Biomedical subjects

S P Bacon

Publications and source records attributed to S P Bacon.

17 recordsLinked to original sources

PubMed1992

Factors influencing temporal effects with notched-noise maskers.

Temporal effects in simulataneous masking were studied by measuring the reduction in the amount of masking produced by a gated masker when that masker was preceded by a 400-ms noise (the precursor) that was usually spectrally identical to the masker. The signal frequency (fs) was 1.0 or 4.0 kHz. Experiment 1 revealed a temporal effect only when there was a spectral notch (centered at fs) in the masker and precursor. For a relative notchwidth of 0.4 fs, the temporal effect was larger at 4.0 than at 1.0 kHz. In experiment 2. where the masker and precursor both consisted of two bands of noise separated by a spectral notch of 0.4 fs, the size of the temporal effect remained essentially constant as the bandwidth of these noise bands increased from 0.2-0.8 kHz. The results from experiment 3 indicated that the temporal effect was largest when the level fo the precursor was equal to the level of the masker. Finally, the results from experiment 4 suggested that the temporal effect may depend upon the frequency region below as well as above fs, but that the frequency region above fs is probably more important.

Acoustic Stimulation↗

PubMed1992

Modulation detection in subjects with relatively flat hearing losses.

Modulation detection thresholds were measured as a function of modulation frequency in 5 normal-hearing subjects and in 8 subjects with relatively flat, slight-to-moderate hearing losses. The carrier was a broadband noise that was sinusoidally amplitude modulated (SAM) in one of two observation intervals. The spectrum level of the carrier ranged from -10 to 50 dB SPL, and, for a given carrier level, modulation frequency varied from 2 to 1024 Hz. The temporal modulation transfer functions (TMTFs) were fitted very well with a simple equation describing a low-pass filter function. The TMTFs from the normal-hearing subjects were relatively independent of carrier level, although the derived time constant tended to increase slightly with decreases in carrier level, from an average value of 2.5 msec at 30 dB SPL to 6.0 msec at -10 dB SPL. In addition, sensitivity to amplitude modulation (AM) decreased by about 4 dB as the pressure spectrum level of the carrier was decreased from 0 to -10 dB SPL. The TMTFs from 7 of the 8 hearing-impaired subjects were similar to those from the normal-hearing subjects when the carriers were presented at equal SPLs, except that the derived time constants were slightly larger in the subjects with hearing impairment. When comparisons were made at comparable sensation levels (SLs), however, the TMTFs from the two groups of subjects were quantitatively similar, with the exception that at the lowest SL (20 dB), hearing-impaired subjects typically were more sensitive to AM than normal-hearing subjects, and the derived time constants from their TMTFs were somewhat smaller. These results, taken together with previously published results, suggest that a broad listening bandwidth is important for normal performance on a temporal resolution task. That the time constant from one of the hearing-impaired subjects was significantly longer than normal, regardless of whether the comparisons were made at equal SPL or equal SL, indicates that other factors can also be important.

Adult↗

PubMed1992

Modulation detection, modulation masking, and speech understanding in noise in the elderly.

Temporal processing of suprathreshold sounds was examined in a group of young normal-hearing subjects (mean age of 26.0 years), and in three groups of older subjects (mean ages of 54.3, 64.8, and 72.2 years) with normal hearing or mild sensorineural hearing loss. Three experiments were performed. In the first experiment (modulation detection), subjects were asked to detect sinusoidal amplitude modulation (SAM) of a broadband noise, for modulation frequencies ranging from 2-1024 Hz. In the second experiment (modulation masking), the task was to detect a SAM signal (modulation frequency of 8 Hz) in the presence of a 100%-modulated SAM masker. Masker modulation frequency ranged from 2-64 Hz. In the final experiment, speech understanding was measured as a function of signal-to-noise ratio in both an unmodulated background noise and in a SAM background noise that had a modulation frequency of 8 Hz and a modulation depth of 100%. Except for a very modest correlation between age and modulation detection sensitivity at low modulation frequencies, there were no significant effects of age once the effect of hearing loss was taken into account. The results of the experiments suggest, however, that subjects with even a mild sensorineural hearing loss may have difficulty with a modulation masking task, and may not understand speech as well as normal-hearing subjects do in a modulated noise background.

Adult↗

PubMed1992

Fringe effects in modulation masking.

Modulation detection thresholds (20 log ms) for a sinusoidally amplitude-modulated (SAM) noise were measured in the presence of a SAM noise masker with a modulation depth (mm) of 1.0 and a modulation frequency of 16 or 64 Hz. The signal and masker carriers were presented continuously, and the signal was modulated during one of the two 500-ms observation intervals. The masker was modulated during both observation intervals and, in some conditions, for a certain amount of time before and after signal modulation. The duration of this "fringe" ranged from 62.5 ms to continuous (masker modulated throughout the thresholds estimate). The first experiment showed that a 500-ms fringe could reduce masked thresholds by 4-6 dB, but only at low signal modulation frequencies (2-8 Hz). In the second and third experiments, it was found that the fringe had to have a duration of 500 ms and a depth of about 0.75 to be maximally effective. A final, supplementary experiment indicated that the fringe effect is not due solely to the fringe that occurs prior to the observation intervals. The results are discussed in terms of both peripheral and central auditory processing.

Adult↗

PubMed1992

Overshoot in normal-hearing and hearing-impaired subjects.

Overshoot was measured in both ears of four subjects with normal hearing and in five subjects with permanent, sensorineural hearing loss (two with a unilateral loss). The masker was a 400-ms broadband noise presented at a spectrum level of 20, 30, or 40 dB SPL. The signal was a 10-ms sinusoid presented 1 or 195 ms after the onset of the masker. Signal frequency was 1.0 or 4.0 kHz, which placed the signal in a region of normal (1.0 kHz) or impaired (4.0 kHz) absolute sensitivity for the impaired ears. For the normal-hearing subjects, the effects of signal frequency and masker level were similar to those published previously. In particular, overshoot was larger at 4.0 than at 1.0 kHz, and overshoot at 4.0 kHz tended to decrease with increasing masker level. At 4.0 kHz, overshoot values were significantly larger in the normal ears: Maximum values ranged from about 7-26 dB in the normal ears, but were always less than 5 dB in the impaired ears. The smaller overshoot values resulted from the fact that thresholds in the short-delay condition were considerably better in the hearing-impaired subjects than in the normal-hearing subjects. At 1.0 kHz, overshoot values for the two groups of subjects more or less overlapped. The results suggest that permanent, sensorineural hearing loss disrupts the mechanisms responsible for a large overshoot effect.

Acoustic Stimulation↗

PubMed1991

Spectral, intensive, and temporal factors influencing overshoot.

Threshold was measured for a 10-msec, 4.0-kHz signal presented near the onset or in the temporal centre of a 400-msec noise masker. Overshoot, the difference (in dB) between these two thresholds, was seen only for masker bandwidths wider than a critical band. The threshold near masker onset, and hence overshoot, could be reduced by the presence of an additional noise that was presented continuously or gated on and off prior to masker onset. The spectral, intensive, and temporal properties of this effect were studied. When the additional noise was continuous and either bandpass filtered with a variable bandwidth or notch filtered with a variable notchwidth, the results indicated that energy both near and remote from the signal frequency contributed to the reduction in overshoot. The effect of this additional noise was highly dependent upon its relative level. When the additional noise was 400 msec in duration and the delay between its offset and the onset of the masker was varied, overshoot "recovered" to its maximum value within about 50 msec. Finally, as the duration of the additional noise was varied from 3 to 400 msec while the time between its offset and masker onset was fixed, the reduction in overshoot was virtually complete for durations of about 25-50 msec. The results are consistent with the notion that overshoot at least partly reflects peripheral adaptation, and that this adaptation is not restricted to the signal frequency channel but, rather, extends in both directions over several channels.

Adult↗

PubMed1991

Binaural modulation masking.

Modulation thresholds were measured in three subjects for a sinusoidally amplitude-modulated (SAM) wideband noise (the signal) in the presence of a second amplitude-modulated wideband noise (the masker). In monaural conditions (Mm-Sm) masker and signal were presented to only one ear; in binaural conditions (M0-S pi) the masker was presented diotically while the phase of modulation of the SAM noise signal was inverted in one ear relative to the other. In experiment 1 masker modulation frequency (fm) was fixed at 16 Hz, and signal modulation frequency (fs) was varied from 2-512 Hz. For monaural presentation, masking generally decreased as fs diverged from fm, although there was a secondary increase in masking for very low signal modulation frequencies, as reported previously [Bacon and Grantham, J. Acoust. Soc. Am. 85, 2575-2580 (1989)]. The binaural masking patterns did not show this low-frequency upturn: binaural thresholds continued to improve as fs decreased from 16 to 2 Hz. Thus, comparing masked monaural and masked binaural thresholds, there was an average binaural advantage, or masking-level difference (MLD) of 9.4 dB at fs = 2 Hz and 5.3 dB at fs = 4 Hz. In addition, there were positive MLDs for the on-frequency condition (fm = fs = 16 Hz: average MLD = 4.4 dB) and for the highest signal frequency tested (fs = 512 Hz: average MLD = 7.3 dB). In experiment 2 the signal was a SAM noise (fs = 16 Hz), and the masker was a wideband noise, amplitude-modulated by a narrow band of noise centered at fs. There was no effect on monaural or binaural thresholds as masker modulator bandwidth was varied from 4 to 20 Hz (the average MLD remained constant at 8.0 dB), which suggests that the observed "tuning" for modulation may be based on temporal pattern discrimination and not on a critical-band-like filtering mechanism. In a final condition the masker modulator was a 10-Hz-wide band of noise centered at the 64-Hz signal modulation frequency. The average MLD in this case was 7.4 dB. The results are discussed in terms of various binaural capacities that probably play a role in binaural release from modulation masking, including detection of varying interaural intensity differences (IIDs) and discrimination of interaural correlation.

Acoustic Stimulation↗

PubMed1991

The effect of pure-tone forward masking on overshoot.

The overshoot effect can be reduced by temporary hearing loss induced by aspirin or exposure to intense sound. The present study simulated a hearing loss at 4.0 kHz via pure-tone forward masking and examined the effect of the simulation on threshold for a 10-ms, 4.0-kHz signal presented 1 ms after the onset of a 400-ms, broadband noise masker whose spectrum level was 20 dB SPL. Masker frequency was 3.6, 4.0, or 4.2 kHz, and masker level was 80 dB SPL. Subject-dependent delays were determined such that 10 or 20 dB of masking at 4.0 kHz was produced. In general, the pure-tone forward masker did not reduce the simultaneous-masked threshold, suggesting that elevating threshold with a pure-tone forward masker does not sufficiently simulate the effect of a temporary hearing loss on overshoot.

Adolescent↗

PubMed1990

Temporal effects in simultaneous masking by vowel and consonant-vowel maskers.

Temporal effects in simultaneous masking were studied using synthetic vowel (V) and consonant-vowel (CV) maskers. For the steady-state V maskers (/i,a,u/), signals were presented at the beginning or in the temporal center of the masker. The masking patterns generally reflected the formant differences among the vowels, and the formant structure of each V masker was more clearly revealed when the signal was presented in the temporal center of the masker. For the CV maskers (/bi,gi/), signals were presented at the beginning of the (consonant portion of the) masker, at the beginning of the vowel portion of the masker, or in the temporal center of the masker. The second-formant difference between the maskers (observed acoustically at their onset) was generally revealed in the masking patterns when the signal was presented at the beginning of the consonant; this difference in the masking patterns was also present, to a lesser extent, when the signal was presented at the beginning of the vowel, where the two maskers were identical acoustically. The masking patterns for the two CV maskers were virtually identical when the signal was presented in the temporal center of the masker. These data extend previous tone-on-tone masking data and suggest that the auditory system requires a certain amount of time to represent most accurately the acoustic spectrum of both steady-state and dynamic complex maskers.

Adult↗

PubMed1990

Regression of tendon xanthomas in patients with familial hypercholesterolemia treated with lovastatin.

Plasma concentrations of total and low-density lipoprotein cholesterol are increased twofold to threefold in patients with heterozygous familial hypercholesterolemia. This sustained increase leads to accelerated rates of cholesterol deposition in the coronary arteries and to the development of tendon xanthomas. To assess whether hypolipidemic therapy with lovastatin, alone and in combination therapy with colestipol hydrochloride or nicotinic acid, results in regression of lipid deposits in the tendons of these patients, we have measured Achilles tendon diameters by xeroradiography before and after treatment. In 20 patients treated for a mean of 43 months (during which time plasma cholesterol concentrations decreased from 430 to 247 mg/dL), the diameter of both the left and right Achilles tendons measured at three different locations decreased by 0.55 to 1.5 mm. Larger reductions were seen in the tendons of seven of these patients who were treated for a mean of 64 months and whose mean concentrations of cholesterol fell from 488 to 279 mg/dL. We conclude that effective long-term hypolipidemic therapy leads to diminution in the size of Achilles tendon xanthomas in patients with heterozygous familial hypercholesterolemia and that such therapy is associated with mobilization of tissue stores of cholesterol in these patients.

Achilles Tendon↗

PubMed1990

Effect of masker level on overshoot.

Overshoot refers to the phenomenon where signal detectability improves for a short-duration signal as the onset of that signal is delayed relative to the onset of a longer duration masker. A popular explanation for overshoot is that it reflects short-term adaptation in auditory-nerve fibers. In this study, overshoot was measured for a 10-ms, 4-kHz signal masked by a broadband noise. In the first experiment, masker duration was 400 ms and signal onset delay was 1 or 195 ms; masker spectrum level ranged from - 10-50 dB SPL. Overshoot was negligible at the lowest masker levels, grew to about 10-15 dB at the moderate masker levels, but declined and approached 0 dB at the highest masker levels. In the second experiment, the masker duration was reduced to 100 ms, and the signal was presented with a delay of 1 or 70 ms; masker spectrum level was 10, 30, or 50 dB SPL. Overshoot was about 10 dB for the two lower masker levels, but about 0 dB at the highest masker level. The results from the second experiment suggest that the decline in overshoot at high masker levels is probably not due to auditory fatigue. It is suggested, instead, that the decline may be attributable to the neural response at high levels being dominated by those auditory-nerve fibers that do not exhibit short-term adaptation (i.e., those with low spontaneous rates and high thresholds).

Adult↗

PubMed1989

The hypolipidemic effects of gemfibrozil in type V hyperlipidemia. A double-blind, crossover study.

Thirteen patients with phenotypic type V hyperlipidemia were treated with either gemfibrozil (Lopid) or a placebo in a randomized, double-blind, crossover study for two 8-week periods. A 4-week baseline period of a low-fat diet preceded the study and served as a dietary control period. A 4-week washout period followed the two 8-week periods. Compared with the placebo phase, gemfibrozil produced a significant reduction in the concentrations of total plasma triglycerides (21.03 vs 5.50 mmol/L) and very low-density lipoprotein triglycerides (14.40 vs 4.59 mmol/L) as well as in total plasma cholesterol levels (10.88 vs 5.62 mmol/L) and very low-density lipoprotein cholesterol (6.66 vs 2.15 mmol/L). Chylomicronemia was virtually abolished by the drug treatment. As expected in treated patients with type V hyperlipidemia, concentrations of low-density lipoprotein cholesterol rose after therapy with gemfibrozil (3.08 mmol/L) as compared with placebo (1.84 mmol/L); high-density lipoprotein cholesterol also increased (0.85 mmol/L after therapy with gemfibrozil, 0.62 mmol/L after placebo). The previously very low values for both of these lipoproteins increased at the same time that the total plasma cholesterol value decreased. We conclude that gemfibrozil is a well-tolerated and effective hypolipidemic agent for the treatment of patients with severe hypertriglyceridemia when used in conjunction with a low-fat diet.

Adult↗

PubMed1989

Modulation masking: effects of modulation frequency, depth, and phase.

Modulation thresholds were measured for a sinusoidally amplitude-modulated (SAM) broadband noise in the presence of a SAM broadband background noise with a modulation depth (mm) of 0.00, 0.25, or 0.50, where the condition mm = 0.00 corresponds to standard (unmasked) modulation detection. The modulation frequency of the masker was 4, 16, or 64 Hz; the modulation frequency of the signal ranged from 2-512 Hz. The greatest amount of modulation masking (masked threshold minus unmasked threshold) typically occurred when the signal frequency was near the masker frequency. The modulation masking patterns (amount of modulation masking versus signal frequency) for the 4-Hz masker were low pass, whereas the patterns for the 16- and 64-Hz maskers were somewhat bandpass (although not strictly so). In general, the greater the modulation depth of the masker, the greater the amount of modulation masking (although this trend was reversed for the 4-Hz masker at high signal frequencies). These modulation-masking data suggest that there are channels in the auditory system which are tuned for the detection of modulation frequency, much like there are channels (critical bands or auditory filters) tuned for the detection of spectral frequency.

Acoustic Stimulation↗

PubMed1989

Temporal overshoot in simultaneous-masked psychophysical tuning curves from normal and hearing-impaired listeners.

Simultaneous-masked psychophysical tuning curves (PTCs) were obtained from normal-hearing and sensorineural hearing-impaired listeners. The 20-ms signal was presented at the onset or at the temporal center of the 400-ms masker. For the normal-hearing listeners, as shown previously [S. P. Bacon and B. C. J. Moore, J. Acoust. Soc. Am. 80, 1638-1645 (1986)], the PTCs were sharper on the high-frequency side for a signal in the temporal center of the masker. For the hearing-impaired listeners, however, the shape of the PTC was virtually independent of the temporal position of the signal. These data suggest that the mechanisms responsible for sharpening the PTC with time in normal-hearing listeners are ineffective in listeners with moderate-to-severe sensorineural hearing loss.

Attention↗

PubMed1988

Intensity discrimination, increment detection, and magnitude estimation for 1-kHz tones.

Intensity difference limens (DLs) were measured over a wide intensity range for 200-ms, 1-kHz gated tones and for 200-ms increments in continuous 1-kHz tones. Magnitude estimates also were obtained for the gated tones over a comparable intensity range. The discrimination data are in general agreement with those from earlier studies but they extend them by showing: (1) good discrimination for gated tones over at least a 115-dB dynamic range; (2) a slight increase in the relative DL (delta I/I) as intensity increases above 95 dB SPL; (3) smaller DLs for increments than for gated tones, with the difference approximately independent of intensity; (4) negligible "negative masking" when thresholds are expressed as intensity differences (delta I). For two of the three subjects, magnitude estimates do not conform to a single-exponent power law for suprathreshold intensities. Over the middle range of intensities where a single exponent is appropriate, the value of the exponent is less than 0.1 for all subjects.

Acoustic Stimulation↗

PubMed1988

Temporal effects in simultaneous pure-tone masking in subjects with high-frequency sensorineural hearing loss.

Temporal effects in simultaneous pure-tone masking were studied in three subjects with a high-frequency sensorineural hearing loss. The masker level was generally 80 dB SPL, and the signal level was varied adaptively to threshold. Masker frequency was always 1.2 times the signal frequency, and three different frequency regions were studied: (1) signal and masker in region of normal hearing; (2) signal in region of normal hearing and masker in region of hearing loss; and (3) signal and masker in region of hearing loss. In the first experiment, the masker was either gated synchronously with the 20-ms signal or was presented continuously. The gated-continuous threshold difference was largest when both the masker and signal were in a region of normal hearing; that difference decreased, though was not eliminated, when either the masker or the signal-plus-masker was in a region of hearing loss. In the second experiment, threshold was measured for the 20-ms signal as a function of its temporal position within a 400-ms masker. Consistent with the first experiment, the biggest change in masking over time generally occurred when the signal and masker were in a region of normal hearing. These data suggest that the mechanisms responsible for temporal effects in normal-hearing subjects (and in regions of normal hearing in subjects with a hearing loss) are adversely affected by (even a mild) sensorineural hearing loss. Moreover, these data suggest that what may be most important for a normal temporal effect is the integrity of the frequency region where the masker is presented.

Aged↗

PubMed1987

Effects of pure-tone forward masker duration on psychophysical measures of frequency selectivity.

The effects of forward masker duration on psychophysical measures of frequency selectivity were investigated in two experiments. In both experiments, masker duration was 50 or 400 ms, signal duration was 20 ms, and there was no delay between masker offset and signal onset. In the first experiment, growth-of-masking functions were measured for a masker whose frequency was below, at, or above the 1000-Hz signal frequency. From those data, input filter patterns (IFPs) were plotted for masker levels from 40-90 dB SPL. In the second experiment, masking patterns (MPs) were measured for a 1000-Hz masker presented at 50, 70, and 90 dB SPL. Both measures of frequency selectivity (IFPs and MPs) indicate that frequency selectivity is greater for the 400-ms masker. These data suggest that there may be a sharpening of frequency selectivity with time at a stage prior to the adaptation observed in forward masking.

Adult↗