturating neural adaptation. Instead the data are well described by a model incorporating a simulation of the basilar-membrane response at characteristic frequency (which is almost linear at low levels and compressive at higher levels) followed by a sliding intensity integrator or temporal window. Taken together with previous results, the findings suggest that the principle nonlinearity in temporal masking may be the basilar membrane response function, and that subsequent to this the auditory system behaves as if it were linear in the intensity domain.

Auditory Perception↗

PubMed1998

Suppression and the upward spread of masking.

The purpose of this study is to clarify the role of suppression in the growth of masking when a signal is well above the masker in frequency (upward spread of masking). Classical psychophysical models assume that masking is primarily due to the spread of masker excitation, and that the nonlinear upward spread of masking reflects a differential growth in excitation between the masker and the signal at the signal frequency. In contrast, recent physiological studies have indicated that upward spread of masking in the auditory nerve is due to the increasing effect of suppression with increasing masker level. This study compares thresholds for signals between 2.4 and 5.6 kHz in simultaneous and nonsimultaneous masking for conditions in which the masker is either at or well below the signal frequency. Maximum differences between simultaneous and nonsimultaneous masking were small (< 6 dB) for the on-frequency conditions but larger for the off-frequency conditions (15-32 dB). The results suggest that suppression plays a major role in determining thresholds at high masker levels, when the masker is well below the signal in frequency. This is consistent with the conclusions of physiological studies. However, for signal levels higher than about 40 dB SPL, the growth of masking for signals above the masker frequency is nonlinear even in the nonsimultaneous-masking conditions, where suppression is not expected. This is consistent with an explanation based on the compressive response of the basilar membrane, and confirms that suppression is not necessary for nonlinear upward spread of masking.

Adult↗

PubMed1997

A behavioral measure of basilar-membrane nonlinearity in listeners with normal and impaired hearing.

This paper examines the possibility of estimating basilar-membrane (BM) nonlinearity using a psychophysical technique. The level of a forward masker required to mask a brief signal was measured for conditions where the masker was either at, or one octave below, the signal frequency. The level of the forward masker at masked threshold provided an indirect measure of the BM response to the signal, as follows. Consistent with physiological studies, it was assumed that the BM responds linearly to frequencies well below the characteristic frequency (CF). Thus the ratio of the slopes of the masking functions between a masker at the signal frequency and a masker well below the signal frequency should provide an estimate of BM compression at CF. Results obtained from normally hearing listeners were in quantitative agreement with physiological estimates of BM compression. Furthermore, differences between normally hearing listeners and listeners with cochlear hearing impairment were consistent with the physiological effects of damage to the cochlea. The results support the hypothesis that BM nonlinearity governs the nonlinear growth of the upward spread of masking, and suggest that this technique provides a straightforward method for estimating BM nonlinearity in humans.

Adult↗

PubMed1997

Short-term temporal integration: evidence for the influence of peripheral compression.

Thresholds for a 6.5-kHz sinusoidal signal, temporally centered in a 400-ms broadband-noise masker, were measured as a function of signal duration for normally hearing listeners and listeners with cochlear hearing loss over a range of masker levels. For the normally hearing listeners, the slope of the function relating signal threshold to signal duration (integration function) was steeper at medium masker levels than at low or high levels by a factor of nearly 2, for signal durations between 2 and 10 ms, while no significant effect of level was found for signal durations of 20 ms and more. No effect of stimulus level was found for the hearing-impaired listeners at any signal duration. For signal durations greater than 10 ms, consistent with many previous studies, the slope of the integration function was shallower for the hearing-impaired listeners than for the normally hearing listeners. However, for shorter durations, there was no significant difference in slope between the results from the hearing-impaired listeners and those from the normally hearing listeners in the high- and low-level masker conditions. A model incorporating a compressive nonlinearity, representing the effect of basilar-membrane (BM) compression, and a short-term temporal integrator, postulated to be a more central process, can account well for changes in the short-term integration function with level, if it is assumed that the compression is greater at medium levels than at low or high levels by a factor of about 4. This is in reasonable agreement with physiological measurements of BM compression, and with previous psychophysical estimates.

Adult↗

PubMed1997

Increment and decrement detection in sinusoids as a measure of temporal resolution.

Measuring thresholds for the detection of brief decrements in the level of a sinusoid is an established method of estimating auditory temporal resolution. Generally, a background noise is added to the stimulus to avoid the detection of the "spectral splatter" introduced by the decrement. Results are often described in terms of a temporal-window model, comprising a band-pass filter, a compressive nonlinearity, a sliding temporal integrator, and a decision device. In this study, thresholds for increments, as well as decrements, in the level of a 55 dB SPL, 4-kHz sinusoidal pedestal were measured as function of increment and decrement duration in the presence of a broadband background noise ranging in spectrum level from -20 to +20 dB SPL. Thresholds were also measured using a 55-dB, 8-kHz pedestal in the absence of background noise. Thresholds for decrements, in terms of the dB change in level (delta L), were found to be more dependent on duration than those for increments. Also, performance was found to be dependent on background-noise level over most levels tested. Neither finding is consistent with the predictions of the temporal-window model or other similar models of temporal resolution. The difference between increment and decrement detection was more successfully simulated by using a decision criterion based on the maximum slope of the temporal-window output.

Adult↗