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Biomedical subjects

S P Bacon

Publications and source records attributed to S P Bacon.

At least 19 recordsLinked to original sources
PubMed1999

Some effects of background noise on modulation detection interference.

Modulation thresholds were obtained for a 2000-Hz signal carrier modulated at a rate of 10 Hz. Thresholds were obtained without a masker carrier and in the presence of a masker carrier that was either unmodulated or modulated at a rate of 10 Hz and a depth of 100% (m(m) = 1.0). Of primary interest was whether the amount of interference caused by the masker was influenced by the frequency proximity of the masker to the signal, and whether background noise had an influence on that proximity effect. In general, for masker carriers higher in frequency than the 2000-Hz signal carrier, there was a tendency for the interference to decline as the masker was moved farther away from the signal for masker carriers lower than 2000 Hz, there was little or no proximity effect. Broadband noise eliminated the proximity effect obtained with an unmodulated masker, but not that obtained with a modulated masker. Results with a narrowband noise suggest that the broadband noise has its effect by masking the high-frequency side of the signal's excitation pattern. These results, as well as the results of an excitation pattern analysis, suggest that the proximity effect with all unmodulated masker may be mediated via a peripheral, within-channel interaction, whereas that with a modulated masker may be mediated via a central, across-channel interaction.

Acoustic Stimulation↗
PubMed1999

Psychophysical measures of auditory nonlinearities as a function of frequency in individuals with normal hearing.

In order to gain a better understanding of how auditory nonlinear phenomena vary as a function of location along the cochlea, several psychophysical measures of nonlinearity were examined as a function of signal frequency. Six normal-hearing individuals completed three experiments, each designed to measure one aspect of nonlinear behavior: (1) the effects of level on frequency selectivity in simultaneous masking, measured using notched-noise maskers at spectrum levels of 30 and 50 dB, (2) two-tone suppression, measured using forward maskers at the signal frequency (fs) and suppressor tones above fs, and (3) growth of masking, measured using forward maskers below fs at a signal/masker frequency ratio of 1.44. Four signal frequencies (375, 750, 1500, and 3000 Hz) were tested to sample the nonlinear behavior at different locations along the basilar membrane, in order to test the hypothesis that the apical (low-frequency) region of the cochlea behaves more linearly than the basal (high-frequency) region. In general, all three measures revealed a progressive increase in nonlinear behavior as signal frequency increased, with little or no nonlinearity at the lowest frequency, consistent with the hypothesis.

Adult↗
PubMed1998

The effects of hearing loss and noise masking on the masking release for speech in temporally complex backgrounds.

Speech recognition was measured in three groups of listeners: those with sensorineural hearing loss of (presumably) cochlear origin (HL), those with normal hearing (NH), and those with normal hearing who listened in the presence of a spectrally shaped noise that elevated their pure-tone thresholds to match those of individual listeners in the HL group (NM). Performance was measured in four backgrounds that differed only in their temporal envelope: steady-state (SS) speech-shaped noise, speech-shaped noise modulated by the envelope of multi-talker babble (MT), speech-shaped noise modulated by the envelope of single-talker speech (ST), and speech-shaped noise modulated by a 10-Hz square wave (SQ). Threshold signal-to-noise ratios (SNRs) were typically best in the ST and especially the SQ conditions, indicating a masking release in those modulated backgrounds. SNRs in the SS and MT conditions were essentially identical to one another. The masking release was largest in the listeners in the NH group, and it tended to decrease as hearing loss increased. In 5 of the 11 listeners in the HL group, the masking release was nearly identical to that obtained in the NM group matched to those listeners; in the other 6 listeners, the release was smaller than that in the NM group. The reduced masking release was simulated best in those HL listeners for whom the masking release was relatively large. These results suggest that reduced masking release for speech in listeners with sensorineural hearing loss can only sometimes be accounted for entirely by reduced audibility.

Adult↗
PubMed1998

Masking by sinusoidally amplitude-modulated tonal maskers.

In experiment 1, masking patterns were obtained with a tonal masker that was sinusoidally amplitude modulated (SAM) at a rate of 8 Hz and a depth (m) of 1.0. The signal was centered at a masker peak or masker valley. Masker frequency (fm) was 750, 1350, or 2430 Hz, and signal frequency (fs) ranged from 0.8 to 1.62 fm. Thresholds were generally higher for a signal in a masker peak than in a masker valley. The magnitude of this peak-to-valley (PV) difference was governed by fs/fm, rather than by fs, and was largest for fs > fm. The PV differences were smallest at the lowest fm, at least when fs > fm. In experiment 2, growth-of-masking functions were measured (fm = 1350 Hz, fs = 1.44fm). The masker was modulated at a depth (m) of 1.0, 0.75, or 0.50. These thresholds were compared with those obtained with an unmodulated masker in forward or simultaneous masking. The comparisons suggest that thresholds for a signal at a peak of an 8-Hz SAM masker are due to simultaneous masking, while those in a valley are due primarily to forward masking when m = 1.0 or simultaneous masking when m = 0.75 or 0.50. For these masker depths, the PV difference first increased but then decreased as masker level increased from 60 to 90 dB SPL. This was a consequence of the slope of the masking function for peak placement changing from a value greater than 2.0 to a value of 1.0 at the highest signal levels (an effect that was also observed with the unmodulated simultaneous masker), a result that may be understood in terms of basilar membrane nonlinearity.

Adult↗
PubMed1998

Psychophysical suppression as a function of signal frequency: noise and tonal maskers.

Physiological studies have suggested that the basal region of the cochlea is more nonlinear than the apical region. To evaluate this possibility psychophysically, suppression was investigated across signal frequency (250, 500, 1000, 2000, and 4000 Hz) in a forward-masking paradigm using both noise and tonal maskers/suppressors. Masker duration was 200 ms, signal duration was 20 or 40 ms, and signal delay was 0 or 20 ms; the longer delay was necessary to eliminate potential confusion effects observed with the (narrow-band) noise masker. When using a noise masker (spectrum level of 40 dB), suppression was determined by comparing the threshold in the presence of a broadband masker with that in the presence of a critical band (ERB) masker. When using a tonal masker (masker level of 50 dB SPL, suppressor level of 70 dB SPL, with the suppressor frequency being 1.2 times the masker/signal frequency), suppression was determined by comparing the threshold in the presence of the masker plus suppressor with that in the presence of the masker alone. The magnitude of suppression was determined either by the measured change in signal threshold or by the inferred change in masker level. Regardless of how suppression was quantified, for both masker types, the amount of suppression increased as signal frequency increased up to about 1000 Hz, but then reached an asymptote or decreased somewhat as signal frequency increased to 4000 Hz. The magnitude of suppression was much larger with a noise masker than with a tonal masker, which could be a result of the different number of components in the masker which might serve as a suppressor.

Adult↗
PubMed1997

Effect of low-frequency gain reduction on speech recognition and its relation to upward spread of masking.

Speech recognition was measured in listeners with normal hearing and in listeners with sensorineural hearing loss under conditions that simulated hearing aid processing in a low-pass and speech-shaped background noise. Differing amounts of low-frequency gain reduction were applied during a high-frequency monosyllable test and a sentence level test to simulate the frequency responses of some commercial hearing aids. The results showed an improvement in speech recognition with low-frequency gain reduction in the low-pass noise, but not in the speech-shaped background noise. Masking patterns also were obtained with the two background noises at 70 and 80 dB SPL to compare with the speech results. There was no correlation observed between the masking results and the improvement in speech recognition with low-frequency gain reduction.

Adult↗
PubMed1997

Masking by modulated and unmodulated noise: effects of bandwidth, modulation rate, signal frequency, and masker level.

The threshold for a sinusoidal signal masked by a band of noise is often times lower when the masking noise is modulated than when it is unmodulated. The difference in masked thresholds is referred to as the modulated-unmodulated difference, or MUD. These present experiments examined the effects of masker bandwidth, masker rate, and masker level on the MUD at several different signal frequencies. The MUD generally increased with increasing masker bandwidth; for masker bandwidths wider than a critical band (or an equivalent rectangular bandwidth-ERB), the results may be influenced by across-channel processes underlying comodulation masking release. The MUD for an ERB masker (MUDERB) was influenced less by masker rate than was the MUD for a broadband (BB) masker (MUDBB). The MUDERB and especially the MUDBB increased significantly with increasing masker level when the modulated masker was modulated at a depth (m) of 1.0, but not when it was modulated at a depth of 0.75. These results have significant implications for extending the MUD paradigm to hearing-impaired subjects. Finally, the MUDERB and the MUDBB increased with increasing signal frequency. This effect for the ERB masker is largely (if not completely) due to the wider absolute bandwidths at higher frequencies. The effect with the BB masker may be influenced by differences in the magnitude of suppression across frequency.

Adult↗
PubMed1997

The modulated-unmodulated difference: effects of signal frequency and masker modulation depth.

The masked threshold for a signal is often times lower when the masker is modulated than when it is unmodulated. The difference in masked thresholds is referred to as the modulated-unmodulated difference, or MUD. The purpose of the present study was to follow up on the results of a previous study [Bacon et al., J. Acoust. Soc. Am. 101, 1600-1610 (1997)] which showed that the MUD is larger for high than for low signal frequencies, both when the masker is no wider than a critical band (and the processing is solely within channel) and when it is broadband (and the processing may be both within and across channel). The present results indicate that the effects of signal frequency primarily exist only when the modulated masker is modulated at a depth greater than about 0.75, and that at these large depths, thresholds in the presence of the modulated masker are governed largely by forward masking. By far, the effect of signal frequency is larger with the broadband masker than with the critical-band masker, suggesting that there may be an across-channel process whose contribution is greater at high than at low signal frequencies. It is argued here that this across-channel process may be related to psychophysical suppression.

Adult↗
PubMed1997

Amplitude modulation depth discrimination of a sinusoidal carrier: effect of stimulus duration.

Discrimination of the change in depth of sinusoidal amplitude modulation (AM) was investigated as a function of stimulus duration. The carrier frequency was 4000 Hz, the standard modulation depth (m) was either 0.1, 0.18, or 0.3, and the modulation rate was either 10, 20, 40, or 80 Hz. For all standard depths and modulation rates, threshold (delta m) decreased by more than a factor o two as stimulus duration doubled from the shortest duration used up to a certain duration (critical duration), beyond which the threshold decreased only slightly or remained constant. The critical duration corresponded to about four cycles of modulation. Psychometric functions were measured for different stimulus durations to examine the extent to which a multiple-looks model could explain the present data. This model provided a reasonable prediction of the change in AM depth discrimination threshold as a function of stimulus duration.

Adult↗
PubMed1996

The effect of level and relative frequency region on the recovery of overshoot.

Overshoot--in particular, threshold for a signal near masker onset--can be reduced by presenting a stimulus (precursor) just prior to masker onset. The recovery of overshoot can be examined by varying the delay between the offset of the precursor and the onset of the masker, where "recovery" denotes an increase in the threshold for a signal near masker onset. The present study examined the effects of stimulus level and relative frequency region on this recovery. In all experiments, the masker was a broadband of noise and the signal was a 4-kHz sinusoid. The first experiment examined the effects of masker level on overshoot in order to choose two levels (one "low" and one "high") that produced similar amounts of overshoot; these levels were used in the remaining experiments. In the second experiment, the precursor was identical to the masker, and recovery functions were measured for both low and high masker and precursor levels. there was no consistent difference in the recovery functions between the two levels. In the third experiment, the precursor was divided into two bands (one below and one above 4 kHz); one was presented continuously while the other was gated as in experiment 2. The recovery was more complete when the band above 4 kHz was gated, although the recovery was usually less than that observed in experiment 2 when (effectively) both bands were gated. The results suggest that the frequency regions on both sides of the signal are important for the recovery of overshoot, but that the frequency region above the signal may be more important than the region below.

Adolescent↗
PubMed1995

Effects of combining maskers in modulation detection interference.

The threshold for detecting 10-Hz amplitude modulation of a 2000-Hz carrier was measured in quiet, in the presence of an unmodulated masker, and in the presence of an amplitude-modulated masker. Two experiments were run; in each, the masker consisted of one or two sinusoidal carriers (chosen from among the frequencies of 800, 1600, 2400, and 3200 Hz). In experiment 1, the modulation rate of the masker ranged from 2 to 80 Hz. The "tuning" in the modulation domain was not affected much by the masker carrier frequency or the increase from one to two carriers. The amount of interference, however, was sometimes greater in the two-carrier condition, although this resulted primarily from the presence of the carriers and not from their modulation. In experiment 2, the modulation rate of each single-carrier masker ranged from 2 to 80 Hz (as in experiment 1), but for the two-carrier conditions, all possible combinations of two carriers (2400 and 3200 Hz) and three masker rates (5, 10, and 20 Hz) were evaluated. In general, the combination of two modulated carriers did not produce more interference than that produced by the more interfering carrier presented alone. Thus the results from both experiments provide little evidence for an additivity of modulation detection interference.

Adult↗
PubMed1995

Some factors influencing comodulation masking release and across-channel masking.

The purpose of this study was to determine whether comodulation masking release (CMR) and across-channel masking (ACM) are by-products of a similar across-channel mechanism. This was addressed by examining how the two are affected by stimulus manipulations expected to influence their magnitude. Subjects were required to detect a 1000-Hz signal in the presence of a masker that consisted of a 1000-Hz (on-frequency) component alone or that component and up to six flanking components (500, 600, 700, 1300, 1400, and 1500 Hz). The on-frequency and flanking components typically were sinusoidally amplitude modulated at 10 Hz, although not necessarily in phase with one another. In experiment 1, the amount of CMR and ACM was highly influenced by whether the signal consisted of one or three 50-ms tone bursts; in fact, ACM was only observed when the signal was a train of three 50-ms tone bursts. In experiments 2 and 3, CMR tended to increase as the modulation depth or the number of flanking components increased, whereas ACM was relatively unaffected by these manipulations. In addition, ACM was observed under dichotic situations, whereas CMR was not. Taken together, the results suggest that ACM and CMR may be mediated by different mechanisms.

Adult↗
PubMed1994

Intensity discrimination and increment detection at 16 kHz.

When presented for several seconds, a very high-frequency tone can decay to inaudibility in subjects with normal hearing. The purpose of the present study was to determine how such a tone behaves once it is inaudible. Intensity difference limens (DLs) at 16 kHz were measured for gated (audible) and continuous (inaudible) pedestals over a range of pedestal sensation levels from about 0-60 dB, and were compared with those obtained in the same two subjects at 1 kHz [N. F. Viemeister and S. P. Bacon, J. Acoust, Soc. Am. 84, 172-178 (1988)]. The results at the two frequencies were remarkably similar, indicating, among other things, that a continuous 16-kHz pedestal--despite being inaudible-behaves as if it were audible. In addition, the results suggest that there is little or no relationship between high-frequency tone decay and intensity DLs. The locus of this long-term adaptation effect is presumably peripheral to the site where binaural interactions occur, and may be at the hair cell or auditory nerve. The intensity DLs are more consistent with a multiplicative model of (long-term) adaptation than with a subtractive model, suggesting that the nature of this adaptation is different from that which characterizes short-term adaptation.

Acoustic Stimulation↗
PubMed1994

Monotic and dichotic modulation detection interference in practiced and unpracticed subjects.

The threshold for detecting 10-Hz amplitude modulation of a 1-kHz carrier was measured in quiet and in the presence of a 4-kHz masker carrier that was either unmodulated or amplitude modulated at a depth of 1.0 and at rates from 2 to 80 Hz. The signal and masker were presented to the same ear (monotic condition) or to opposite ears (dichotic condition), and the subjects either had no previous experience with psychoacoustic experiments (n = 10) or had from 16 to about 70 h of experience with modulation-detection tasks (n = 4). There were no significant differences between the two groups of subjects, although there was a significant effect of presentation mode. Thresholds generally were higher in the monotic condition, particularly when the masker rate was similar to the signal rate. According to excitation-pattern analyses, the greater interference in the monotic condition is unlikely due to peripheral interactions.

Acoustic Stimulation↗
PubMed1993

Modulation detection interference under conditions favoring within- or across-channel processing.

A series of experiments was conducted to examine modulation detection interference (MDI) under two general conditions: one where the processing was likely to be across channel (2.0-kHz signal, 4.0-kHz masker), and the other where the possibility of within-channel processing was much more likely (2.0-kHz signal, 1.8-kHz masker). In the first experiment, MDI was measured as the modulation frequency of the signal and masker was increased from 5 to 100 Hz. The amount of MDI decreased as modulation frequency increased from 5 to 20 Hz, beyond which it remained approximately constant. This effect of common modulation frequency was somewhat smaller for the 1.8-kHz masker. The second experiment examined the effects of relative modulator phase between the masker and signal for modulation frequencies of 10 or 100 Hz. In general, there was not a consistent effect of phase. However, when the signal and masker modulators were in-phase, there was a tendency, on average, for the amount of MDI to be at a maximum when the masker frequency was 1.8 kHz and at a minimum when it was 4.0 kHz. In the third experiment, increases in masker modulation depth usually resulted in nearly proportional increases in signal modulation depth at threshold; this was true for both masker carriers, although the increase was slightly greater for the 1.8-kHz masker. The final experiment examined the effects of level for conditions where the signal and masker carriers were either equal or unequal in level. When they were equal in level, the amount of MDI increased somewhat with increases in level for both masker carriers. Comparing the amount of MDI in these conditions with those where the carriers were unequal in level revealed that the amount of MDI was considerably less when the signal was more intense than the masker. This occurred almost exclusively for the 1.8-kHz masker, however, suggesting that it may reflect a within-channel effect. Taken together, the results from the four experiments suggest that the processing underlying MDI was generally the same in the presence of both the 1.8- and 4.0-kHz masker carriers, although within-channel processing via spread of excitation probably influenced some of the results with the 1.8-kHz masker.

Acoustic Stimulation↗
PubMed1993

Modulation detection interference: some spectral effects.

Several experiments are described in which the threshold for detecting 10-Hz amplitude modulation (AM) of a 2000-Hz signal carrier was measured in the presence of a masker that consisted of one to ten carrier frequencies. The masker was either unmodulated or amplitude modulated at a depth of 0.5 and a frequency of 10 Hz. In experiment 1, threshold was measured as a function of the frequency of a single masker carrier. The patterns of the interference effect were similar whether the masker was modulated or not, suggesting that some of the interference was due to a within-channel effect (spread of excitation). In experiment 2, it was shown that a masker consisting of two carrier frequencies could produce more interference than either carrier alone. However, there was generally no additional interference when more than two carriers were presented, at least up to the ten tested here (experiments 3 and 4). In experiment 5, the masker carrier frequencies were either harmonically or nonharmonically related to the signal carrier frequency. There was little difference in the size of the interference effect between the two maskers, suggesting that harmonicity may not play an important role in modulation detection interference.

Acoustic Stimulation↗
PubMed1993

Detection and identification of a single modulated carrier in a complex sound.

Hall and Grose [J. Acoust. Soc. Am. 90, 3028-3035 (1991)] reported that subjects had difficulty in deciding which carrier in a two-carrier complex sound was modulated. The present experiments examined how the ability to identify a single modulated carrier was affected by the number of carriers in the complex and by harmonicity. Initially, thresholds were measured for detecting amplitude modulation of a single carrier in a complex sound. Thresholds were higher when that carrier was one of the inner carriers in a six-carrier harmonic or inharmonic complex than when it formed part of a two-carrier complex. Thresholds were only slightly, if at all, higher when the modulated carrier was varied randomly from trial to trial than when its frequency was fixed within a block of trials. Next, subjects were required to decide whether the frequency of a single modulated carrier (with a suprathreshold modulation depth) in a complex sound was the same as or different from the frequency of a probe composed of a single modulated carrier. They generally performed well above chance. Performance was not greatly affected by whether the probe was presented before or after the complex, but was generally slightly better for a modulation depth of 100% than for a depth of 50%. Randomly varying the level of each carrier in the complexes from one stimulus to the next produced only a slight impairment of performance, indicating that short-term across-frequency differences in level were not used to identify the modulated carrier in experiment 2. Overall, performance was best for the six-carrier harmonic complex, less good for the six-carrier inharmonic complex, and worst for the two-carrier complex. The results are interpreted in terms of perceptual grouping.

Auditory Perception↗